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Kim, K., Lees, J.M. and Ruiz, M., (2014) Source mechanism of Vulcanian eruption at Tungurahua Volcano, Ecuador, derived from seismic moment tensor inversions, J. Geophys. Res.. : pp. in press.
BibTeX:
@article{Kim2014,
  author = {Keehoon Kim and Jonathan M. Lees and Mario Ruiz},
  title = {Source mechanism of Vulcanian eruption at Tungurahua Volcano, Ecuador, derived from seismic moment tensor inversions},
  journal = {J. Geophys. Res.},
  year = {2014},
  pages = {in press}
}
Anderson Jake F. and Lees, J.M., (2013) Instrument Corrections by Time-Domain Deconvolution, Seismological Research Letters. Vol. 85(1): pp. 197-201.
BibTeX:
@article{Anderson2013,
  author = {Anderson, Jake F., and Jonathan M. Lees},
  title = {Instrument Corrections by Time-Domain Deconvolution},
  journal = {Seismological Research Letters},
  year = {2013},
  volume = {85},
  number = {1},
  pages = {197-201},
  doi = {10.1785/0220130062}
}
Bowman Daniel and Lees, J.M., (2013) The Hilbert-Huang Transform: A High Resolution Spectral Method for Nonlinear and Nonstationary Time Series, Seismological Research Letters., November/December, 2013. Vol. 84(6): pp. 1074-1080.
BibTeX:
@article{Bowman2013,
  author = {Bowman, Daniel, and Jonathan M. Lees},
  title = {The Hilbert-Huang Transform: A High Resolution Spectral Method for Nonlinear and Nonstationary Time Series},
  journal = {Seismological Research Letters},
  year = {2013},
  volume = {84},
  number = {6},
  pages = {1074-1080},
  doi = {10.1785/0220130025}
}
Shi, L., Song, W.-Z., Lees, J.M. and Xing, G., (2013) Imaging Seismic Tomography in Sensor Networks, IEEE SECON 2013 TPC.
Abstract: Tomography imaging, applied to volcano seismology, requires a new, decentralized approach if high resolution calculations are to be performed in a sensor network configuration. The real-time data retrieval from a network of large amount wireless seismic stations to a central server is virtually impossible due to the sheer data amount and resource limitations. In this paper, we propose and design a distributed algorithm for processing data and inverting volcano tomography in the network, while avoiding costly data collections and centralized computations. Based on an innovative partition of the tomographic inversion problem, the new algorithms distribute the computational burden to sensor nodes and perform real-time tomographic inversion in the network, so that we can recover a high resolution tomographic model in real-time under the constraints of network resources. Our emulation results indicate that our distributed algorithms successfully reconstruct the synthetic models, while reducing and balancing the communication and computation cost to a large extent.
BibTeX:
@article{LeiShi2013,
  author = {Lei Shi and Wen-Zhan Song and Jonathan M. Lees and Guoliang Xing},
  title = {Imaging Seismic Tomography in Sensor Networks},
  journal = {IEEE SECON 2013 TPC},
  year = {2013}
}
Liu, G., Tan, R., Zhou, R., Xing, G., Song, W.-Z. and Lees, J.M., (2013) Volcanic Earthquake Timing using Wireless Sensor Networks, ACM/IEEE International Conference on Information Processing in Sensor Networks 2013 (IPSN 2013).
Abstract: Recent years have witnessed pilot deployments of inexpen- sive wireless sensor networks (WSNs) for active volcano mon- itoring. This paper studies the problem of picking arrival times of primary waves (i.e., P-phases) received by seis- mic sensors, one of the most critical tasks in volcano mon- itoring. Two fundamental challenges must be addressed. First, it is virtually impossible to download the real-time high-frequency seismic data to a central station for P-phase picking due to limited wireless network bandwidth. Sec- ond, accurate P-phase picking is inherently computation- intensive, and is thus prohibitive for many low-power sensor platforms. To address these challenges, we propose a new P-phase picking approach for hierarchical volcano monitor- ing WSNs where a large number of inexpensive sensors are used to collect fine-grained, real-time seismic signals while a small number of powerful coordinator nodes process col- lected data and pick accurate P-phases. We develop a suite of new in-network signal processing algorithms for accurate P-phase picking, including lightweight signal pre-processing at sensors, sensor selection at coordinators as well as signal compression and reconstruction algorithms. Testbed experi- ments and extensive simulations based on real data collected from a volcano show that our approach achieves accurate P- phase picking while only 16% of the sensor data are trans- mitted.
BibTeX:
@inproceedings{Liu2013,
  author = {Guojin Liu and Rui Tan and Ruogu Zhou and Guoliang Xing and Wen-Zhan Song and Jonathan M. Lees},
  title = {Volcanic Earthquake Timing using Wireless Sensor Networks},
  journal = {ACM/IEEE International Conference on Information Processing in Sensor Networks 2013 (IPSN 2013)},
  year = {2013},
  url = {http://www.iti.uni-luebeck.de/IPSN2013/paper/3?cap=03azCEoxBQH4mk}
}
Lyons, J.J., Ichihara, M., A. Kurokawa and Lees, J.M., (2013) Switching between seismic and seismo-acoustic harmonic tremor simulated in the laboratory: Insights into the role of open degassing channels and magma viscosity, J. Geophys. Res.. Vol. 118: pp. 277–289.
Abstract: Switching between seismic-only harmonic tremor (SHT) and seismo-acoustic harmonic tremor (SAHT) has been reported at few volcanoes worldwide, but its occurrence may indicate important changes in shallow conduit conditions. Switching was simulated in a laboratory experiment in which harmonic signals were produced with a flow-driven valve and compressed air. The harmonic signals were passed through a tank of shear-thinning viscoelastic fluid, and the resulting signals were recorded. At high fluid stiffness, a stable, open conduit was produced, and the harmonic signals generated within the experimental apparatus were efficiently transmitted into the atmosphere. At lower fluid stiffness, bubbling dominated the activity, stable pathways were not generated in the fluid, and HT was not recorded in the atmosphere. These results are compared to observations of switching at Fuego volcano, Guatemala. We conclude that at intermediate magma viscosities, the development of stable degassing pathways open to the atmosphere will allow HT generated in the conduit to be transmitted into the atmosphere. Further, subtle changes in magma properties and supply rate may control whether SHT or SAHT is recorded, providing information about the state of the shallow conduit and vent at active volcanoes.
BibTeX:
@article{Lyons2013,
  author = {Lyons, J. J. and M. Ichihara and A. Kurokawa, and J. M. Lees},
  title = {Switching between seismic and seismo-acoustic harmonic tremor simulated in the laboratory: Insights into the role of open degassing channels and magma viscosity},
  journal = {J. Geophys. Res.},
  year = {2013},
  volume = {118},
  pages = {277–289},
  doi = {10.1002/jgrb.50067}
}
Anderson, J., Lees, J., Waite, G. and Johnson, J., (2012) Source and Propagation Effects on Near-Field Co-Eruptive Ground Motion at Santiaguito Volcano, Guatemala, Bull, Seismol. Soc. Am.. Vol. 102: pp. 696-706.
Abstract: Data from a December 2008–January 2009 seismometer deployment show substantial variation in seismic signal characteristics among different sites and eruptive events at Santiaguito volcano. A station on one of the inactive domes typically records higher amplitudes and a more peaked spectrum than does another station closer to the vent on lower ground. Amplitude ratios between the stations and spectral peakedness of the station on the dome vary substantially and depend on vertical signal polarity, which is controlled by seismic source mechanism. Finite?difference models considering two likely source mechanisms and several hypothetical subsurfaces show that the topography of the free surface and subsurface units can increase or decrease amplitudes by focusing or obstructing body waves. A homogeneous velocity structure is shown not to be a close approximation to the actual shallow subsurface, while a subsurface including a dipping reflective layer produces results that fit observations better. These models also indicate that path effects act more strongly on waves originating from an implosion than from a downward force, possibly due to differences in body and surface wave proportions and differing susceptibilities of body and surface waves to these path effects. Thus, recorded data and models both indicate that the level of wave?field distortion associated with propagation effects is controlled by the source mechanism.
BibTeX:
@article{Anderson2012,
  author = {Anderson, J. and J. Lees and G. Waite and J. Johnson},
  title = {Source and Propagation Effects on Near-Field Co-Eruptive Ground Motion at Santiaguito Volcano, Guatemala},
  journal = {Bull, Seismol. Soc. Am.},
  year = {2012},
  volume = {102},
  pages = {696-706},
  doi = {10.1785/0120110106}
}
Kim, K., Lees, J. and Ruiz, M., (2012) Acoustic multipole source model for volcanic explosions and inversion for source parameters, Geoph. J. Int.. Vol. 191: pp. 1192-1204.
Abstract: Volcanic explosions are accompanied by strong acoustic pressure
disturbances in the atmosphere. With a proper source model, these
acoustic signals provide invaluable information about volcanic
explosion dynamics. Far-field solutions to volcanic infrasound
radiation have been derived above a rigid half-space boundary, and a
simple inversion method was developed based on the half-space model.
Acoustic monopole and dipole sources were estimated simultaneously
from infrasound waveforms. Stability of the inversion procedure was
assessed in terms of variances of source parameters, and the procedure
was reliable with at least three stations around the infrasound
source. Application of this method to infrasound observations recorded
at Tungurahua volcano in Ecuador successfully produced a reasonable
range of source parameters with acceptable variances. Observed strong
directivity of infrasound radiation from explosions at Tungurahua are
successfully explained by the directivity of a dipole source
model. The resultant dipole axis, in turn, shows good agreement with
the opening direction of the vent at Tungurahua which is considered to
be the origin of the dipole source. The method is general and can be
utilized to study any monopole, dipole, or combined sources generated
by explosions.
BibTeX:
@article{Kim2012,
  author = {Kim, K. and J.M. Lees and M. Ruiz},
  title = {Acoustic multipole source model for volcanic explosions and inversion for source parameters},
  journal = {Geoph. J. Int.},
  year = {2012},
  volume = {191},
  pages = {1192-1204},
  doi = {10.1111/j.1365-246X.2012.05696.x}
}
Lees, J.M., (2012) Open and Free: Software and Scientific Reproducibility (opinion), Seismological Research Letters. Vol. 83: pp. 751-752.
BibTeX:
@article{Lees2012,
  author = {Lees, J. M.},
  title = {Open and Free: Software and Scientific Reproducibility (opinion)},
  journal = {Seismological Research Letters},
  year = {2012},
  volume = {83},
  pages = {751-752},
  doi = {10.1785/0220120091}
}
Lyons, J., Waite, G., Ichihara, M. and Lees, J., (2012) Tilt prior to explosions and the effect of topography on ultra-long-period seismic records at Fuego volcano, Guatemala, Geophys. Res. Lett.. Vol. 39: pp. L08305.
Abstract: Ground tilt is measured from broadband seismic records prior to frequent explosions at Fuego volcano, Guatemala. We are able to resolve tilt beginning 20–30 minutes prior to explosions, followed by a rapid reversal in deformation coincident with explosion onsets. The tilt amplitude and polarity recorded on the horizontal channels vary from station to station such that the steep and unusual topography of the upper cone of Fuego appears to affect the ultra-long-period signals. We account for the effect of topography and attempt to constrain the tilt source depth and geometry through finite-difference modeling. The results indicate a shallow spherical pressure source, and that topography must be considered when attempting to model tilt sources at volcanoes with steep topography. The tilt signals are interpreted as pressurization of the shallow conduit beneath a crystallized plug followed by elastic deflation concurrent with explosive pressure release.
BibTeX:
@article{Lyons2012,
  author = {Lyons, J.J. and G.P. Waite and M. Ichihara and J.M. Lees},
  title = {Tilt prior to explosions and the effect of topography on ultra-long-period seismic records at Fuego volcano, Guatemala},
  journal = {Geophys. Res. Lett.},
  year = {2012},
  volume = {39},
  pages = {L08305},
  doi = {10.1029/2012GL051184}
}
Garcia-March, J., Surge, D., Lees, J. and Kersting, D., (2011) Stable isotope ratios in Pinna nobilis shells record ecological information and water mass properties in the Mediterranean, Journal of Geophysical Research Biogeosciences. Vol. 116: pp. G02009.
Abstract: Sclerochronologic and stable isotope records in Pinna nobilis shells potentially record ecological and oceanographic information. P. nobilis is a subtidal bivalve adapted to live in a variety of environments in the Mediterranean. We hypothesized that stable isotope ratios (?18O and ?13C) and growth increment patterns from individuals living in different environments serve as ecological indicators. Using a new methodology for calcite sampling, we (1) identified annual growth features (nacre tongues) and (2) compared monthly resolved variations in ?18O and ?13C values and calcification temperatures recorded in animals located above and below the thermocline (16 and 30 m depth). The specimens from 16 m showed more negative ?18O values than the specimen from 30 m, likely reflecting differences in salinity. The specimens from 30 m recorded ?13C values less positive than the specimens from 16 m, which we interpreted as an ontogenetic effect observed in previous studies. Estimated calcification temperatures were offset relative to measured water temperature by ?6.1°C (?1.4‰). This finding is evident in earlier proxy studies of P. nobilis, although it was not discussed in those studies. Using the seasonal pattern of ?18O and ?13C values, we demonstrated that nacre tongues are deposited annually and that their formation is independent of temperature. Food availability rather than temperature may control nacre tongue formation. An alternative explanation for nacre tongue formation is gonad maturation during spring. Our findings support the idea that sclerochronology in P. nobilis can be used to reconstruct environmental, ecological, and climate archives of the Mediterranean.
BibTeX:
@article{Garcia-March2011,
  author = {Garcia-March, J.R. and Surge, D. and Lees, J. and Kersting, D.},
  title = {Stable isotope ratios in Pinna nobilis shells record ecological information and water mass properties in the Mediterranean},
  journal = {Journal of Geophysical Research Biogeosciences},
  year = {2011},
  volume = {116},
  pages = {G02009},
  doi = {10.1029/2010JG001461}
}
Jefferson, A.L., Lees, J. and McClinton, T., (2011) Synthesizing knowledge of ocean islands, Eos Trans. AGU. Vol. 92: pp. 44.
Abstract: AGU Chapman Conference on the Galápagos as a Laboratory for the Earth Sciences; Puerto Ayora, Galápagos, Ecuador, 25–30 July 2011 An inspiration for Darwin's theory of evolution, the Galápagos Islands and surrounding waters are a natural laboratory for a wide range of Earth science topics. The Galápagos are perfectly situated for geophysical and geochemical investigations of deep-Earth processes at a hot spot, and proximity to a spreading center allows exploration of hot spot-ridge interactions. Several highly active volcanoes show rapid deformation facilitating investigation of melt transport paths and volcanic structure. The islands exhibit a range of ages, eruptive styles, and climatic zones that allow analysis of hydrogeologic and geomorphic processes. The Galápagos Islands are a World Heritage Site and are an ideal setting for developing an integrated biological and geological understanding of ocean island evolution.
BibTeX:
@article{Jefferson2011,
  author = {Jefferson, A. L. and J.M. Lees and T. McClinton},
  title = {Synthesizing knowledge of ocean islands},
  journal = {Eos Trans. AGU},
  year = {2011},
  volume = {92},
  pages = {44},
  note = {Chapman Conference on The Galápagos as a Laboratory for the Earth Sciences},
  doi = {10.1029/2011EO440004}
}
Johnson, J.B., Lees, J.M. and Varley, N., (2011) Characterizing complex eruptive activity at Santiaguito Volcano, Guatemala using infrasound semblance in networked arrays, Journal of Volcanology and Geothermal Research. Vol. 199(1-2): pp. 1-14.
Abstract: We implement an infrasound semblance technique to identify acoustic sources originating from volcanic vents and apply the technique to the generally low-amplitude infrasound (< 3 Pa at 1 km) signals produced by Santiaguito dome in Guatemala. Semblance detection is demonstrated with data collected from two-element miniature arrays with ~ 30 m spacing between elements. The semblance technique is effective at identifying a range of eruptive phenomena, including pyroclastic-laden eruptions, vigorous degassing events, and rockfalls, even during periods of high wind contamination Many of the detected events are low in amplitude (tens of mPa) such that they are observed only by select arrays positioned with proximity and line-of-sight to the source. Larger events, such as the pyroclastic-laden eruptions, which occurred bi-hourly in 2009, were detected by all five arrays and produced an infrasonic signal that was correlated across the network. Network correlated events can be roughly located and map to the summit of the Caliente Vent where pyroclastic-laden eruptions originate. In general, the degree of Santiaguito infrasound event correlation is poor across the network, suggesting that complex source geometry contributes to asymmetric sound radiation.
BibTeX:
@article{Johnson2011,
  author = {Johnson, J. B. and Lees, J. M. and Varley, N.},
  title = {Characterizing complex eruptive activity at Santiaguito Volcano, Guatemala using infrasound semblance in networked arrays},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2011},
  volume = {199},
  number = {1-2},
  pages = {1-14},
  url = {http://www.sciencedirect.com/science/article/pii/S0377027310002519}
}
Kim, K. and Lees, J.M., (2011) Finite-difference time-domain modeling of transient infrasonic wavefields excited by volcanic explosions, Geophys. Res. Lett.. Vol. 38: pp. L06804.
Abstract: Numerical modeling of waveform diffractions along the rim of a volcano vent shows high correlation to observed explosion signals at Karymsky Volcano, Kamchatka, Russia. The finite difference modeling assumed a gaussian source time function and an axisymmetric geometry. A clear demonstration of the significant distortion of infrasonic wavefronts was caused by diffraction at the vent rim edge. Data collected at Karymsky in 1997 and 1998 were compared to synthetic waveforms and variations of vent geometry were determined via grid search. Karymsky exhibited a wide range of variation in infrasonic waveforms, well explained by the diffraction, and modeled as changing vent geometry. Rim diffraction of volcanic infrasound is shown to be significant and must be accounted for when interpreting source physics from acoustic observations.
BibTeX:
@article{Kim2011,
  author = {Kim, K. and J. M. Lees},
  title = {Finite-difference time-domain modeling of transient infrasonic wavefields excited by volcanic explosions},
  journal = {Geophys. Res. Lett.},
  year = {2011},
  volume = {38},
  pages = {L06804},
  doi = {10.1029/2010GL046615}
}
Johnson, J.B. and Lees, J.M., (2010) Sound produced by the rapidly inflating Santiaguito lava dome, Guatemala, Geophysical Research Letters. Vol. 37(L22305): pp. 6 .
Abstract: Vertical inflation of the Caliente lava dome at Santiaguito (Guatemala) occurs coincidentally with the onset of explosive eruptions and produces infrasound that is generally peaked between 0.5 and 2 Hz with amplitude of up to 5 Pa (at ?1 km from vent). Inflation of up to ?1 m progresses rapidly (within ?2 s) and can encompass the entire surface of the ?200-m-diameter dome. We use particle image velocimetry to quantify the time history of dome uplift and demonstrate that deflection of a volcano's solid surface can generate infrasound waves with amplitudes of a few Pa. The volumetric displacement history is used to model linear volumetric acoustic sources, both compact and of finite extent. Synthetic waveforms match recorded infrasound in terms of timing and frequency content. Amplitude fit is very good for a station located 3.3 km from the vent and less good for stations shielded by near vent topography.
BibTeX:
@article{Johnson2010a,
  author = {Johnson, Jeffrey B. and Lees, Jonathan M.},
  title = {Sound produced by the rapidly inflating Santiaguito lava dome, Guatemala},
  journal = {Geophysical Research Letters},
  year = {2010},
  volume = {37},
  number = {L22305},
  pages = {6 },
  url = {http://www.agu.org/pubs/crossref/2010/2010GL045217.shtml}
}
Madlazim, K., Santosa, B., Lees, J. and Utama, W., (2010) Earthquake Source Parameters at the Sumatran Fault Zone: Identification of the Activated Fault Plane, Central European Journal of Geosciences. : pp. 1896-1517.
Abstract: Fifteen earthquakes (Mw 4.1-6.4) occurring at ten major segments of the Sumatran Fault Zone (SFZ) were analyzed to identify their respective fault planes. The events were relocated in order to assess hypocenter uncertainty. Earthquake source parameters were determined from three-component local waveforms recorded by IRIS-DMC and GEOFON broadband lA networks. Epicentral distances of all stations were less than 10°. Moment tensor solutions of the events were calculated, along with simultaneous determination of centroid position. Joint analysis of hypocenter position, centroid position, and nodal planes produced clear outlines of the Sumatran fault planes. The preferable seismotectonic interpretation is that the events activated the SFZ at a depth of approximately 14-210 km, corresponding to the interplate Sumatran fault boundary. The identification of this seismic fault zone is significant to the investigation of seismic hazards in the region.
BibTeX:
@article{Madlazim2010,
  author = {Madlazim, K. and Santosa, B.J. and Lees, J.M. and Utama, W.},
  title = {Earthquake Source Parameters at the Sumatran Fault Zone: Identification of the Activated Fault Plane},
  journal = {Central European Journal of Geosciences},
  year = {2010},
  pages = {1896-1517},
  url = {http://www.springerlink.com/content/0pu8j423t25328vr/},
  doi = {10.2478/v10085-010-0016-5}
}
Sanderson, R.W., Johnson, J.B. and Lees, J.M., (2010) Ultra-long period seismic signals and cyclic deflation coincident with eruptions at Santiaguito volcano, Guatemala, Journal of Volcanology and Geothermal Research. Vol. 198(1-2): pp. 35-44.
Abstract: Broadband seismic observations made near the active vent at Santiaguito provide constraints on changing pressures inside the edifice. During January 2009 we recorded episodic ultra-long period signals (ULPs, > 30 s) that were coincident with eruptive activity centered on the active El Caliente vent. These signals lasted 2–5 min and corresponded to depressurization of a volume beneath the dome. Coincident shorter-period seismic signals (> 0.5 Hz) appeared to be associated with shallow level pyroclastic emissions and dome surface motions. These shorter period signals varied significantly from event to event, unlike the ULP waveforms, which were self-similar. As opposed to the short period signals, which were recorded across the network, the ULPs were only recorded on the horizontal channels of 3 stations that were within 1.1 km of the vent. This and other factors suggest that they were a result of long period rotational motion rather than horizontal translation. ULP source locations derived from horizontal particle motions point to the northwestern part of El Caliente 150 m from the vent axis, the general locus of activity from 1967 until flank collapses in 1989/1990. We modeled the ULP signal as a Mogi source and solved for three dimensional location and equivalent source strength. Through analysis of 78 events, we identify a mean Mogi source that is 200 m west of and 250 m beneath the centre of the vent with the peak volume changes accompanying eruptions ranging from 125 to 1825 m3. These inferred volume losses coincide with the onset of gas venting and/or explosive eruptions at the surface of Caliente. Based upon these observations we develop a model to describe the pre-eruptive, eruptive and inter-eruptive physical processes occurring at the surface and inside Santiaguito.
BibTeX:
@article{Sanderson2010,
  author = {Sanderson, R. W. and Johnson, J. B. and Lees, Jonathan M.},
  title = {Ultra-long period seismic signals and cyclic deflation coincident with eruptions at Santiaguito volcano, Guatemala},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2010},
  volume = {198},
  number = {1-2},
  pages = {35-44},
  doi = {10.1016/j.jvolgeores.2010.08.007}
}
Chung, T.W., Lees, J.M., Yoshimoto, K., Fujita, E. and Ukawa, M., (2009) Intrinsic and Scattering Attenuation the Mt. Fuji Region, Japan, Geophysical Journal International. Vol. 177: pp. 1366-1382.
Abstract: Mount Fuji is the focus of intense study because of its potential hazard signaled by seismic, geologic and historical activity. Based on extensive seismic data recorded in the vicinity of Mt Fuji, coda quality factor (Q?1C) using a single scattering model hypothesis, and intrinsic and scattering quality factor (Q?1i and Q?1S) using the Multiple Lapse Time Window (MLTW) method were measured. To focus the study on the magmatic structure below Mt Fuji, the data were separated into two groups: a near-Fuji region of rays traversing an area with radius 5 km around the summit (R < 5 km), and a far-Fuji region of rays beyond a radius of 20 km around the summit (R > 20 km). This classification shows the largest discrepancy of Q?1C at a range of sampling volumes corresponding to overlapped sampling depth of about 80 km. Further, the spatial division shows significant difference of Q?1i and Q?1s at hypocentral distance of 80 km. The large difference of Q?1s in bandwidths 2–4, and 4–8 Hz indicates lithospheric heterogeneity beneath Mt Fuji with a characteristic heterogeneity scale length of about 1 km. The results have a small error range due to the large data sample, showing that all Q?1 values in the near-Fuji area are greater than those of the far-Fuji area, and Q?1i for both the near and far-Fuji areas is higher than Q?1s at high frequencies. The Q?1i and Q?1s values for far-Fuji are in the range of values for typical non-volcanic areas. The Q?1i values of the near-Fuji area are lower than those of other volcanic areas considered, where as values of Q?1s are not. The low Q?1i for the volcanic region of near-Fuji suggests that the magmatic activity, indicated by percent partial melt, in the vicinity of Mt Fuji is not as active as hot spot volcanoes, such as Kilauea, Hawaii.
BibTeX:
@article{Chung2009,
  author = {Chung, Tae Woong and Lees, Jonathan M. and Yoshimoto, Kazuo and Fujita, Eisuke and Ukawa, M.},
  title = {Intrinsic and Scattering Attenuation the Mt. Fuji Region, Japan},
  journal = {Geophysical Journal International},
  year = {2009},
  volume = {177},
  pages = {1366-1382},
  url = {http://onlinelibrary.wiley.com/doi/10.1111/j.1365-246X.2009.04121.x/full},
  doi = {10.1111/j.1365-246X.2009.04121.x}
}
Johnson, J., Sanderson, R., Lyons, J., Escobar-Wolf, R., Waite, G. and Lees, J., (2009) Dissection of a composite volcanic earthquake at Santiaguito, Guatemala, Geophysical Research Letters. Vol. 36: pp. L16308.
Abstract: Volcano earthquake sources associated with eruptions at Santiaguito volcano in Guatemala are complex. Rock fracture, fluid flow, and gas expansion occur at variable time scales and induce superposed ground motions, including both static and dynamic deformation, and atmospheric pressure disturbances. Dissection of this composite event is facilitated through extra-seismic observations, such as infrasound, geodetic, and visual monitoring. Multi-parametric investigation of an eruptive event on Jan. 4th 2009 reveals increased degassing, apparent as both geodetic tilt and harmonic seismo-infrasonic tremor, preceding an explosive event. The explosive event itself entails surface dome uplift, multiple eruptive pulses, and subsequent re-equilibration of the volcanic edifice manifested in derived tilt. We report here on an integrated approach to discerning the physical processes at the actively effusing and exploding Santiaguito volcano and describe the composite earthquake that occurs here.
BibTeX:
@article{Johnson2009,
  author = {Johnson, J.B. and Sanderson, R. and Lyons, J. and Escobar-Wolf, R. and Waite, G.P. and Lees, J.M.},
  title = {Dissection of a composite volcanic earthquake at Santiaguito, Guatemala},
  journal = {Geophysical Research Letters},
  year = {2009},
  volume = {36},
  pages = {L16308},
  url = {http://www.agu.org/pubs/crossref/2009/2009GL039370.shtml}
}
Chung, T., Lees, J.M. and Yoon, S., (2008) Seismic data analysis using R, Mulli-Tamsa. Vol. 11: pp. 378-384.
Abstract: R is a free software for statical computing and graphics. It compiles and runs not only on UNIX platforms but MS Windows. The R commands are easy and offer interactive help. R is used in extensive field by implementing packages. RSEIS, the package of R, enables us to do easy graphical processing of seismic data. Here we illustrate by showing an example of seismic data processing using RSEIS.
BibTeX:
@article{Chung2008,
  author = {Chung, T.W. and Lees, J. M. and Yoon, S.},
  title = {Seismic data analysis using R},
  journal = {Mulli-Tamsa},
  year = {2008},
  volume = {11},
  pages = {378-384}
}
Chung, T.W. and Lees, J.M., (2008) Preparation of Tomographic maps based on the R Package, Mulli-Tamsa. Vol. 11(4): pp. 373-378 .
Abstract: Although widely used for preparation of geographic maps in the field of earth sciences, the Generic Mapping Tools(GMT) software is difficult for users to understand, and does not work well with Microsoft (MS) Window PC. By utilizing R package, GEOmap, we can do mapping work at MS window PC with commands easier than those of GMT. In addition, the R commands provide interactive help. Here we briefly introduce a few features of GEOmap, and illustrate a procedure for preparing topographic maps as an example.
BibTeX:
@article{Chung2008a,
  author = {Chung, Tae Woong and Lees, Jonathan M.},
  title = {Preparation of Tomographic maps based on the R Package},
  journal = {Mulli-Tamsa},
  year = {2008},
  volume = {11},
  number = {4},
  pages = {373-378 }
}
Johnson, J., Lees, J.M., Gerst, A., Sahagian, D. and Varley, N., (2008) Long-period earthquake and co-eruptive dome inflation seen with particle image velocimetry, Nature. Vol. 456
Abstract: Dome growth and explosive degassing are fundamental processes in the cycle of continental arc volcanism1, 2. Because both processes generate seismic energy, geophysical field studies of volcanic processes are often grounded in the interpretation of volcanic earthquakes3. Although previous seismic studies have provided important constraints on volcano dynamics4, 5, 6, such inversion results do not uniquely constrain magma source dimension and material properties. Here we report combined optical geodetic and seismic observations that robustly constrain the sources of long-period volcanic earthquakes coincident with frequent explosive eruptions at the volcano Santiaguito, in Guatemala. The acceleration of dome deformation, extracted from high-resolution optical image processing, is shown to be associated with recorded long-period seismic sources and the frequency content of seismic signals measured across a broadband network. These earthquake sources are observed as abrupt subvertical surface displacements of the dome, in which 20–50-cm uplift originates at the central vent and propagates at ~50 m s-1 towards the 200-m-diameter periphery. Episodic shifts of the 20–80-m thick dome induce peak forces greater than 109 N and reflect surface manifestations of the volcanic long-period earthquakes, a broad class of volcano seismic activity that is poorly understood and observed at many volcanic centres worldwide7. On the basis of these observations, the abrupt mass shift of solidified domes, conduit magma or magma pads may play a part in generating long-period earthquakes at silicic volcanic systems.
BibTeX:
@article{Johnson2008,
  author = {Johnson, J. and Lees, J. M. and Gerst, A. and Sahagian, D. and Varley, N.},
  title = {Long-period earthquake and co-eruptive dome inflation seen with particle image velocimetry},
  journal = {Nature},
  year = {2008},
  volume = {456},
  doi = {10.1038/nature07429}
}
Lees, J.M., Johnson, J.B., Ruiz, M., Troncoso, L. and Welsh, M., (2008) Reventador Volcano 2005: Eruptive Activity Inferred from Seismo-Acoustic Observation, Journal of Volcanology and Geothermal Research. Vol. 176(1): pp. 179-190.
Abstract: Reventador Volcano entered an eruptive phase in 2005 which included a wide variety of seismic and infrasonic activity. These are described and illustrated: volcano-tectonic, harmonic tremor, drumbeats, chugging and spasmodic tremor, long period and very long period events. The recording of this simultaneous activity on an array of three broadband, seismo-acoustic instruments provides detailed information of the state of the conduit and vent during this phase of volcanic eruption. Quasi-periodic tremor at Reventador is similar to that observed at other volcanoes and may be used as an indicator of vent aperture. Variations in the vibration modes of the volcano, frequency fluctuations and rapid temporal fluctuations suggest the influx of new material, choking of the vent and possible modification of the conduit geometry during explosions and effusion over a period of six weeks.
BibTeX:
@article{Lees2008,
  author = {Lees, J. M. and Johnson, J. B. and Ruiz, M. and Troncoso, L. and Welsh, M.},
  title = {Reventador Volcano 2005: Eruptive Activity Inferred from Seismo-Acoustic Observation},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2008},
  volume = {176},
  number = {1},
  pages = {179-190},
  doi = {10.1016/j.jvolgeores.2007.10.006}
}
Lees, J.M. and Ruiz, M., (2008) Non-linear Explosion Tremor at Sangay, Volcano, Ecuador, Journal of Volcanology and Geothermal Research. Vol. 176(1): pp. 170-178 .
Abstract: A detailed analysis of discrete degassing pulses, chugs, at Sangay volcano, was performed on seismic and infrasonic records to determine the physics of the conduit. Infrasonic chugging signals appear as repetitive pulses with small variations in amplitude and time lag. An automated time-domain analysis was developed to measure with high precision time intervals and amplitudes at different wave arrivals, reducing the possibility error associated with hand picking. Using this automated method, a strong positive correlation of acoustic amplitude with repose time between individual pulses on chugging signals of Sangay was found on numerous oscillating sequences. Frequency gliding of apparent harmonic frequencies generally trends from high to low frequency at Sangay, in contrast to trends at Karymsky Volcano, Russia. A new description of chugging events using wavelet transform methods, appropriate for non-stationary signals, shows subtle changes in the waveforms relate to physical processes in the volcano. A system of non-linear feedback, based on choked flow at the vent, is postulated as the most likely source of this volcanic tremor.
BibTeX:
@article{Lees2008a,
  author = {Lees, Jonathan M. and Ruiz, Mario},
  title = {Non-linear Explosion Tremor at Sangay, Volcano, Ecuador},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2008},
  volume = {176},
  number = {1},
  pages = {170-178 },
  doi = {10.1016/j.jvolgeores.2007.08.012}
}
Scharff, L., Hort, M., Harris, A.J.L., Ripepe, M., Lees, J.M. and Seyfried, R., (2008) Eruption dynamics of the SW crater of Stromboli volcano, Italy, An interdisciplinary approach, Journal of Volcanology and Geothermal Research. Vol. 176(4): pp. 565-570.
Abstract: We report on the evaluation of in situ measurements of eruption velocities and relative mass flux, collected by Doppler radar, as well as acoustic and infrared data, that were recorded at Stromboli volcano. Doppler radar observations were made alongside thermal, acoustic and seismic measurements, to (1) further investigate the complex waveforms of the SW crater by combining infrasonic, infrared and Doppler radar measurements, (2) establish a relationship between infrared, acoustic and Doppler radar measurements and (3) verify that all instruments observe the same behavior in terms of relative mass eruption rate. We also explore the relationship between kinetic and acoustic energy released during an eruption. Comparing the different methods to each other we are able to show that the mass erupted can be estimated either from the total reflected Doppler radar energy or from infrared observations. However, neither thermal nor reflected energy can provide a value in terms of absolute mass in kg. The erupted masses of different eruptions can only be evaluated relative to each other. Using the combined three data sets we especially focus on the eruption dynamics of the SW crater of Stromboli, namely its fluctuations in eruption strength. These pulses in one eruptive event, so called ’pulsations’ dominate more than 40% of the eruptions of SW crater. Previous models that explain pulsations to be generated by multiple consecutively exploding bubbles are supported by our combined analysis of infrasound, thermal data, particle velocities, and the reflected energy.
BibTeX:
@article{Scharff2008,
  author = {Scharff, L. and Hort, M. and Harris, A. J. L. and Ripepe, M. and Lees, J. M. and Seyfried, R.},
  title = {Eruption dynamics of the SW crater of Stromboli volcano, Italy, An interdisciplinary approach},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2008},
  volume = {176},
  number = {4},
  pages = {565-570},
  doi = {10.1016/j.jvolgeores.2008.05.008}
}
Tang, C., Rial, J.A. and Lees, J.M., (2008) Seismic Imaging of the Geothermal Field at Krafla, Iceland Using Shear-wave Splitting, Journal of Volcanology and Geothermal Research. Vol. 176(2): pp. 315-324.
Abstract: Shear-wave splitting is emerging as a useful exploration method for geothermal reservoirs as it can detect the geometry of the fracture system, the intensity of cracking and possibly, changes in fluid pressure within the reservoir. The method is based on the analyses of polarizations and time delays of shear-waves that have been distorted by the anisotropy of the medium through which the seismic waves have propagated. Observations of shear-wave splitting within the Krafla–Leirhnúkur geothermal field, Iceland, using a 20-station 3-component portable seismic array have provided evidence for at least two major crack systems of microfractures, oriented approximately N–S and E–W. Located microearthquakes align roughly along the E–W direction of the geothermal field, with shallow focal depths mostly around the injection well, probably related to the ongoing injection. This unexpected direction is however consistent with results from a simultaneous MT (magnetotelluric) survey.
BibTeX:
@article{Tang2008,
  author = {Tang, C. and Rial, J. A. and Lees, J. M.},
  title = {Seismic Imaging of the Geothermal Field at Krafla, Iceland Using Shear-wave Splitting},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2008},
  volume = {176},
  number = {2},
  pages = { 315-324},
  doi = {10.1016/j.jvolgeores.2008.04.017}
}
Chung, T.W., Noh, M.-H., Kim, J.-K., Park, Y.-K., Yoo, H.-J. and Lees, J.M., (2007) A Study of the Regional Variation of Low Frequency QLg-1 Around The Korean Peninsula, Bulletin of the Seismological Society of America. Vol. 97(6): pp. 2190-2197.
Abstract: We studied regional Graphic at 1 Hz (Graphic) around the Korean Peninsula based on broadband vertical component seismic records of six Incorporated Research Institutions for Seismology (IRIS) Global Seismographic Network stations and 19 Korean stations of the Korea Institute of Geoscience and Mineral Resources (KIGAM). Using 177 seismic events with magnitudes between 5.3 and 5.7 and depths less than 50 km, the reversed two station method was applied, and 94 high quality interstation paths were selected from 869 possible pairs. These results show high Graphic paths around the Sea of Japan (East Sea) reflecting the typical oceanic structure and low Graphic paths around northeastern China related to inactive seismicity. Assigning these path values into 193 cells around South Korea with a size of 1° by 1°, we observed that the regional Graphic decreased gradually from east to west between 2 and 1?10-3.
BibTeX:
@article{Chung2007,
  author = {Chung, Tae Woong and Noh, Myung-Hyun and Kim, Junk-Kyong and Park, Yun-Kyong and Yoo, Hyun-Jae and Lees, Jonathan M.},
  title = {A Study of the Regional Variation of Low Frequency QLg-1 Around The Korean Peninsula},
  journal = {Bulletin of the Seismological Society of America},
  year = {2007},
  volume = {97},
  number = {6},
  pages = {2190-2197},
  doi = {10.1785/0120070066}
}
Lees, J.M., (2007) Seismic tomography of magmatic systems, Journal of Volcanology and Geothermal Research. Vol. 167(1-4): pp. 37-56 .
Abstract: Seismic inversion for three-dimensional variations of velocity and attenuation are often used to delineate magma bodies in the crust and upper mantle. Problems related to spatial resolution and data noise can obscure details relevant to investigating magma chambers, and the introduction of smoothing constraints, or damping, causes blurring. Tomographic inversions for P- and S-wave velocity/attenuation are summarized including large calderas, rift zones and smaller scale subduction zone volcanoes. While results vary considerably from place to place, most anomalies are found to be in the range of ± 10% perturbation, a range often controlled by the method of smoothing or regularization imposed during analysis. At many volcanoes high velocity anomalies are observed in the shallow regions below active areas where conduits, dykes or sills are expected to be present. At other locations low velocity perturbations are seen and interpreted as magma accumulation. Resolution limitations and regularization play a significant role in determining the level of perturbation observed in tomographic studies, although there may be regions where diffuse accumulations of magma do not exhibit strong anomalies and their identification will be elusive.
BibTeX:
@article{Lees2007,
  author = {Lees, J. M.},
  title = {Seismic tomography of magmatic systems},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2007},
  volume = {167},
  number = {1-4},
  pages = {37-56 },
  doi = {10.1016/j.jvolgeores.2007.06.008}
}
Lees, J.M., (2007) SeisR & FocR: Earthquake and Seismic Analysis in R, In useR!. Iowa State University, Ames, Iowa
Abstract: I present several new packages for analyzing seismic data for time series analysis and earthquake focal mechanisms. The packages consists of modules that 1) read in seismic waveform data in various common exchange formats, 2) display data as either event or continuous recordings and 3) performs numerous standard analyses applied to earthquake and volcano monitoring. SeisR is designed as a research tool aimed at investigators who need to quickly assess large amounts of time-series as they are related to the spatial distribution of geologic structure and wave propagation. In addition to time series analysis, a spatial mapping program is included that ties waveforms and radiation patterns to geographical data-base and mapping programs.
BibTeX:
@inproceedings{Lees2007a,
  author = {Lees, J. M.},
  title = {SeisR & FocR: Earthquake and Seismic Analysis in R},
  booktitle = {useR!},
  year = {2007}
}
Lees, J.M., Symons, N., Chubarova, O., Gorelchik, V. and Ozerov, A., (2007) Tomographic Images of Kliuchevskoi Volcano P-wave Velocity, In Volcanism and Subduction: The Kamchatka Region. Washington, D.C. Vol. 172: pp. 293-302. American Geophysical Union.
Abstract: Three-dimensional structural images of the P-wave velocity below the edifice of the great Klyuchevskoy group of volcanoes in central Kamchatka are derived via tomographic inversion. The structures show a distinct low velocity feature extending from around 20 km depth to 35 km depth, indicating evidence of magma ponding near the Moho discontinuity. The extensive low velocity feature represents, at least to some degree, the source of the large volume of magma currently erupting at the surface near the Klyuchevskoy group.
BibTeX:
@incollection{Lees2007f,
  author = {Lees, Jonathan M. and Symons, Neill and Chubarova, Olga and Gorelchik, Valentina and Ozerov, Alexei},
  editor = {Eichelberger, J. and Gordeev, E. and Kasahara, M. and Izbekov, P. and Lees, J. M.},
  title = {Tomographic Images of Kliuchevskoi Volcano P-wave Velocity},
  booktitle = {Volcanism and Subduction: The Kamchatka Region},
  publisher = {American Geophysical Union},
  year = {2007},
  volume = {172},
  pages = {293-302}
}
Lees, J.M., VanDecar, J., Gordeev, E., Ozerov, A., Brandon, M., Park, J. and Levin, V., (2007) Three Dimensional Images of the Kamchatka-Pacific Plate Cusp, In Volcanism and Subduction: The Kamchatka Region. Washington, D.C. Vol. 172: pp. 65-75. American Geophysical Union.
Abstract: First arrivals of seismic waves were recorded along the Kamchatka arc using broadband seismic stations deployed for one year in 1998-1999. Cross correlation methods were used from a high resolution data set for tomographic inversion of body waves. The P-wave teleseismic tomography shows evidence of slab shoaling along the northern terminus of the Kamchatka subduction zone. Tomographic anomalies corroborate trends in seismicity, geochemistry, heat flow, shear wave splitting, and surface wave inversions. Thermal ablation via contact with asthenosphere, under the proper conditions, is offered as a possible explanation of the observed shoaling of the Kamchatka slab edge.
BibTeX:
@incollection{Lees2007g,
  author = {Lees, Jonathan M. and VanDecar, John and Gordeev, Evgenii and Ozerov, Alexei and Brandon, Mark and Park, Jeff and Levin, Vadim},
  editor = {Eichelberger, J. and Gordeev, E. and Kasahara, M. and Izbekov, P. and Lees, J. M.},
  title = {Three Dimensional Images of the Kamchatka-Pacific Plate Cusp},
  booktitle = {Volcanism and Subduction: The Kamchatka Region},
  publisher = {American Geophysical Union},
  year = {2007},
  volume = {172},
  pages = {65-75}
}
Ruppert, N.A., Lees, J.M. and Kozyreva, N.P., (2007) Seismicity, Earthquakes and Structure along the Alaska-Aleutian and Kamchatka-Kurile Subduction zones: A Review, In Volcanism and Subduction: The Kamchatka Region. Washington, D.C. Vol. 172: pp. 129-144. American Geophysical Union.
Abstract: We present a review of great earthquakes and seismicity patterns along the Alaska-Aleutian and Kamchatka-Kurile arcs as an overview of one of the longest subduction zone complexes on the planet. Seismicity patterns, double seismic zones and focal mechanism solutions are described and used to illustrate the distribution of stress in the Pacific plate as it collides with North America and Eurasia. Seismicity along the Alaska-Aleutian arc is relatively shallow as compared to the Kamchatka-Kurile arc where the plate is considerably older and thicker prior to entering the subduction zone. Tomographic inversions of the slab generally show high velocity anomalies where seismicity is high, presumably tracking the cold subducting lithosphere.
BibTeX:
@incollection{Ruppert2007,
  author = {Ruppert, Natalia A. and Lees, Jonathan M. and Kozyreva, Natalia P.},
  editor = {Eichelberger, J. and Gordeev, E. and Kasahara, M. and Izbekov, P. and Lees, J. M.},
  title = {Seismicity, Earthquakes and Structure along the Alaska-Aleutian and Kamchatka-Kurile Subduction zones: A Review},
  booktitle = {Volcanism and Subduction: The Kamchatka Region},
  publisher = {American Geophysical Union},
  year = {2007},
  volume = {172},
  pages = {129-144}
}
Fang, W.-C., Kedar, S., Owen, S., Wei, G.-Y., Brooks, D. and Lees, J., (2006) System-on-Chip Architecture Design for Intelligent Sensor Networks, In International Conference on Intelligent Information Hiding and Multimedia. Vol. iih-msp: pp. 579-582.
Abstract: While wireless sensor networks can generically be used for a wide variety of applications, breakthrough innovations are most often achieved when driven by a genuine need or application, with its specific system-level and science-related requirements and objectives. Hence, our work focuses on the development of wireless sensor network system-on-chip devices and supporting software for volcano monitoring, which we call Sensor Network for Active Volcanoes (SNAV). In this paper we present preliminary results of our research and development work on intelligent sensor networks for monitoring hazardous environments especially the SNAV system-on-chip design for active volcanoes monitoring.
BibTeX:
@inproceedings{Fang2006,
  author = {Fang, Wai-Chi and Kedar, Sharon and Owen, Susan and Wei, Gu-Yeon and Brooks, David and Lees, Jonathan},
  title = {System-on-Chip Architecture Design for Intelligent Sensor Networks},
  booktitle = {International Conference on Intelligent Information Hiding and Multimedia},
  year = {2006},
  volume = {iih-msp},
  pages = {579-582}
}
Johnson, J.B., Lees, J.M. and Yepes, H., (2006) Volcanoes, lightning, and a waterfall: Differentiating the menagerie of infrasound in the Ecuadorian jungle, Geophysical Research Letters. Vol. 33(L06308)
Abstract: In northeastern Ecuador, near Reventador Volcano, the airwaves are filled with infrasound. Here we identify the locations and characterize three distinct sources of local infrasound, including two types of infrasonic sources, which are not commonly discussed in the literature. The first of these novel sources is an intense and continuous radiator with a fixed location corresponding to San Rafael Waterfall. The signal from the river exhibits a tremor-like envelope that is well correlated across the 3-element infrasound network. Beyond the river, we also observe and map spatially variable sources corresponding to thunder. These transient signals have impulsive onsets, but are not well correlated across the network and are attributable to spatially-distributed source regions. Finally, we identify plentiful infrasound corresponding to Reventador's volcanic vent that is associated with unrest. This study demonstrates the utility of dispersed infrasound networks for distinguishing variable sources and improving interpretation of mechanisms of infrasound radiators.
BibTeX:
@article{Johnson2006a,
  author = {Johnson, Jeffrey B. and Lees, Jonathan M. and Yepes, Hugo},
  title = {Volcanoes, lightning, and a waterfall: Differentiating the menagerie of infrasound in the Ecuadorian jungle},
  journal = {Geophysical Research Letters},
  year = {2006},
  volume = {33},
  number = {L06308},
  doi = {10.1029/2005GL025515}
}
Werner-Allen, G., Lorincz, K., Johnson, J.B., Lees, J.M. and Welsh, M., (2006) Fidelity and Yield in a Volcano Monitoring Sensor Network, In Proc. of the 7th Symposium on Operating Systems Design and Implementation (OSDI '06). Seattle, WA, USA : pp. 381-396.
Abstract: We present a science-centric evaluation of a 19-day sensor network deployment at Reventador, an active volcano in Ecuador. Each of the 16 sensors continuously sampled seismic and acoustic data at 100 Hz. Nodes used an event-detection algorithm to trigger on interesting volcanic activity and initiate reliable data transfer to the base station. During the deployment, the network recorded 229 earthquakes, eruptions, and other seismoacoustic events. The science requirements of reliable data collection, accurate event detection, and high timing precision drive sensor networks in new directions for geophysical monitoring. The main contribution of this paper is an evaluation of the sensor network as a scientific instrument, holding it to the standards of existing instrumentation in terms of data fidelity (the quality and accuracy of the recorded signals) and yield (the quantity of the captured data). We describe an approach to time rectification of the acquired signals that can recover accurate timing despite failures of the underlying time synchronization protocol. In addition, we perform a detailed study of the sensor network?s data through a direct comparison to a standalone data logger, as well as an investigation of seismic and acoustic wave arrival times across the network.
BibTeX:
@inproceedings{Werner-Allen2006,
  author = {Werner-Allen, G. and Lorincz, K. and Johnson, J. B. and Lees, J. M. and Welsh, M.},
  title = {Fidelity and Yield in a Volcano Monitoring Sensor Network},
  booktitle = {Proc. of the 7th Symposium on Operating Systems Design and Implementation (OSDI '06)},
  year = {2006},
  pages = {381-396}
}
Werner-Allen, G., Lorincz, K., Ruiz, M.C., Marcillo, O., Johnson, J.B., Lees, J.M. and Welsh, M., (2006) Deploying a wireless sensor network on an active volcano, March/April, 2006. Vol. 10(2): pp. 18-25.
Abstract: Augmenting heavy and power-hungry data collection equipment with lighter, smaller wireless sensor network nodes leads to faster, larger deployments. Arrays comprising dozens of wireless sensor nodes are now possible, allowing scientific studies that aren?t feasible with traditional instrumentation. Designing sensor networks to support volcanic studies requires addressing the high data rates and high data fidelity these studies demand. The authors? sensor-network application for volcanic data collection relies on triggered event detection and reliable data retrieval to meet bandwidth and data-quality demands.
BibTeX:
@article{Werner-Allen2006a,
  author = {Werner-Allen, G. and Lorincz, K. and Ruiz, M. C. and Marcillo, O. and Johnson, J. B. and Lees, J. M. and Welsh, M.},
  title = {Deploying a wireless sensor network on an active volcano},
  year = {2006},
  volume = {10},
  number = {2},
  pages = {18-25}
}
Johnson, J.B., Ruiz, M.C., Lees, J.M. and Ramon, P., (2005) Poor scaling between elastic energy release and eruption intensity at Tungurahua Volcano, Ecuador, Geophysical Research Letters. Vol. 32( L15304)
Abstract: An important objective in volcanology is the quantification of eruption intensity through the study of elastic energy propagated into the atmosphere and ground. To better understand the relation between elastic wave radiation and eruptive activity we deployed seismic, acoustic, and video instrumentation at the active Tungurahua Volcano (Ecuador) in Nov.–Dec. of 2004. Our data show that plume expansion scales very poorly with both seismic and acoustic trace energy and only the initial amplitude of the acoustic signal is perhaps correlated with initial plume rise speeds. In general, the eventual size of Tungurahua eruption plumes does not appear to be controlled by elevated material accelerations within the conduit, which are primary influences on elastic energy radiation. This result has vital implications for volcano observatories, which are interested in using seismic and acoustic trace amplitudes as proxies for eruption magnitudes.
BibTeX:
@article{Johnson2005a,
  author = {Johnson, J. B. and Ruiz, M. C. and Lees, J. M. and Ramon, P.},
  title = {Poor scaling between elastic energy release and eruption intensity at Tungurahua Volcano, Ecuador},
  journal = {Geophysical Research Letters},
  year = {2005},
  volume = {32},
  number = { L15304},
  doi = {10.1029/2005GL022847}
}
Ruiz, M.C., Lees, J.M. and Johnson, J.B., (2005) Source constraints of Tungurahua explosion events, Bulletin of Volcanology. Vol. 68(5): pp. 480-490.
Abstract: The most recent eruptive cycle of Tungurahua volcano began in May 2004, and reached its highest level of activity in July 2004. This activity cycle is the last one of a series of four cycles that followed the reawakening and major eruption of Tungurahua in 1999. Between June 30 and August 12, 2004, three temporary seismic and infrasonic stations were installed on the flanks of the volcano and recorded over 2,000 degassing events. The events are classified by waveform character and include: explosion events (the vast majority, spanning three orders of pressure amplitudes at 3.5 km from the vent, 0.1–180 Pa), jetting events, and sequences of repetitive infrasonic pulses, called chugging events. Travel-time analysis of seismic first arrivals and infrasonic waves indicates that explosions start with a seismic event at a shallow depth (<200 m), followed ?1 s later by an out-flux of gas, ash and solid material through the vent. Cluster analysis of infrasonic signals from explosion events was used to isolate four groups of similar waveforms without apparent correlation to event size, location, or time. The clustering is thus associated with source mechanism and probably spatial distribution. Explosion clusters do not exhibit temporal dependence.
BibTeX:
@article{Ruiz2005,
  author = {Ruiz, M. C. and Lees, J. M. and Johnson, J. B.},
  title = {Source constraints of Tungurahua explosion events},
  journal = {Bulletin of Volcanology},
  year = {2005},
  volume = {68},
  number = {5},
  pages = {480-490},
  doi = {10.1007/s00445-005-0023-8}
}
Tang, C., Rial, J.A. and Lees, J.M., (2005) Shear-wave splitting: A diagnostic tool to monitor fluid pressure in geothermal fields, Geophysical Research Letters. Vol. 32(21): pp. L21317.
Abstract: An experiment on the uses of shear-wave splitting as an imaging tool in fracture-controlled geothermal reservoirs was conducted at Krafla, Iceland. Fifteen days after the beginning of the seismic recording the injection was stopped for eleven days and then restarted, a sequence designed to determine whether shear-wave splitting measurements can detect the transient response of the subsurface crack system to changes in fluid pressure. It was observed that time delays between the fast and slow split shear waves changed significantly and promptly with the stoppage and resumption of injection. Large time delays occurred only during injection, decreased substantially during the stoppage phase, and increased again as injection restarted. Comparisons of these results with similar observations at the Coso geothermal field in California strongly suggest that the time delay of split shear waves can be a useful proxy to monitor fluid pressure in the cracks and changes in crack density.
BibTeX:
@article{Tang2005,
  author = {Tang, Chuanhai and Rial, Jose A. and Lees, Jonathan M.},
  title = {Shear-wave splitting: A diagnostic tool to monitor fluid pressure in geothermal fields},
  journal = {Geophysical Research Letters},
  year = {2005},
  volume = {32},
  number = {21},
  pages = {L21317},
  doi = {10.1029/2005GL023551}
}
Tang, C., Rial, J.A., Lees, J.M. and Thompson, E., (2005) Seismic Imaging of the geothermal field at Krafla, Iceland, In Proceedings, Thirteenth Workshop on Geothermal Reservoir Engineering. Stanford University, Stanford, CA : pp. SGP-TR-176.
Abstract: During the summer of 2004 we recorded the seismicity at the Krafla geothermal field for forty days with an array of twenty PASSCAL L-28 4.5-Hz sensors. The Krafla field is located approximately 60 km East of Akureyri in northern Iceland. The array covered an area approximately 5 km N-S by 4 km EW. The field area is located on Holocene lava flows on the Mid-Atlantic Ridge. The array recorded approximately 5 micro-earthquakes per day at a sampling rate of 500 Hz. This high sampling rate is required to exploit newly developed theories on the frequency-dependence of shear-wave splitting (SWS). During the experiment, the injection well was stopped for ten days to study the response of the subsurface crack system to changes in water pressure. SWS is an exploration method based on the analyses of polarizations and time delays of shear waves that have been distorted by the anisotropy of the medium through which the seismic waves have propagated. Epicenters roughly align along the E-W direction, while hypocenters are shallow around the injection well and appear to be related to the on-going injection. Observations of SWS at Krafla have provided evidence for at least two major crack systems oriented approximately N-S and E-W. This last, rather unexpected direction is consistent with results from a simultaneous MT (magneto-telluric) survey. Further SWS study will lead to a more detailed understanding of the fracture locations, sizes, and orientations in the geothermal field.
BibTeX:
@inproceedings{Tang2005a,
  author = {Tang, Chuanhai and Rial, Jose A. and Lees, Jonathan M. and Thompson, Eric},
  title = {Seismic Imaging of the geothermal field at Krafla, Iceland},
  booktitle = {Proceedings, Thirteenth Workshop on Geothermal Reservoir Engineering},
  year = {2005},
  pages = {SGP-TR-176}
}
Werner-Allen, G., Johnson, J., Ruiz, M., Lees, J. and Welsh, M., (2005) Monitoring Volcanic Eruptions with a Wireless Sensor Network, In Proc. Second European Workshop on Wireless Sensor Networks (EWSN'05).
Abstract: This paper describes our experiences using a wireless sensor network to monitor volcanic eruptions with low-frequency acoustic sensors. We developed a wireless sensor array and deployed it in July 2004 at Volc´an Tungurahua, an active volcano in central Ecuador. The network collected infrasonic (low-frequency acoustic) signals at 102 Hz, transmitting data over a 9 km wireless link to a remote base station. During the deployment, we collected over 54 hours of continuous data which included at least 9 large explosions. Nodes were time-synchronized using a separate GPS receiver, and our data was later correlated with that acquired at a nearby wired sensor array. In addition to continuous sampling, we have developed a distributed event detector that automatically triggers data transmission when a well-correlated signal is received by multiple nodes. We evaluate this approach in terms of reduced energy and bandwidth usage, as well as accuracy of infrasonic signal detection.
BibTeX:
@inproceedings{Werner-Allen2005,
  author = {Werner-Allen, Geoff and Johnson, Jeff and Ruiz, Mario and Lees, Jonathan and Welsh, Matt},
  title = {Monitoring Volcanic Eruptions with a Wireless Sensor Network},
  booktitle = {Proc. Second European Workshop on Wireless Sensor Networks (EWSN'05)},
  year = {2005}
}
Davaille, A. and Lees, J.M., (2004) Thermal modeling of subducted plates: tear and hot spot at the Kamchatka corner, Earth and Planetary Science Letters. Vol. 226(3-4): pp. 293-304.
Abstract: Pacific plate subduction at the Aleutian–Kamchatka juncture, or corner, could be accommodated by either a large bend or a tear in the oceanic lithosphere. In this paper, we describe a number of observations which suggest that the Pacific plate terminates abruptly at the Bering transform zone (TZ). Seismicity shoals along the subduction zone from Southern Kamchatka (600 km) to relatively shallow depths near the Kamchatka–Bering Fault intersection (100–200 km). This seismicity shoaling is accompanied by an increase in the heat flow values measured on the Pacific plate. Moreover, unusual volcanic products related to adakites are erupted on Kamchatka peninsula at the juncture. Simple thermal modeling shows that a slab torn and thinner along the northern edge of the Pacific plate would be compatible with the observations. Delayed thickening of the lithosphere due to the Meiji–Hawaiian hotspot may be responsible for the required thinning.
BibTeX:
@article{Davaille2004,
  author = {Davaille, A. and Lees, J. M.},
  title = {Thermal modeling of subducted plates: tear and hot spot at the Kamchatka corner},
  journal = {Earth and Planetary Science Letters},
  year = {2004},
  volume = {226},
  number = {3-4},
  pages = {293-304},
  doi = {10.1016/j.epsl.2004.07.024}
}
Lees, J.M., (2004) Scattering from a fault interface in the Coso geothermal field, Journal of Volcanology and Geothermal Research. Vol. 130(1-2): pp. 61-75.
Abstract: Large-amplitude, secondary arrivals are modeled as scattering anomalies near the Coso, California, geothermal field. Polarization and ray tracing methods determine the orientation and location of the scattering body. Two models are proposed for the scatterer: (1) a point scatterer located anywhere in a one-dimensional (1-D), layered velocity model; and (2) a dipping interface between two homogeneous half spaces. Each model is derived by non-linear, grid search inversion for the optimal solution which best predicts observed travel times. In each case the models predict a nearly vertical scatterer southwest of stations S4 and Y4, each southeast of Sugarloaf Mountain, a large rhyolite dome. The interface model includes five unknown parameters describing the location and orientation of the interface in addition to the S-wave velocity of the half space. The S-wave velocity, 3.25 km/s, agrees with independently derived 1-D models in this area. The large amplitude, vertical impedance contrast interface coincides with steep gradients of heat flow measured near the surface and with structural boundaries observed in surface geology. The reflector is most probably the sharp boundary between the northern part of the field where there is significant fluid flow and the southern part where hydrothermal fluids are absent. The interface coincides with geological boundaries and faults recently observed in this region, most likely representing the hydrothermal barrier which channels hot fluids northward.
BibTeX:
@article{Lees2004,
  author = {Lees, Jonathan M.},
  title = {Scattering from a fault interface in the Coso geothermal field},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2004},
  volume = {130},
  number = {1-2},
  pages = {61-75}
}
Lees, J.M., Gordeev, E.I. and Ripepe, M., (2004) Explosions and Periodic Tremor at Karymsky Volcano, Kamchatka, Russia, Geophysical Journal International. Vol. 158(3): pp. 1151–1167.
Abstract: The explosions of Karymsky volcano often produce signals containing a sequence of repeating pulses recorded on acoustic and seismic sensors, known as chugging. The amplitudes of these pulses correlate with the time interval between pulses. For a given measured acoustic pressure, seismic amplitudes take on arbitrary values up to a specific, empirically determined threshold. Conversely, events with a small seismic amplitude yielded acoustic waves with large variations and large-amplitude seismic events corresponded to large acoustic waves. These observations are not consistent with a source modelled by a resonating conduit. Rather, a model consisting of a sequence of discrete pulses explains the data and provides a framework for understanding the dynamics of degassing at the vent. The physical model for chugging involves a time-varying narrowing vent where gasses are released in a series of oscillations which appear to be harmonic but instead are modelled as short-term transients, or discrete pulses, suggestive of choked flow.
BibTeX:
@article{Lees2004a,
  author = {Lees, J. M. and Gordeev, E. I. and Ripepe, M.},
  title = {Explosions and Periodic Tremor at Karymsky Volcano, Kamchatka, Russia},
  journal = {Geophysical Journal International},
  year = {2004},
  volume = {158},
  number = {3},
  pages = {1151–1167},
  doi = {10.1111/j.1365-246X.2004.02239.x}
}
McGreger, A.D. and Lees, J.M., (2004) Vent Discrimination At Stromboli Volcano, Italy, Journal of Volcanology and Geothermal Research. Vol. 137(1-3): pp. 169-185.
Abstract: Eruptive activity at Stromboli Volcano was significantly elevated over background levels in May 2001. During 63 h of observation, eight vents produced, on average, 17 explosions/h with an average repose interval of 3 min. During this period, the Stromboli vents exhibited consistent seismic and acoustic signatures, based on cross-correlation cluster analysis. Dendrogram clustering, based on waveform cross-correlation, was used to illustrate the complexity of the near surface plumbing system of Stromboli's multi-vent edifice. Cross-correlations of displacement seismograms produced by explosions at specific craters, such as the Northeast crater (NEC), form dense waveform clusters with correlation coefficients between 0.96 and 0.99, while displacement waveforms from other craters, such as the Southwest crater (SWC), exhibit loose clusters with correlations between 0.88 and 0.96. The inconsistency of SWC events, as compared to the NEC, suggests that the vent system there is more heterogeneous. Cluster linkage distance between the NEC cluster and the Central crater (CC) cluster is shorter than the linkage distance between the NEC and SWC clusters, indicating that NEC and CC are more closely related. Infrasonic observations were used to locate vent explosions confirming that the clusters of events are associated with specific vents or craters. Qualitative analyses of acoustic waveforms from approximately 500 explosions reveal that impulsive acoustic signals were associated with short, mechanically simple ground displacement responses. These events may correspond to the bursting of an individual gas slug. Similar degassing mechanisms from vents in the NEC and the CC show common characteristics in their displacement waveforms.
BibTeX:
@article{McGreger2004,
  author = {McGreger, A. D. and Lees, J. M.},
  title = {Vent Discrimination At Stromboli Volcano, Italy},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2004},
  volume = {137},
  number = {1-3},
  pages = {169-185},
  doi = {10.1016/j.jvolgeores.2004.05.007}
}
Shalev, E. and Lees, J.M., (2004) Three dimensional tomographic analysis of the Loma Prieta region, In USGS Professional Paper 1550-E: The Loma Prieta, California, Earthquake of October 17, 1989 - Geologic Setting and Crustal Structure. Reston, VA : pp. 127-142. U.S. Geological Survey.
Abstract: A high resolution tomographic study, using cubic B splines parameterization and employing a systematic approach to the choosing of appropriate damping and smoothing parameters, provided a three dimensional P wave velocity map of the Loma Prieta area. 11,977 high quality raypaths from 844 aftershocks of the 1989 Loma Prieta earthquake were used in the inversion. The velocity model exhibits a low velocity feature between the San Andreas and Zayante Vergeles faults in the top 10 km of the crust. This low velocity feature is interpreted as a sedimentary unit exposed to the northwest and separated from the Salinian block by the Zayante Vergeles fault. Below 10 km no consistent change is observed between the Salinian and the Franciscan blocks. There appears to be a high correlation of aftershock activity and localized high velocity anomalies southeast of the Loma Prieta main shock. Whereas this anomaly may represent brittle rocks associated with a fault zone asperity that failed after the main shock, there is evidence to suggest it may be a body of serpentinite. The serpentinite exhibits high velocities and is potentially less competent than surrounding country rock, thus providing a sector along the fault more likely to be associated with many smaller earthquakes or creep behavior.
BibTeX:
@incollection{Shalev2004,
  author = {Shalev, E. and Lees, Jonathan M.},
  editor = {Wells, Ray E.},
  title = {Three dimensional tomographic analysis of the Loma Prieta region},
  booktitle = {USGS Professional Paper 1550-E: The Loma Prieta, California, Earthquake of October 17, 1989 - Geologic Setting and Crustal Structure},
  publisher = {U.S. Geological Survey},
  year = {2004},
  pages = {127-142}
}
Johnson, J.B., Aster, R.C., Ruiz, M.C., Malone, S.D., McChesney, P.J., Lees, J.M. and Kyle, P.R., (2003) Interpretation and utility of infrasonic records from erupting volcanoes, Journal of Volcanology and Geothermal Research. Vol. 121(1-2): pp. 15-63.
Abstract: In the most basic seismo-acoustic studies at volcanoes, infrasound monitoring enables differentiation between sub-surface seismicity and the seismicity associated with gas release. Under optimal conditions, complicated degassing signals can be understood, relative explosion size can be assessed, and variable seismo-acoustic energy partitioning can be interpreted. The extent to which these points may be investigated depends upon the quality of the infrasonic records (a function of background wind noise, microphone sensitivity, and microphone array geometry) and the type of activity generated by the volcano (frequency of explosions, bandwidth of the signals, and coupling efficiency of the explosion to elastic energy). To illustrate the features, benefits, and limitations of infrasonic recordings at volcanoes, we showcase acoustic and seismic records from five volcanoes characterized by explosive degassing. These five volcanoes (Erebus in Antarctica, Karymsky in Russia, and Sangay, Tungurahua, and Pichincha in Ecuador) were the focus of seismo-acoustic experiments between 1997 and 2000. Each case study provides background information about the volcanic activity, an overview of visual observations during the period of monitoring, and examples of seismo-acoustic data. We discuss the benefits and utility of the infrasound study at each respective volcano. Finally, we compare the infrasound records and eruptive activity from these volcanoes with other volcanoes that have been the focus of previous seismo-acoustic experiments.
BibTeX:
@article{Johnson2003,
  author = {Johnson, J. B. and Aster, R. C. and Ruiz, M. C. and Malone, S. D. and McChesney, P. J. and Lees, J. M. and Kyle, P. R.},
  title = {Interpretation and utility of infrasonic records from erupting volcanoes},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2003},
  volume = {121},
  number = {1-2},
  pages = {15-63}
}
Ozerov, A., Ispolatov, I. and Lees, J., (2003) Modeling Strombolian eruptions of Karymsky volcano, Kamchatka, Russia, Journal of Volcanology and Geothermal Research. Vol. 122(3-4): pp. 265-280.
Abstract: A model is proposed to explain temporal patterns of activity in a class of periodically exploding Strombolian-type andesite volcanoes. These patterns include major events (explosions) which occur every 3–30 min and subsequent tremor with a typical period of 1 s. This two-periodic activity is thought to be caused by two distinct mechanisms of accumulation of the elastic energy in the moving magma column: compressibility of the magma in the conduit and viscoelastic response of the almost solid magma plug on the top. A release of the elastic energy occurs during a stick–slip dynamic phase transition in a boundary layer along the walls of the conduit; this phase transition is driven by the shear stress accumulated in the boundary layer. The intrinsic hysteresis of this first-order phase transition explains the long periods of inactivity in the explosion cycle. Temporal characteristics of the model are found to be qualitatively similar to the acoustic and seismic signals recorded at Karymsky volcano in Kamchatka.
BibTeX:
@article{Ozerov2003,
  author = {Ozerov, A. and Ispolatov, I. and Lees, J.},
  title = {Modeling Strombolian eruptions of Karymsky volcano, Kamchatka, Russia},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2003},
  volume = {122},
  number = {3-4},
  pages = {265-280},
  doi = {10.1016/S0377-0273(02)00506-1}
}
Bhattacharyya, J. and Lees, J.M., (2002) Seismicity, stress and triggering in the Coso/Indian Wells Valley region, In Geologic Evolution of the Central Mojave Desert and Southern Basin and Range. Vol. 195: pp. 243-258. Geological Society of America.
Abstract: The temporal and spatial distribution of seismicity in the Coso and Indian Wells valley region of eastern California are discussed in this study. An analysis of fault-related seismicity in the region let us conclude that the Little Lake fault and the Airport Lake faults are the most significant seismogenic zones. The faulting pattern clearly demarcates the region as a transition between the San Andreas type strike-slip regime to the west and Basin and Range extension to the east. We present the spatial and temporal variations in seismicity immediately following significant earthquakes in nearby regions over the last fifteen years with special emphasis on larger earthquakes (M 5) over the last five years. The Ridgecrest earthquakes of 1995 show a complicated faulting pattern as the rupture changes from normal-slip to right-slip at depth. The inter-relationships between the Coso earthquakes of 1996 and 1998 are presented as a set of conjugate events. Analysis of earthquake source mechanisms shows evidence for lateral variations of faulting pattern in eastern California. Earthquake focal mechanisms are used to estimate local stress orientation within the Coso geothermal field. We have identified a boundary between a transpressional regime and a transtensional regime inside the field which correlates with observed spatial variations of heat flow and seismic attenuation, velocity and anisotropy.
BibTeX:
@incollection{Bhattacharyya2002,
  author = {Bhattacharyya, J. and Lees, J. M.},
  editor = {J., A. Glazner and Walker, D. and Bartley, J. M.},
  title = {Seismicity, stress and triggering in the Coso/Indian Wells Valley region},
  booktitle = {Geologic Evolution of the Central Mojave Desert and Southern Basin and Range},
  publisher = {Geological Society of America},
  year = {2002},
  volume = {195},
  pages = {243-258}
}
Lees, J.M., (2002) Three-Dimensional Anatomy of a Geothermal Field, In Geologic Evolution of the Central Mojave Desert and Southern Basin and Range. Boulder, CO. Vol. 195: pp. 259-276. Geological Society of America.
Abstract: This paper reviews geophysical and seismological imaging in the Coso geothermal field, located in east central California. The Coso geothermal production area covers an area approximately 6 10 km2. Although regional seismicity is addressed, as it sheds light on the magma, or heat, sources in the field, the main focus of this paper is on the main production area. Three-dimensional inversions for P- and S-wave velocity variations, distribution of attenuation, and anisotropy are presented side-by-side so that anomalies can be compared spatially in a direct manner. Velocity inversions for P- and S-waves are combined for direct determination of Poisson's ratio and indirect estimation of variations of porosity in the field. Anomalies southeast of Sugarloaf Mountain are prominent on nearly all analyses. The anomalies coincide with high levels of seismicity and with stress anomalies as determined from earthquake focal mechanism analysis and seismic anisotropy distribution. They also correlate with high heat flow in the field and the termination of geothermal production to the south. I speculate that an intrusion is present in this region that causes significant perturbation of stress in the field.
BibTeX:
@incollection{Lees2002,
  author = {Lees, J. M.},
  editor = {Glazner, A. and Walker, J. D. and Bartley, J. M.},
  title = {Three-Dimensional Anatomy of a Geothermal Field},
  booktitle = {Geologic Evolution of the Central Mojave Desert and Southern Basin and Range},
  publisher = {Geological Society of America},
  year = {2002},
  volume = {195},
  pages = {259-276}
}
Levin, V., Park, J., Brandon, M., Lees, J.M., Peyton, V., Gordeev, E. and Ozerov, A., (2002) Crust and upper mantle of Kamchatka from teleseismic receiver functions, In Tectonophysics. Amsterdam : pp. 233-265. Elsevier.
Abstract: Teleseismic receiver functions (RFs) from a yearlong broadband seismological experiment in Kamchatka reveal regional variations in the Moho, anisotropy in the supra-slab mantle wedge, and, along the eastern coast, Ps converted phases from the steeply dipping slab. We analyze both radial- and transverse-component RFs in bin-averaged epicentral and backazimuthal sweeps, in order to detect Ps moveout and polarity variations diagnostic of interface depth, interface dip, and anisotropic fabric within the shallow mantle and crust. At some stations, the radial RF is overprinted by near-surface resonances, but anisotropic structure can be inferred from the transverse RF. Using forward modeling to match the observed RFs, we find Moho depth to range between 30 and 40 km across the peninsula, with a gradational crust–mantle transition beneath some stations along the eastern coast. Anisotropy beneath the Moho is required to fit the transverse RFs at most stations. Anisotropy in the lower crust is required at a minority of stations. Modeling the amplitude and backazimuthal variation of the Ps waveform suggests that an inclined axis of symmetry and 5–10% anisotropy are typical for the crust and the shallow mantle. The apparent symmetry axes of the anisotropic layers are typically trench-normal, but trench-parallel symmetry axes are found for stations APA and ESS, both at the fringes of the central Kamchatka depression. Transverse RFs from east-coast stations KRO, TUM, ZUP and PET are fit well by two anisotropic mantle layers with trench-normal symmetry axes and opposing tilts. Strong anisotropy in the supraslab mantle wedge suggests that the mantle ‘‘lithosphere’’ beneath the Kamchatka volcanic arc is actively deforming, strained either by wedge corner flow at depth or by trenchward suction of crust as the Pacific slab retreats.
BibTeX:
@incollection{Levin2002,
  author = {Levin, Vadim and Park, Jeffrey and Brandon, Mark and Lees, Jonathan M. and Peyton, Valerie and Gordeev, Evgenii and Ozerov, Alexei},
  editor = {Artemieva, Irina M. and Thybo, Hans and Mooney, Walter D. and Perchuc, Edward},
  title = {Crust and upper mantle of Kamchatka from teleseismic receiver functions},
  booktitle = {Tectonophysics},
  publisher = {Elsevier},
  year = {2002},
  pages = {233-265}
}
Park, J., Levin, V., Brandon, M., Lees, J., Peyton, V., Gordeev, E. and Ozerov, A., (2002) A dangling slab, amplified arc volcanism, mantle flow and seismic anisotropy in the Kamchatka Plate Corner, In Plate Boundary Zones. Vol. 30: pp. 295-324. AGU.
Abstract: The Kamchatka peninsula in Russian East Asia lies at the junction of a transcurrent plate boundary, aligned with the western Aleutian Islands, and a steeply-dipping subduction zone with near-normal convergence. Seismicity patterns and P-wave tomography argue that subducting Pacific lithosphere terminates at the Aleutian junction, and that the downdip extension (>150km depth) of the slab edge is missing. Seismic observables of elastic anisotropy (SKS splitting and Love-Rayleigh scattering) are consistent with asthenospheric strain that rotates from trench-parallel beneath the descending slab to trench-normal beyond its edge. Present-day arc volcanism is concentrated near the slab edge, in the Klyuchevskoy and Sheveluch eruptive centers. Loss of the downdip slab edge, whether from thermo-convective or ductile instability, and subsequent ``slab-window'' mantle return flow is indicated by widespread Quaternary volcanism in the Sredinny range inland of Klyuchevskoy and Sheveluch, as well as the inferred Quaternary uplift of the central Kamchatka depression. The slab beneath Klyuchevskoy has shallower dip (35o) than the subduction zone farther south (55o) suggesting a transient lofting of the slab edge, either from asthenospheric flow or the loss of downdip load. Such lofting may induce pressure-release melting to supply the Klyuchevskoy and Sheveluch eruptive centers. Petrologic indicators of high magma-peridotite equilibrium temperatures, long residence times for the hydrous arc-volcanic component, and weak expression of subducted sediment flux support the lofting hypothesis, and discourage an alternate interpretation in terms of accelerated slab rollback and/or a heightened influx of subducted volatiles. Over the late Cenozoic, the Komandorsky Basin subducted beneath northern Kamchatka and produced arc volcanics in the Sredinny Range. Several lines of evidence suggest the northeast migration of a plate triple junction (North America/Pacific/Komandorsky) along the southern Kamchatka coast in Oligocene-Miocene times. Three ``cape terranes'' (Shipunsky, Kronotsky, Kamchatka) along the coastline are exotic, with geologic similarities to present-day Western Aleutian islands, and may have accreted in a ``caulking-gun'' process as the triple junction migrated NE. The late Cenozoic transfer of arc volcanism from the Sredinny range to the eastern volcanic front of Kamchatka may have been facilitated by the progressive replacement of a shallow-dipping Komandorsky slab with a steeply-dipping Pacific slab.
BibTeX:
@incollection{Park2002,
  author = {Park, J. and Levin, V. and Brandon, M. and Lees, J. and Peyton, V. and Gordeev, E. and Ozerov, A.},
  editor = {Stein, S. and Freymuller, J.},
  title = {A dangling slab, amplified arc volcanism, mantle flow and seismic anisotropy in the Kamchatka Plate Corner},
  booktitle = {Plate Boundary Zones},
  publisher = {AGU},
  year = {2002},
  volume = {30},
  pages = {295-324}
}
Lees, J.M., Brandon, M., Park, J., Levin, V., Ozerov, A. and Gordeev, E., (2001) Kamchatka: Edge of the Plate, Iris News Letter. Vol. 2000(1): pp. 17-19.
Abstract: New PASSCAL data have been acquired along the extent of the Kamchatka Peninsula to examine the interaction of the Pacific Plate and the mantle in the corner junction of the Aleutian and Kamchatka trenches. The project, called the Side Edge of Kamchatka Slab, is a collaborative effort between researchers at Yale University and the Russian Academy of Sciences Institutes in Petropavlovsk-Kamchatski, in particular the Institute of Volcanology (Alexei Ozerov) and KOMSP (Evgenii Gordeev). In Russia, Kamchatka is known as the caboose, the last car on the train. It is practically the farthest one can get from Moscow, the center of cultural life in Russia. Being nine time zones away from the financial centers creates a sense of isolation and liberation. For Russians, Kamchatka is a land of dreams and possibilities, much as Alaska is the last frontier for Americans. This gives one the sense that being in Kamchatka is like being on the edge of the world. But the feeling has more significance than the mere waywardness of the place. The majestic volcanoes, the continuous seismicity and the incredible geology are omnipresent reminders that this is a special place. The relentless subduction of the Pacific plate plunges beneath Kamchatka, 70mm per year. Kamchatka is home to the most magnificent volcanoes in the Pacific Rim (28 active volcanoes) and neighbor to the Bering strike-slip fault, marking the western end of the Aleutian-Komandorsky Islands. This land mass provides an exceptional platform for investigating the interactions of volcanism, tectonics, and mantle dynamics.
BibTeX:
@article{Lees2000c,
  author = {Lees, J. M. and Brandon, M. and Park, J. and Levin, V. and Ozerov, A. and Gordeev, E.},
  title = {Kamchatka: Edge of the Plate},
  journal = {Iris News Letter},
  year = {2001},
  volume = {2000},
  number = {1},
  pages = {17-19}
}
Peyton, V., Levin, V., Park, J., Brandon, M., Lees, J., Gordeev, E. and Ozerov, A., (2001) Mantle Flow at a Slab Edge: Seismic Anisotropy in the Kamchatka Region, Geophysical Research Letters. Vol. 28(2): pp. 379-382.
Abstract: The junction of the Aleutian Island and the Kamchatka peninsula defines a sharp turn in the boundary of the Pacific and North American plates, terminating the subduction zones of the northwest Pacific. The regional pattern of shear-wave birefringence near the junction indicates that trench-parallel strain follows the seismogenic Benioff zone, but rotates to trench-normal elsewhere. Asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge, and may influence the shallowing dip of the Benioff zone at the Aleutian junction.
BibTeX:
@article{Peyton2001,
  author = {Peyton, V. and Levin, V. and Park, J. and Brandon, M. and Lees, J. and Gordeev, E. and Ozerov, A.},
  title = {Mantle Flow at a Slab Edge: Seismic Anisotropy in the Kamchatka Region},
  journal = {Geophysical Research Letters},
  year = {2001},
  volume = { 28},
  number = {2},
  pages = {379-382}
}
Yogodzinski, G.M., Lees, J.M., Churikova, T., Dorendorf, F., Woerner, G. and Volynets, O., (2001) Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges, Nature. Vol. 409: pp. 500-504.
Abstract: Data are presented to show that slab melt-related volcanism (adakitic volcanism) is occurring in the Aleutian-Kamchatka junction, and in the western Aleutians where the plate boundary switches from convergent to strike-slip motion. The subducting lithosphere in these areas is relatively old (>40Ma), and is therefore not expected to melt during subduction (i.e., Defant and Drummond, 1990). It is argued that the slab is melting in these areas because the side edge of the down-going plate is warmed by asthenospheric flow on three sides. The absence of deep or intermediate-depth seismicity in these areas supports a warm slab interpretation. An anomalously large slab melt contribution to arc volcanic rocks in the Aleutian-Kamchatka junction and in the western Aleutians is interpreted to be a geochemical edge effect. The presence of this effect in these areas is consistent with the idea that the Pacific plate is being torn in an unzipping motion as it moves westward from the convergent to the strike-slip portions of the Aleutian arc.
BibTeX:
@article{Yogodzinski2001,
  author = {Yogodzinski, G. M. and Lees, J. M. and Churikova, T.G. and Dorendorf, F. and Woerner, G. and Volynets, O.N.},
  title = {Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges},
  journal = {Nature},
  year = {2001},
  volume = {409},
  pages = {500-504}
}
Johnson, J.B. and Lees, J.M., (2000) Plugs and Chugs - Strombolian activity at Karymsky, Russia, and Sangay, Ecuador, Journal of Volcanology and Geothermal Research. Vol. 101: pp. 67-82.
Abstract: Frequent degassing explosions, occuring at intervals of minutes to tens of minutes, are common at many active basaltic and andesitic volcanoes worldwide. In August 1997, April 1998, and September 1998 we recorded seismic and acoustic signals generated at two andesitic volcanoes with 'Strombolian-type' activity. Despite variations in explosion frequency (5 to 15 per hour at Karymsky as opposed to 1 to 3 per hour at Sangay), the signatures of the explosions are remarkably similar at these two, diverse field sites. In all explosions, gas emission begins rapidly and is correlated with an impulsive acoustic pressure pulse. Seismic waveforms are extremely emergent and begin 1 to 2 seconds before the explosion itself. We classify explosion events at the two volcanoes as either short-duration (less than 1 minute) simple impulses or long-duration (up to 5 minutes) tremor events. Many tremor events have harmonic frequency spectra and correspond to regular 1 second acoustic pulses, often audible, that sound like chugging from a locomotive. Chugging events are intermittent, suggesting that the geometry or geochemistry of the process is variable over short time scales. We attribute the 1 Hz periodic chugs to a resonant phenomena in the upper section of conduit.
BibTeX:
@article{Johnson2000,
  author = {Johnson, J. B. and Lees, J. M.},
  title = {Plugs and Chugs - Strombolian activity at Karymsky, Russia, and Sangay, Ecuador},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2000},
  volume = {101},
  pages = {67-82}
}
Lees, J.M., (2000) Geotouch: Software for Three and Four Dimensional GIS in the Earth Sciences, Computers & Geosciences. Vol. 26(7): pp. 751-761.
Abstract: A new program for exploratory data analysis in three and four dimensions is presented and described. Interactive communication between diverse datasets is stressed as the main gestalt of the Geotouch program. The primary kinds of data include point, vector, raster and wireframe data sets, in addition to specialized forms such as focal mechanisms and ellipsoidal information. The software includes methods for cutting cross sections at arbitrary angles, spinning objects in three space and animating time series of punctual data, such as hypocenter series and volcano eruptions. The program is written in POSIX compliant C using X-windows for Unix systems and has been ported to Linux. Free access, via the Internet, to executable binary, source code and documentation make this package an attractive alternative to expensive or unwieldy commercial options.
BibTeX:
@article{Lees2000,
  author = {Lees, J. M.},
  title = {Geotouch: Software for Three and Four Dimensional GIS in the Earth Sciences},
  journal = {Computers & Geosciences},
  year = {2000},
  volume = {26},
  number = {7},
  pages = {751-761}
}
Lees, J.M. and Wu, H., (2000) Poisson's ratio and porosity at Coso Geothermal Area, California, Journal of Volcanology and Geothermal Research. Vol. 95: pp. 157-173.
Abstract: High resolution three-dimensional compressional and shear wave velocity models, derived from microearthquake traveltimes, are used to map the distribution of Poisson's ratio and porosity at Coso Geothermal Area, Inyo County, California. Spatial resolution of the three-dimensional Poisson's ratio and porosity distributions is estimated to be 0.5 km horizontally and 0.8 km vertically. Model uncertainties, pm 1% in the interior and pm 2.3% around the edge of the model, are estimated by a jackknife method. We use perturbations of r = V_p / V_s ratio and Psi = V_p * V_s product to derive distributions of Poisson's ratio, and porosity, which are then used to constrain and delineate possible zones of intense heat, fracture accumulation and fluid saturation. Poisson's ratio at Coso ranges from 0.15 to 0.35 with an average of 0.224, lower than the crustal average of 0.25. High Poisson's ratios are more extensive in shallower depths (<.5 km) while lower Poisson's ratios are found in the deeper section (1.5--3.0 km) of the target area. Two major features with low Poisson's ratio are identified at geothermal production depth (1--3 km) around stations S2-S6 and S1-S3-S4. The two low sigma features are separated by a northwest-southeast trending high sigma belt with variable width of 1 ~ 3 km. A high Psi body is found around S2 and S6, and extends down in depth. A circular, low Psi belt corresponding to the high sigma belt, is located around S2-S6 and is linked to a previously reported structure in V_s tomography. This low Psi (highly porous) belt is probably a horizontal conduit/reservoir of geothermal fluid. A vertical, low sigma and high Psi channel beneath triangle S1-S3-S4 corresponds to a high attenuation, dome-like feature. We propose an upwell-and-spread magma intrusion model for the last major magmatism in the Coso region. The magmatic upwelling is centered in the S1-S3-S4 area. The model predicts potential geothermal resources to the south and west of triangle S1-S3-S4 based on local faulting patterns.
BibTeX:
@article{Lees2000d,
  author = {Lees, J. M. and Wu, H.},
  title = {Poisson's ratio and porosity at Coso Geothermal Area, California},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {2000},
  volume = {95},
  pages = {157-173}
}
Bhattacharyya, J., Grosse, S., Lees, J.M. and Hasting, M., (1999) Recent earthquake sequences at Coso: Evidence for conjugate faulting and stress loading near a geothermal field, Bulletin of the Seismological Society of America. Vol. 89(3): pp. 785-795.
Abstract: Two recent earthquake sequences near the Coso geothermal field show clear evidence of faulting along conjugate planes. We present results from analyzing an earthquake sequence occurring in 1998, and compare it with a similar sequence that occurred in 1996. The two sequences followed mainshocks which occurred on November 27, 1996 and March 6, 1998. Both mainshocks ruptured approximately co-located regions of the same fault system. Following a comparison with the background seismicity of the Coso region, we have detected evidence of stress loading within the geothermal field that appears to be in response to the 1998 earthquakes. The M_L = 5.2 mainshock in the 1998 sequence occurred at 5:47 am UTC, and waslocated approximately 45 km north of the town of Ridgecrest in the Coso range. The mainshock of the 1996 sequence had a M_L magnitude of 5.3. There have been no observable surface ruptures associated with either of these sequences. Though the mainshocks for both sequences were located about 900 m apart and have nearly the same local magnitudes, the sequences differ in both their temporal and spatial characteristics. An analysis of the fault plane solutions of the mainshocks and the aftershock locations suggests that the two sequences ruptured fault planes which are perpendicular to one another. We observe a much faster temporal decay of the 1998 sequence compared to the one in 1996; moreover, while the 1996 sequence was not followed by any sizeable (i.e., M_L > 4.0) aftershocks, the 1998 sequence had four such events. From an estimate of the tectonic stressing rate on the fault that produced the 1998 sequence, we infer a repeat cycle of 135 years for an earthquake of comparable magnitude at Coso.
BibTeX:
@article{Bhattacharyya1999,
  author = {Bhattacharyya, J. and Grosse, S. and Lees, J. M. and Hasting, M.},
  title = {Recent earthquake sequences at Coso: Evidence for conjugate faulting and stress loading near a geothermal field},
  journal = {Bulletin of the Seismological Society of America},
  year = {1999},
  volume = {89},
  number = {3},
  pages = {785-795}
}
Hough, S.E., Lees, J.M. and Monastero, F., (1999) Attenuation and source properties at the Coso geothermal area, California, Bulletin of the Seismological Society of America. Vol. 89(6): pp. 1606-1619.
Abstract: We use a multiple-empirical Green's function method to determine source properties of small (M-0.4 to 1.3) earthquakes and P and S-wave attenuation at the Coso Geothermal field, California. Source properties of a previously-identified set of clustered events from the Coso geothermal region are first analyzed using an empirical Green's function (EGF) method. Stress drops values of at least 0.5-1 MPa are inferred for all of the events; in many cases, the corner frequency is outside the usable bandwidth and can only be constrained as being higher than ~3 MPa. P- and S-wave stress drop estimates are identical to the resolution limits of the data. These results are indistinguishable from numerous EGF studies of M2-5 earthquakes, suggesting a similarity in rupture processes that extends to events that are both tiny and induced and providing further support for Byerlee's Law. Whole-path Q estimates for P and S waves are determined using the multiple-empirical Green's function (MEGF) method of it Hough, (1997), whereby spectra from clusters of colocated events at a given station are inverted for a single attenuation parameter, with source parameters constrained from EGF analysis. The kappa estimates, which we infer to be resolved to within 0.003 sec or better, exhibit almost as much scatter as a function of hypocentral distance as do values from previous single-spectrum studies for which much higher uncertainties in individual kappa estimates are expected. The variability in kappa estimates determined here therefore suggests real lateral variability in Q structure. Although the raypath coverage is too sparse to yield a complete three-dimensional attenuation tomographic image, we invert the inferred kappa value for three-dimensional structure using a damped least-squares method and the results do reveal significant lateral variability in Q structure. The inferred attenuation variability corresponds to the heat-flow variations within the geothermal region. A central low-Q region corresponds well with the central high-heat flow region; additional detailed structure is also suggested.
BibTeX:
@article{Hough1999,
  author = {Hough, S. E. and Lees, J. M. and Monastero, F.},
  title = {Attenuation and source properties at the Coso geothermal area, California},
  journal = {Bulletin of the Seismological Society of America},
  year = {1999},
  volume = {89},
  number = {6},
  pages = {1606-1619}
}
Lees, J.M. and Wu, H., (1999) P-wave anisotropy, stress, and crack distribution at Coso Geothermal Field, California, Journal of Geophysical Research. Vol. 104(8): pp. 17,955-17,973.
Abstract: A new inversion method for P-wave anisotropy [Wu and Lees, 1999a] has been applied to high-precision, microseismic travel-time data collected at Coso geothermal region, California. Direction-dependent P-wave velocity, and thus its perturbation, are represented by a symmetric positive definite matrix A instead of a scalar. The resulting anisotropy distribution is used to estimate variations in crack density, stress distribution and permeability within the producing geothermal field. A circular dome-like structure is observed at the southwestern part of the geothermal region southwest of Sugarloaf Mountain. Using a linear stress-bulk modulus relation and deviatoric stress is estimated to be 3-6 MPa at geothermal production depths (1-2 km), assuming all the anisotropy is realted to stress. The stress field is compressional NNE-SSW and dilational WNW-ESE, coinciding with a previous, independent study using earthquake focal mechanisms. Following a theory on flat, elliptic cracks, residual crack density estimated from P anisotropy is about 0.0078, assuming crack aspect ratios >>1:60 and is ~0.041 when crack aspect ratios are close to 1:60. Residual crack orientation distribution is related to velocity anisotropy. Based on the anisotropic part of crack density distribution function, the anisotropic part of permeability distribution may be calculated by a statistical approach via simple parallel fluid flow along cracks.
BibTeX:
@article{Lees1999c,
  author = {Lees, J. M. and Wu, H.},
  title = {P-wave anisotropy, stress, and crack distribution at Coso Geothermal Field, California},
  journal = {Journal of Geophysical Research},
  year = {1999},
  volume = {104},
  number = {8},
  pages = {17,955-17,973}
}
Moran, S.C., Lees, J.M. and Malone, S.D., (1999) P-wave velocity structure in the greater Mount Rainier area from local earthquake tomography, Journal of Geophysical Research. Vol. 104(10): pp. 10,775-10,786.
Abstract: We present results from a local earthquake tomographic imaging experiment in the greater Mount Rainier area. We inverted P-wave arrival times from local earthquakes recorded at permanent and temporary Pacific Northwest Seismograph Network seismographs between 1980 and 1996. We used a method similar to that described by Lees and Crosson [1989], modified to incorporate the parameter separation method for decoupling the hypocenter and velocity problems. In the upper 7 km of the resulting model there is good correlation between velocity anomalies and surface geology. Many focal mechanisms within the St. Helens seismic zone have nodal planes parallel to the epicentral trend as well as to a north-south trending low-velocity trough, leading us speculate that the trough represents a zone of structural weakness in which a moderate (M 6.5-7.0) earthquake could occur. In contrast, the western Rainier seismic zone (WRSZ) does not correlate in any simple way with anomaly patterns or focal mechanism fault planes, leading us to infer that the WRSZ is less likely to experience a moderate earthquake. A ~10 km-wide low-velocity anomaly occurs 5 to 18 km beneath the summit of Mount Rainier, which we interpret to be a signal of a region composed of hot, fractured rock with possible small amounts of melt or fluid. No systematic velocity pattern is observed in association with the southern Washington Cascades conductor. A mid-crustal anomaly parallels the Olympic-Wallowa lineament (OWL) as well as several other geophysical trends, indicating that the OWL may play an important role in regional tectonics.
BibTeX:
@article{Moran1999a,
  author = {Moran, S. C. and Lees, J. M. and Malone, S. D.},
  title = {P-wave velocity structure in the greater Mount Rainier area from local earthquake tomography},
  journal = {Journal of Geophysical Research},
  year = {1999},
  volume = {104},
  number = {10},
  pages = {10,775-10,786}
}
Wu, H. and Lees, J.M., (1999) Cartesian Parameterization of Anisotropic traveltime tomography, Geophysical Journal International. Vol. 137(1): pp. 64-80.
Abstract: A new method for inverting P-wave travel times for seismic anisotropy on a local scale is presented and tested. In this analysis, direction-dependent seismic velocity is represented by a second- or fourth-order Cartesian tensor, which is shown to be equivalent to decomposing a velocity surface using a basis set of Cartesian products of unit vectors. The new inversion method for P- and S-wave anisotropy from traveltime data is based on the tensor decomposition. The formulation is formally derived from a Taylor series expansion of a continuously extended, three-dimensional velocity function originally defined on the surface of the unit sphere. This approach allows us to solve a linear inversion instead of the standard nonlinear method. The resultant, linearized, fourth-order traveltime equation is similar to a previous, fourth-order result (Chapman and Pratt, 1992) although our representation offers a natural second-order simplification. Conventional isotropic traveltime tomography is a special case of our tensorial representation of velocities. P-wave velocity can be represented by a second-order tensor(matrix) as a first approximation, although S-wave traveltime tomography is intrinsically fourth order because of S-wave solution duality. Differences of isotropic and anisotropic parameterizations are investigated when velocity is represented by a matrix A.

The tradeoff between isotropy and anisotropy in practical tomography, which differs from the fundamental deficiency of anisotropic traveltime tomography (Mochizuki, 1997), is shown to be ~1, i.e., their effects are of the same order. We conclude that anisotropic considerations may be important in velocity inversions where ray coverage is less than optimal. On the other hand, when the ray directional coverage is complete and balanced, effects of anisotropy sum to zero and the isotropic part gives the result obtained from inverting for isotropic variations of velocity alone. Synthetic test datasets are inverted, demonstrating the effectiveness of the new inversion approach. When ray coverage is fairly complete, original anisotropy is well recovered, even with random noise introduced, although anisotropy ambiguities arise where ray coverage is limited. Random noise was found to be less important than ray directional coverage in anisotropic inversions.

BibTeX:
@article{Wu1999,
  author = {Wu, H. and Lees, J. M.},
  title = {Cartesian Parameterization of Anisotropic traveltime tomography},
  journal = {Geophysical Journal International},
  year = {1999},
  volume = {137},
  number = {1},
  pages = {64-80}
}
Wu, H. and Lees, J.M., (1999) Three-dimensional P and S wave velocity structures of the Coso Geothermal Area, California, from microseismic traveltime data, Journal of Geophysical Research. Vol. 104: pp. 13,217-13,233.
Abstract: High precision P and S wave travel times for 2104 microearthquakes with focus <6 km are used in a non-linear inversion to derive high-resolution three-dimensional compressional and shear velocity structures at the Coso Geothermal Area in eastern California. Block size for the inversion is 0.2 km horizontally and 0.5 km vertically and inversions are investigated in the upper 5 km of the geothermal area. Spatial resolution, calculated by synthetic modeling of a cross model at critical locations, is estimated to be 0.35 km for Vp and 0.5 km for Vs. Model uncertainties are estimated by a jackknife approach and simulation of random and associated picking errors. Low-velocity zones for both P and S waves are identified at geothermal production depths (1-3 km). A large, low Vp (-6%) zone is found at depth 2-2.5 km 2 km southwest of Sugarloaf Mountain where high attenuation has been previously reported. However, a general high-Vp zone is seen under Coso Hot Springs with a slightly low Vs zone, which is characteristic of fluid saturation. The overall distributions of Vp and Vs perturbations do not correlate. An isolated high-Vs (+9%) feature, about 2 km in diameter, is unambiguously seen 2 km due west of Sugarloaf extending from surface to depth. This feature is surrounded by a circular, low-Vs belt of ~1 km width. The surrounding belt is probably the cracked, high-porosity reservoir/conduit of geothermal fluid flow. In the 2 km southwest Sugarloaf region, we found low Vp and high Vs at geothermal production depths from 1 to 2.5 km. Combined with attenuation results, this may represent a hot, fluid-depleted center of magmatic activity.
BibTeX:
@article{Wu1999a,
  author = {Wu, H. and Lees, J. M.},
  title = {Three-dimensional P and S wave velocity structures of the Coso Geothermal Area, California, from microseismic traveltime data},
  journal = {Journal of Geophysical Research},
  year = {1999},
  volume = {104},
  pages = {13,217-13,233}
}
Feng, Q. and Lees, J.M., (1998) Microseismicity, stress, and fracture within the Coso geothermal Field, California, Tectonophysics. Vol. 289(1-3): pp. 221-238.
Abstract: Microseismicity, stress, and fracture in the Coso geothermal field are investigated using seismicity, focal mechanisms and stress analysis. Comparison of hypocenters of microearthquakes with locations of development wells indicates that microseismic activity has increased since the commencement of fluid injection and circulation. Microearthquakes in the geothermal field are proposed as indicators of shear fracturing associated with fluid injection and circulation along major pre-existing fractures. High seismicity zones are associated with the main fluid flow paths within the geothermal system. Calculated stress patterns from focal mechanisms provide direct evidence for the boundary between significantly different stress regimes within the Coso geothermal field.

Microseismicity in the Coso geothermal field is spatially but not temporally related to regional seismicity extending southeast of the field. The spatial distribution of these events defines a northwest-trending seismic-fracture zone, consistent with a previously defined northwest-striking zone. The abrupt decrease of seismicity below this fracture zone may provide seismic evidence for the existence of a brittle and ductile transition zone at shallow depth beneath the Coso geothermal field.

BibTeX:
@article{Feng1998,
  author = {Feng, Q and Lees, J. M.},
  title = {Microseismicity, stress, and fracture within the Coso geothermal Field, California},
  journal = {Tectonophysics},
  year = {1998},
  volume = {289},
  number = {1-3},
  pages = {221-238}
}
Johnson, J.B., Lees, J.M. and Gordeev, E., (1998) Degassing explosions at Karymsky Volcano, Kamchatka, Geophysical Research Letters. Vol. 25(21): pp. 3999-4042.
Abstract: During the summer of 1997, Karymsky Volcano produced summit explosions about six times each hour. Typical explosive episodes lasted between 30 seconds and three minutes, produced gas and ash columns several hundred meters high, and ejected some incandescent material. To better understand the physical source mechanisms responsible, we recorded hundreds of explosions with a three component broad-band seismometer and an infrasonic pressure sensor located 1650 meters from the active vent. Nearly every explosion is recorded as an emergent yet identical seismic wavelet which is followed 4.15 s later by an impulsive acoustic arrival. We interpret the signals as a near-surface gas volume burst which fractures the vent `plug,' lowers the lithostatic pressure within the magma column, and often induces further degassing. When degassing continues, it is generally manifested as either a series of regular one second `chugging' explosions, steady higher frequency `jetting', or a hybrid combination. We believe that the seismic signature for `chugs,' short duration harmonic tremor with integer overtones, is the result of repeated gas volume bursts at the vent. In contrast, seismograms for jetting are non-harmonic and contain higher frequencies. We believe that the competing degassing behaviors are influenced by the gas flux as well as the plug/conduit characteristics. We propose that a plug exists due to a viscosity gradient caused by volatile depletion in the upper conduit.
BibTeX:
@article{Johnson1998,
  author = {Johnson, J. B. and Lees, J. M. and Gordeev, E.},
  title = {Degassing explosions at Karymsky Volcano, Kamchatka},
  journal = {Geophysical Research Letters},
  year = {1998},
  volume = {25},
  number = {21},
  pages = {3999-4042}
}
Lees, J.M., (1998) Multiplet analysis at Coso Geothermal, Bulletin of the Seismological Society of America. Vol. 88(5): pp. 1127-1143.
Abstract: Microseismicity in the Coso geothermal field has been searched for seismic doublets, hypocenters co-located that appear to have identical waveforms. Using 1085 high quality events from 1993-1994, numerous doublets were identified, some occurring within minutes of each other. Hypocentral data were subdivided into spatial clusters to reduce the computational burden and multiple cross correlation pairs were evaluated, assigning scores to each pair. In one spatial cluster including 183 events (16471 pairs) yielded 96 high correlation (>0.6) paired events. Equivalence class analysis and cluster analysis routines were used to isolate potential multiplets. Among the 96 high correlation pairs 24 equivalence classes were isolated. While most of these are doublets, 8 classes included 3 or more cluster members and one class included 16 members. Relative locations are calculated using phase shifts between corresponding events. Detailed analysis of hypocenter relocations shows elongate, vertical structure aligned along ambient stress fields as measured by conventional focal mechanism analysis. Multiplet hypocenters are generally oriented sub-parallel to faults observed at the surface.
BibTeX:
@article{Lees1998,
  author = {Lees, J. M.},
  title = {Multiplet analysis at Coso Geothermal},
  journal = {Bulletin of the Seismological Society of America},
  year = {1998},
  volume = {88},
  number = {5},
  pages = {1127-1143}
}
Shalev, E. and Lees, J.M., (1998) Cubic B-Splines tomography at Loma Prieta, Bulletin of the Seismological Society of America. Vol. 88(1): pp. 256-269.
Abstract: A high resolution tomographic study, using cubic B-splines parameterization and employing a systematic approach to the choosing of appropriate damping and smoothing parameters, provided a three-dimensional P-wave velocity map of the Loma Prieta area. 11,977 high quality raypaths from 844 aftershocks of the 1989 Loma Prieta earthquake were used in the inversion. The velocity model exhibits a low-velocity feature between the San Andreas and Zayante-Vergeles faults in the top 10 km of the crust. This low-velocity feature is interpreted as a sedimentary unit exposed to the northwest and separated from the Salinian block by the Zayante-Vergeles fault. Below 10 km no consistent change is observed between the Salinian and the Franciscan blocks. There appears to be a high correlation of aftershock activity and localized high-velocity anomalies southeast of the Loma Prieta main shock. Whereas this anomaly may represent brittle rocks associated with a fault zone asperity that failed after the main shock, there is evidence to suggest it may be a body of serpentinite. The serpentinite exhibits high velocities and is potentially less competent than surrounding country rock, thus providing a sector along the fault more likely to be associated with many smaller earthquakes or creep behavior.
BibTeX:
@article{Shalev1998,
  author = {Shalev, E. and Lees, J. M.},
  title = {Cubic B-Splines tomography at Loma Prieta},
  journal = {Bulletin of the Seismological Society of America},
  year = {1998},
  volume = {88},
  number = {1},
  pages = {256-269}
}
Lees, J.M., (1997) Waveform and spatial clustering in high frequency seismograms, In Inverse Problems in Geophysical Applications. Philadelphia : pp. 109-130. Society for the Industrial Applications of Mathematics.
Abstract: Coherency between seismic signals recorded in tight arrays is investigated by reorganizing data via waveform cluster analysis. Multitaper spectral estimates and coherency functions suggest that pair-wise coherency between stations located only a few meters apart at Pinyon Flat, CA, is low above approximately 15 Hz. This observation is examined in detail by searching for clusters of similar waveforms using single link cluster analysis on suites of stations located in the 1990 high frequency, 60 station Pinyon Flat, CA, deployment. The analysis requires that noisy line spectra, introduced by instrument noise and shallow resonances be removed prior to analysis. Spectrum reshaping is applied to remove biases associated with smearing periodic signals in the frequency domain. Cluster analysis is performed on coherency and correlation scores derived from multi-taper analysis of 60 signals in the surface and borehole arrays. Cluster analysis shows that while simple pair-wise comparisons of coherency suggests that local scattering dominates the spectrum above 15 Hz, spatial clustering indicates that there is more structure in arriving waveforms than is apparent in the simple analysis. Stations that are clustered together spatially also cluster in coherency score, at frequencies as high as 50 Hz. By re-arranging seismograms according to associated clusters, signals along north-south and east-west arms of the array are separable at frequencies ranging from 5-50 Hz. These observations have important implications for the use of high frequency seismic spectra in determination of source and path effects for small magnitude, local earthquake analyses.
BibTeX:
@incollection{Lees1997,
  author = {Lees, Jonathan M.},
  editor = {Engl, H. W. and Louis, A. K. and Rundell, W.},
  title = {Waveform and spatial clustering in high frequency seismograms},
  booktitle = {Inverse Problems in Geophysical Applications},
  publisher = {Society for the Industrial Applications of Mathematics},
  year = {1997},
  pages = {109-130}
}
Wu, H. and Lees, J.M., (1997) Boundary conditions on a finite grid: Applications with pseudo-spectral wave propagation, Geophysics. Vol. 62(5): pp. 1544-1555.
Abstract: A new method for calculating boundary conditions at the free surface and along absorbing boundaries of a finite grid is presented. A finite, twice differentiable reduction function which achieves a 99% reduction over 3 wavelengths is proposed and tested. In the context of pseudospectral wave propagation, this implies a boundary layer of at least 6 grid nodes. The method is analyzed in one and two dimensions and the problems of waves impinging on corners are addressed. The reduction function recommended is gamma_R = alpha (1+cos(pi x))^2 where alpha is a parameter to be determined by optimization. Tests of the performance of the new method versus other common schemes are presented and analyzed. We provide a strategy for determining the optimal parameter in the reduction function. Synthetic Rayleigh waves are observed at the free surface of the simulation. Experiments with a vertical fault plane show the presence of direct, reflected, transmitted and head waves. The presence of head waves may be used to analyze velocity contrasts across fault zones.
BibTeX:
@article{Wu1997,
  author = {Wu, H. and Lees, J. M.},
  title = {Boundary conditions on a finite grid: Applications with pseudo-spectral wave propagation},
  journal = {Geophysics},
  year = {1997},
  volume = {62},
  number = {5},
  pages = {1544-1555}
}
Iversen, E.S. and Lees, J.M., (1996) A statistical Technique for validating velocity models, Bulletin of the Seismological Society of America. Vol. 86(6): pp. 1853-1862.
Abstract: This study investigates the use of a station influence statistic to identify velocity model shortcomings in the earthquake hypocenter location problem. Two groups of microearthquake events are examined. The first is a group of 81 events from the Mount St. Helens region which occurred between November 1987 and September 1991; the second, 110 well located events from the 1992 Joshua Tree aftershock sequence. We describe a method for validating a postulated earth model. Let lambda denote the hypocenter estimates that Geiger's method obtains. Systematically remove each station observation from the location problem and recompute the location estimate. Call this estimate lambda(i) when the i-th station is removed. For a single event define a station's influence (SI) as a weighted difference between lambda and lambda(i). Distributional summaries of SI statistics across events are used to identify model shortcomings: given a specified velocity model, SI distributions which are not homogeneous across stations provide evidence of model inadequacies and/or failures in the weighting scheme. We show that velocity model shortcomings detected using SI statistics for the Mount St. Helens sequence under a one-dimensional model appear to correlate with known physical anomalies; while SI distributions evaluated under a 3-dimensional model are more homogeneous and reflect a modest improvement over the 1-dimensional model. SI distributions provide evidence of model failure for the Joshua Tree sequence under a 1-dimensional model, but no evidence of failure under a 3-dimensional model. Finally, the weighting scheme's validity is verified for the Joshua Tree sequence under the 3-dimensional model.
BibTeX:
@article{Iversen1996,
  author = {Iversen, E. S. and Lees, Jonathan M.},
  title = {A statistical Technique for validating velocity models},
  journal = {Bulletin of the Seismological Society of America},
  year = {1996},
  volume = {86},
  number = {6},
  pages = {1853-1862}
}
Mann, M.E. and Lees, J.M., (1996) Robust estimation of background noise and signal detection in climatic time series, Climate Change. Vol. 33: pp. 409-445.
Abstract: We present a new technique for isolating climate signals in time series with a characteristic ``red'' noise background which arises from temporal persistence. This background is estimated by a ``robust'' procedure that, unlike conventional techniques, is largely unbiased by the presence of signals immersed in the noise. Making use of multiple-taper spectral analysis methods, the technique further provides for a distinction between purely harmonic (periodic) signals, and broader-band (``quasiperiodic'') signals. The effectiveness of our signal detection procedure is demonstrated with synthetic examples that simulate a variety of possible periodic and quasiperiodic signals immersed in red noise. We apply our methodology to historical climate and paleoclimate time series examples. Analysis of a $ 3 million year sediment core reveals significant periodic components at known astronomical forcing periodicities and a significant quasiperiodic 100 year peak. Analysis of a roughly 1500 year tree-ring reconstruction of Scandinavian summer temperatures suggests significant quasiperiodic signals on a near-century timescale, an interdecadal 16-18 year timescale, within the interannual El Nino/Southern Oscillation (ENSO) band, and on a quasibiennial timescale. Analysis of the 144 year record of Great Salt Lake monthly volume change reveals a significant broad band of significant interdecadal variability, ENSO-timescale peaks, an annual cycle and its harmonics. Focusing in detail on the historical esimated global-average surface temperature record, we find a highly significant secular trend relative to the estimated red noise background, and weakly significant quasiperiodic signals within the ENSO band. Decadal and quasibiennial signals are marginally significant in this series.
BibTeX:
@article{Mann1996,
  author = {Mann, Michael E. and Lees, Jonathan M.},
  title = {Robust estimation of background noise and signal detection in climatic time series},
  journal = {Climate Change},
  year = {1996},
  volume = {33},
  pages = {409-445}
}
Wu, H. and Lees, J.M., (1996) Attenuation structure of the Coso Geothermal area, California, from pulse width data of P-wave, Bulletin of the Seismological Society of America. Vol. 86: pp. 1574-1590.
Abstract: Pulse width data are used to invert for attenuation structure in the Coso geothermal area, California. The dataset consists of pulse width measurements of 838 microseismic events recorded on a seismic array of 16 downhole stations between August 1993 and March 1994. The quality factor Q correlates well with surface geology and surface heat flow observations. A broad region of low Q(~30-37) is located at 0.5-1.2 km depth below Devil's Kitchen, Nicol Prospects and Coso Hot Springs. A vertical, low Q(~36) in contrast with surrounding rock of 80) region is interpreted as a channel through which hydrothermal energy is transported from depth to the surface. The location of the channel is between stations S1 and S4 and its dimension is about 1 km. At the deep end of the channel, a large, broad body of low Q is also located at 3 km depth 2-4 km to the southwest of Nicol Prospects and Devil's Kitchen. Since it lies at the bottom of the target region and beyond the scope of seismicity, further research is needed to constrain its extent. Numerical modeling with a pseudospectral method is also done to investigate the applicability of the inversion scheme to fractured regions. A linear relationship between pulse width broadening and travel time is upheld, and the proportional constants are estimated.
BibTeX:
@article{Wu1996,
  author = {Wu, Huatao and Lees, J. M.},
  title = {Attenuation structure of the Coso Geothermal area, California, from pulse width data of P-wave},
  journal = {Bulletin of the Seismological Society of America},
  year = {1996},
  volume = {86},
  pages = {1574-1590}
}
Lees, J.M., (1995) Reshaping spectrum estimates by removing periodic noise: application to seismic spectral ratios, Geophysical Research Letters. Vol. 22(4): pp. 513-516.
Abstract: An automated method for removing line spectrum elements embedded in colored spectra is presented. Since smooth spectrum estimates are desired, line spectra tend to smear out over an effective smoothing window. This introduces a bias in the spectrum estimation that can seriously degrade determination of signal-to-noise ratios, spectral deconvolution or any other operation where spectrum shape is important in analysis. Multi-taper analysis provides a simple algorithmic approach to this problem and a simple method of determining where spectral peaks are both significant and contain signal power is suggested. While the method is completely general, an illustration of the technique applied to seismic signals is provided. Examples include estimation of signal-to-noise ratio at the high frequency array at, Pinyon Flat, CA. A comparison of noise spectra line segments and signal spectra line spectra reveals similarities associated with instrument noise and shallow resonances that are stimulated by incoming seismic signals. Identification and removal of the resonances provides a better means of estimating background noise spectrum for the purposes of modeling earthquake source spectra and path effects associated with attenuation.
BibTeX:
@article{Lees1995a,
  author = {Lees, Jonathan M.},
  title = {Reshaping spectrum estimates by removing periodic noise: application to seismic spectral ratios},
  journal = {Geophysical Research Letters},
  year = {1995},
  volume = {22},
  number = {4},
  pages = {513-516}
}
Lees, J.M., (1995) Xmap8: Three-dimensional GIS for geology and geophysics, Seismological Research Letters. Vol. 66(4): pp. 33-37.
Abstract: A new program for exploratory data analysis in three and four dimensions is presented and described. Interactive communication between diverse datasets is stressed as the main gestalt of the Xmap8 program. The primary kinds of data include point, vector, raster and wireframe data sets, in addition to specialized forms such as focal mechanisms and ellipsoidal information. The software includes methods for cutting cross sections at arbitrary angles, spinning objects in three space and animating time series of punctual data, such as hypocenter series and volcano eruptions. The program is written in POSIX compliant C using X-windows for Unix systems and has been ported to Linux. Free access, via the Internet, to executable binary, source code and documentation make this package an attractive alternative to expensive or unwieldy commercial options.
BibTeX:
@article{Lees1995c,
  author = {Lees, J. M.},
  title = {Xmap8: Three-dimensional GIS for geology and geophysics},
  journal = {Seismological Research Letters},
  year = {1995},
  volume = {66},
  number = {4},
  pages = {33-37},
  url = {http://srl.geoscienceworld.org/content/66/4/33.extract}
}
Lees, J.M. and Park, J., (1995) Multiple-taper spectral analysis: A stand-alone C-subroutine, Computers & Geosciences. Vol. 21(2): pp. 199-236.
Abstract: A simple set of subroutines in ANSI-C are presented for multiple taper
spectrum estimation. The multitaper approach provides an optimal
spectrum estimate by minimizing spectral leakage while reducing the
variance of the estimate by averaging orthogonal eigenspectrum
estimates. The orthogonal tapers are Slepian $n prolate functions used
as tapers on the windowed time series. Since the taper functions are
orthogonal, combining them to achieve an average spectrum does not
introduce spurious correlations as standard smoothed single-taper
estimates do. Furthermore, estimates of the degrees of freedom and
$F$-test values at each frequency provide diagnostics for determining
levels of confidence in narrow band (single frequency) periodicities.
The program provided is portable and has been tested on both Unix and
Macintosh systems.
BibTeX:
@article{Lees1995d,
  author = {Lees, J. M. and Park, Jeffrey},
  title = {Multiple-taper spectral analysis: A stand-alone C-subroutine},
  journal = {Computers & Geosciences},
  year = {1995},
  volume = {21},
  number = {2},
  pages = {199-236}
}
Ohmi, S. and Lees, J.M., (1995) Three-dimensional P and S-wave velocity structure below Unzen Volcano, Journal of Volcanology and Geothermal Research. Vol. 65: pp. 1-26.
Abstract: Unzen Volcano, located in south west Japan, erupted on November 17, 1990 after 198 years of dormancy and has been ejecting lava since May 20, 1991. In this paper, we present three-dimensional P and S-wave velocity variations below Unzen Volcano using 22,473 P-wave, and 14,349 S-wave arrival times from 3,457 local earthquakes recorded on a local network of 12 seismic stations. The model was parameterized by 24,000 approximately 2.0 km cubic blocks, targeting a volume of 120X100X20 km. A prominent low velocity anomaly (greater than 4% slowness perturbation) starting from 2.5 km to 5.0 km depth beneath the volcano is observed on both the P and S wave inversions. Below Unzen volcano slightly lower amplitude low velocity anomalies are further observed to a depth of 12.5 km for P and 7.5 km for S-waves. Shallow low velocity anomalies are observed below the Chijiwa Bay area (northwest of Unzen), and deeper anomalies are seen below Shimabara Bay (southwest off the Shimabara Peninsula). This northwest to southeast trending feature represents an elongated system of dykes which supply the shallow magma reservoirs.
BibTeX:
@article{Ohmi1995,
  author = {Ohmi, S. and Lees, J. M.},
  title = {Three-dimensional P and S-wave velocity structure below Unzen Volcano},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {1995},
  volume = {65},
  pages = {1-26}
}
Lees, J.M. and Lindley, G.T., (1994) Three-dimensional attenuation tomography at Loma Prieta: Inverting t* for Q, Journal of Geophysical Research. Vol. 99(B4): pp. 6843-6863.
Abstract: Three-dimensional Q-1 variations in the aftershock region of Loma Prieta are derived by tomographic inversion. The data set consists of over 4000 aftershock recordings at 22 PASSCAL (Program for Array Seismic Studies of the Continental Lithosphere) stations deployed after the Loma Prieta mainshock of 1989. Estimates of attenuation are determined from nonlinear least squares best fits to the Fourier amplitude spectrum of P and S wave arrivals. The linear attenuation inversion is accomplished by using three-dimensional velocity variations derived previously in nonlinear velocity inversions. Low Q is observed near the surface and Q generally increases with depth. The southwest side of the San Andreas fault exhibits lower Q than does the northeast side and this feature apparently extends to approximately 7 km depth. The fault zone, as determined by the dipping plane of aftershock activity, is characterized by slightly higher Qp and lower Qs, compared to regions immediately adjacent to the fault. These correlate with high- velocity anomalies associated with seismicity at depth. The results are in agreement with earlier observations regarding the association of high-velocity anomalies, seismicity, and fault zone asperities.
BibTeX:
@article{Lees1994a,
  author = {Lees, J. M. and Lindley, G. T.},
  title = {Three-dimensional attenuation tomography at Loma Prieta: Inverting t* for Q},
  journal = {Journal of Geophysical Research},
  year = {1994},
  volume = {99},
  number = {B4},
  pages = {6843-6863}
}
Fischer, R. and Lees, J.M., (1993) Shortest path raytracing with sparse graphs, Geophysics. Vol. 58(7): pp. 987-996.
Abstract: A technique for improving the efficiency of shortest path raytracing (SPR) [Moser, 1991] is presented. We analyze situations where SPR fails and provide quantitative measures to assess the performance of SPR raytracing with varying numbers of nodes. Our improvements include perturbing the ray at interfaces according to Snell's Law, and a method to find correct rays efficiently in regions of low velocity contrast. This approach allows the investigator to use fewer nodes in the calculation, thereby increasing the computational efficiency. In two dimensional cross-borehole experiments we find that with our improvements, we need only use 2/3 as many nodes, saving up to 60% in time. Savings should be even greater in three dimensions. These improvements make SPR more attractive for tomographic applications in three dimensions.
BibTeX:
@article{Fischer1993,
  author = {Fischer, R. and Lees, J. M.},
  title = {Shortest path raytracing with sparse graphs},
  journal = {Geophysics},
  year = {1993},
  volume = {58},
  number = {7},
  pages = {987-996}
}
Lees, J.M. and Nicholson, C., (1993) Three-dimensional tomography of the 1992 Southern California sequence: Constraints on dynamic earthquake ruptures?, Geology. Vol. 21(5): pp. 385-480.
Abstract: Tomographic inversion of P-wave arrival times from aftershocks of recent 1992 Southern California earthquakes is used to produce three-dimensional images of subsurface velocity. The preliminary 1992 dataset, augmented by the 1986 M 5.9 North Palm Springs sequence, consists of 6458 high-quality events recorded by the permanent regional networkÑproviding 76, 306 raypaths for inversion. The target area consisted of a 104 ´ 104 ´ 32 km3 volume divided into 52 ´ 52 ´ 10 rectilinear blocks. Significant velocity perturbations appear to correlate with rupture properties of recent major earthquakes. Preliminary results indicate a low-velocity anomaly separates the dynamic rupture of the M 6.5 Big Bear event from the M 7.4 Landers mainshock; a similar low-velocity region separates the M 6.1 Joshua Tree sequence from the Landers rupture. High-velocity anomalies occur at or near nucleation sites of all 4 recent mainshocks (North Palm Springs- Joshua Tree-Landers-Big Bear). A high-velocity anomaly is present along the San Andreas fault between 5 and 12 km depth through San Gorgonio Pass; this high-velocity area may define an asperity where stress is concentrated and where future large earthquakes may begin.
BibTeX:
@article{Lees1993a,
  author = {Lees, J. M. and Nicholson, C.},
  title = {Three-dimensional tomography of the 1992 Southern California sequence: Constraints on dynamic earthquake ruptures?},
  journal = {Geology},
  year = {1993},
  volume = {21},
  number = {5},
  pages = {385-480}
}
Ligdas, N. and Lees, J.M., (1993) Seismic velocity constraints in the Thessaloniki and Chalkidiki areas (Northern Greece) from a 3-D tomographic study, Tectonophysics. Vol. 228: pp. 97-121.
Abstract: Three-dimensional tomographic inversion of P-wave travel-time data is used to investigate the seismic velocity structure of the crust in Thessaloniki and Chalkidiki, N. Greece. Local earthquakes recorded by two networks operating in the area are used as natural seismic sources. Two different target volumes, defined on the surface by 39o50' - 41o50'N and 21o25' - 24o20'E, and 40o10' - 41o 00'N and 22o 45' - 23o 50'E, are investigated. The first dataset is recorded by 13 stations and the second by 29. The size of the blocks used to parameterize the areas is 10 x 10 km and 3 x 3 km in the horizontal, respectively, with varying depth layering. The major seismic velocity anomalies within the crust, obtained by the tomographic inversion, are resolved with a horizontal spatial resolution of about 20 km and 7 km for the first and second target volume, respectively. Our particular interest is to illuminate velocity anomalies and more detailed characteristics of the two main Neogene- Quaternary basins in this region (Vardar-Axios and Struma-Strymon). These basins are identified as low velocity features overlying relatively higher P-wave velocity structures in the lower crust. The complex Mygdonian area reveals a similar pattern of low-velocity basin overlying higher-velocity basement. Overall the velocity patterns correlate well with the location and strike of the main geological and tectonic units in the area, as well as the basic assumptions on basin development. This highlights the utility of local tomography to illuminate structural, tectonic and rheological properties within the crust.
BibTeX:
@article{Ligdas1993,
  author = {Ligdas, N. and Lees, J. M.},
  title = {Seismic velocity constraints in the Thessaloniki and Chalkidiki areas (Northern Greece) from a 3-D tomographic study},
  journal = {Tectonophysics},
  year = {1993},
  volume = {228},
  pages = {97-121}
}
Lees, J. and Ukawa, M., (1992) The South Fossa Magna, Japan, Revealed by High Resolution P and S-Wave Travel Time Tomography, Tectonophysics. Vol. 207: pp. 377-396.
Abstract: Detailed tomographic images of the collision zone between the northern edge of the Philippine Sea and the Eurasian plates reveal a high correlation between tectonic features inferred from seismicity and P and S-wave velocity structures. The linear tomographic inversion covered a 150x150x60 km region in the south Fossa Magna centered on the northern Izu Peninsula. Thirty three 3-component stations were located in the target region and 3823 high quality earthquakes were selected from the catalogues of the NRIESDP, giving rise to 53,593 P and 50,059 S-wave phase arrivals used in the inversion. The model was parameterized by 60x60x10 rectilinear blocks measuring 2.5 km per side horizontally and 5-10 km varying thicknesses in depth. Three-dimensional perturbations from the one dimensional, NRIESDP, layered model were derived by minimizing the squared misfit of the travel time residuals. Regularization was employed to reduce the effects of noisy data by constraining the 2-dimensional Laplacian of the model, within horizontal layers, to be small. The large sparse matrix was solved using the conjugate gradient algorithm LSQR. Reduction of misfit was 50% for the P-wave inversion and 57% for the S-wave inversion.

There is a high correlation between the P and S-wave inversions. Between 5-15 km depth both 3D images indicate low velocity anomalies trending northward in the west along the Fujikawa, from Suruga Bay to northwest of Mt. Fuji. North of Izu Peninsula there is a broad high velocity zone. These results correlate with Bouguer gravity anomalies previously observed. Starting at 5 km depth, a high velocity, northeast dipping anomaly beneath Sagami Bay is inferred to be the shallow subducting PHS plate. To the north, a high velocity zone in the vicinity of the East Yamanashi seismic swarm (15-32 km depth) is clearly observed on the P-wave inversion and to a lesser extent on the S-wave inversion. Because of the high seismicity in this region, we speculate this is the zone where the PHS plate is grazing the EUR plate to the northwest, and the high velocity anomaly represents a large scale asperity. To the west, below the Mt. Fuji area, a broad low velocity is observed between 15-32 km depth. We interpret the deep, low velocity structure, which exhibits little seismicity, to be either the subduction of the volcanic arc or a region of elevated temperature associated with hot rising magma. Near the swarm area on the eastern edge of Izu Peninsula, low velocity anomalies are observed near the surface on both P and S wave inversions. Along the eastern part of Izu Peninsula a prominent low velocity anomaly is observed between 10-20 km on the S-wave image but is less pronounced on the P-wave inversion.

BibTeX:
@article{Lees1992,
  author = {Lees, J.M. and Ukawa, M.},
  title = {The South Fossa Magna, Japan, Revealed by High Resolution P and S-Wave Travel Time Tomography},
  journal = {Tectonophysics},
  year = {1992},
  volume = {207},
  pages = {377-396}
}
Lees, J.M., (1992) The magma system of Mount St. Helens: Non-linear high resolution P-wave tomography, Journal of Volcanology and Geothermal Research. Vol. 53(1-4): pp. 103-116.
Abstract: High resolution, three-dimensional images of P-wave velocity anomalies below Mt. St. Helens, Washington, were derived using tomographic inversion. The model is a 27.5´21´20 km target volume parameterized by blocks .5 km per side. The area included 39 stations and 5454 local events leading to 35,475 rays used in the inversion. To diminish the effects of noisy data, the Laplacian was constrained to be small within horizontal layers, providing smoothing of the model. Non-linear effects were compensated for by iterating three-dimensional ray tracing (using pseudo-bending) between inversions and relocating earthquakes relative to the updated three-dimensional model. The structural differences between the linear and non-linear inversions appear to be insignificant, although the amplitudes of the anomalies are larger in the non-linear models. Results indicate a low- velocity anomaly (>7%), approximately 1 km in lateral extent, from 1.5 to 3 km depths. Between 3 and 6 km depth the anomaly appears to spread out. Below 6 km depth the low velocity feature changes to a higher velocity perturbation with lower velocity perturbations flanking around the perimeter of the volcano. The higher velocity material, which correlates with the higher seismicity at that depth, is interpreted as being a plug capping the low velocity magma chamber which begins below 9 km depth.
BibTeX:
@article{Lees1992a,
  author = {Lees, J. M.},
  title = {The magma system of Mount St. Helens: Non-linear high resolution P-wave tomography},
  journal = {Journal of Volcanology and Geothermal Research},
  year = {1992},
  volume = {53},
  number = {1-4},
  pages = {103-116}
}
Lees, J.M. and Shalev, E., (1992) On the stability of P-wave tomography at Loma Prieta: A comparison of parameterizations, linear and non-linear inversions, Bulletin of the Seismological Society of America. Vol. 82(4): pp. 1821-1839.
Abstract: We investigate the stability of tomographic analysis by comparing the results of two different methods of parameterizing the three-dimensional P-wave velocity variations in the vicinity of the 1989 Loma Prieta earthquake. A block inversion is implemented using 55x45x10 blocks of 1 km width and varying thickness to a depth of 25 km below the surface. Linear and non-linear analysis are presented. The non-linear analysis is achieved by iterating over three-dimensional raytracing and earthquake relocation relative to current three-dimensional models until solutions show only small improvements. A second parameterization is achieved by using cubic B-spline functions to span the space of the model which is rotated by 46.5û. Non-linear results are presented with several different starting models illustrating the robustness of the technique to the initial conditions. All the non-linear results produced essentially the same final model, which was structurally the same as the model obtained by linear analysis using a reasonable starting model.
BibTeX:
@article{Lees1992c,
  author = {Lees, J. M. and Shalev, E.},
  title = {On the stability of P-wave tomography at Loma Prieta: A comparison of parameterizations, linear and non-linear inversions},
  journal = {Bulletin of the Seismological Society of America},
  year = {1992},
  volume = {82},
  number = {4},
  pages = {1821-1839}
}
Nicholson, C. and Lees, J.M., (1992) Travel-time tomography in the northern Coachella Valley using aftershocks of the 1986 ML 5.9 North Palm Springs earthquake, Geophysical Research Letters. Vol. 19(1): pp. 1-4.
Abstract: Tomographic inversion is applied to delay times from aftershocks of the 1986 ML 5.9 North Palm Springs (NPS) earthquake to image 3-D velocity variations within the northern Coachella Valley. P-wave arrival times from 1074 earthquakes, with depths ranging from 3 to 20 km, were used as sources recorded by 12 portable and 4 permanent stations. Preliminary results show well-defined high- and low-velocity anomalies (2-7%) that correlate with the rupture distribution of the 1986 mainshock. At depths less than 8 km, a low-velocity anomaly predominates between the two NE-dipping Banning and Mission Creek faults. From 8 to 12 km, where the NPS mainshock and most of the aftershocks occur, a high-velocity anomaly is observed. This high-velocity feature is interpreted as imaging the asperity responsible for the 1986 rupture; and suggests that velocity information may help to define important elements, such as asperities, that control fault rupture, and thus, may help to predict the location and size of future events.
BibTeX:
@article{Nicholson1992,
  author = {Nicholson, C. and Lees, J. M.},
  title = {Travel-time tomography in the northern Coachella Valley using aftershocks of the 1986 ML 5.9 North Palm Springs earthquake},
  journal = {Geophysical Research Letters},
  year = {1992},
  volume = {19},
  number = {1},
  pages = {1-4}
}
Lees, J. and Vandecar, J., (1991) Seismic tomography constrained by Bouguer gravity anomalies: Applications in Western Washington, Pure and Applied Geophysics. Vol. 135(1): pp. 31-52.
Abstract: Tomographic inversions for velocity variations in western Washington indicate a high correlation with surface geology and geophysical measurements, including gravity observations. By assuming a simple linear relationship between density and velocity (Birch's law) it is possible to calculate the gravity field predicted from the velocity perturbations obtained by local tomographic inversion. While the predicted gravity matches observations in parts of the model the overall correlation is not satisfactory. In this paper we suggest a method of constraining the tomographic inversion to fit the gravity observations simultaneously with the seismic travel time data. The method is shown to work well with synthetic data in 3 dimensions where the assumption of Birch's Law holds strictly. If the sources of the gravity anomalies are assumed to be spatially localized, integration can be carried out over a relatively small volume below the observation points and sparse matrix techniques can be applied. We have applied the constrained inversion method to western Washington using 4,387 shallow earthquakes, to depths of 40.0 km, (36,865 raypaths) covering a 150´250 km region and found that the gravitational constraints may be satisfied with minor effect on the degree of misfit to the seismic data.
BibTeX:
@article{Lees1991,
  author = {Lees, J.M. and Vandecar, J.C.},
  title = {Seismic tomography constrained by Bouguer gravity anomalies: Applications in Western Washington},
  journal = {Pure and Applied Geophysics},
  year = {1991},
  volume = {135},
  number = {1},
  pages = {31-52}
}
Lees, J.M. and Crosson, R., (1991) Bayesian ART versus conjugate gradient methods in tomographic seismic imaging: An application at Mount St. Helens, Washington, In Spatial Statistics and Imaging. Hayward, CA Vol. 20: pp. 186-208. Inst. of Math. Statistics.
Abstract: We compare two approaches for solving the large, sparse linear systems that arise in tomographic velocity inversion problems. When noise is present in the data, the system typically is inconsistent and quasi-overdetermined, and some form of regularization must be implemented to avoid the strong, undesired influence of small singular values dominating the solutions. First, a Bayesian ART (Algebraic Reconstruction Technique) algorithm is applied to the system where we solve for a set of model parameters and residuals simultaneously. Careful choice of relaxation parameters and smoothing filters insures convergence and acceptable results. This approach avoids the undesirable effects of implicit row weighting inherent in simple ART or SIRT (Simultaneous Iterative Reconstruction Technique) applications. Second, a conjugate gradient approach is implemented via algorithm LSQR where regularization is achieved by augmenting the system with additional constraint equations which minimize the roughness of the model. Specifically, applied to a 3-dimensional tomographic inversion we constrain the second derivative (the Laplacian) to be zero within horizontal layers. Comparison on synthetic data reveals that these techniques produce nearly equivalent results. By applying these methods to local earthquake data in the vicinity of Mount St. Helens, Washington, we have produced a 3-dimensional, laterally varying velocity structure in the top 40 km of the crust which correlates well with known geological and geophysical features and delineates possible accumulation of magma beneath the crater.
BibTeX:
@incollection{Lees1991a,
  author = {Lees, J. M. and Crosson, R.S.},
  editor = {Possolo, A.},
  title = {Bayesian ART versus conjugate gradient methods in tomographic seismic imaging: An application at Mount St. Helens, Washington},
  booktitle = {Spatial Statistics and Imaging},
  publisher = {Inst. of Math. Statistics},
  year = {1991},
  volume = {20},
  pages = {186-208}
}
Lees, J.M., (1990) Tomographic P-wave velocity images of the Loma Prieta earthquake asperity, Geophysical Research Letters. Vol. 17: pp. 1433-1436.
Abstract: Tomographic inversion is applied to delay times from local earthquakes to image 3-D velocity variations surrounding the main rupture of the 1989 Loma Prieta earthquake. The 55x45 square km region is represented by blocks of 1 km per side laterally and by 8 layers of varying thickness to 18 km depth. High quality P-wave arrival times recorded on the USGS CALNET array from 549 crustal earthquakes with depths of 0 to 25 km were used as sources. Preliminary results several velocity variations (5-12%) that correlate with specific characteristics of the 1989 rupture. These include prominent high-velocity anomalies near the mainshock hypocenter and prominent low-velocity anomalies where the dip of the San Andreas fault appears to change significantly. The termination of prominent low velocity features existing primarily in the hanging wall to depths of 7-9 km, correlates with the top of the rupture zone. High-velocity variations along the fault dominate where aftershock activity is high. The high velocity anomaly located at depth along the fault is interpreted as imaging the asperity on which the Loma Prieta earthquake occurred.
BibTeX:
@article{Lees1990,
  author = {Lees, J. M.},
  title = {Tomographic P-wave velocity images of the Loma Prieta earthquake asperity},
  journal = {Geophysical Research Letters},
  year = {1990},
  volume = {17},
  pages = {1433-1436}
}
Lees, J.M. and Crosson, R.S., (1990) Tomographic imaging of local earthquake delay times for 3-D velocity variation in western Washington, Journal of Geophysical Research. Vol. 95(B4): pp. 4763-4776.
Abstract: Tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations in the Puget Sound region of Western Washington. The 37,500 square km region is represented by nearly cubic blocks of 5 km per side. P-wave arrival time observations from 4,387 crustal earthquakes, with depths of 0 to 40 km, were used as sources producing 36,865 rays covering the target region. A conjugate gradient method (LSQR) is used to invert the large, sparse system of equations. To diminish the effects of noisy data, the Laplacian is constrained to be zero within horizontal layers, providing smoothing of the model. The resolution is estimated by calculating impulse responses at blocks of interest and estimates of standard errors are calculated by the jackknife statistical procedure. Results of the inversion are correlated with some known geologic features and independent geophysical measurements. High P-wave velocities along the eastern flank of the Olympic Peninsula are interpreted to reflect the subsurface extension of Crescent terrane. Low velocities beneath the Puget Sound further to the east are inferred to reflect thick sediment accumulations. The Crescent terrane appears to extend beneath Puget Sound, consistent with its interpretation as a major accretionary unit. In the southern Puget Sound basin, high velocity anomalies at depths of 10-20 km are interpreted as Crescent terrane and are correlated with a region of low seismicity. Near Mt. Rainier, high velocity anomalies may reflect buried plutons.
BibTeX:
@article{Lees1990a,
  author = {Lees, J. M. and Crosson, R. S.},
  title = {Tomographic imaging of local earthquake delay times for 3-D velocity variation in western Washington},
  journal = {Journal of Geophysical Research},
  year = {1990},
  volume = {95},
  number = {B4},
  pages = {4763-4776}
}
Lees, J.M. and Malin, P.E., (1990) Tomographic images of P-Wave velocity variation at Parkfield, California, Journal of Geophysical Research. Vol. 95(B13): pp. 21,793-21,804.
Abstract: Tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations near Parkfield, California. The 25 ´ 20 square km region is represented by nearly cubic blocks of 0.5 km per side. Arrival times of P waves from 551 local earthquakes, with depths of 0 to 15 km, were used as sources producing 3135 rays covering the target region. The data were recorded on low-noise downhole seismographs. A conjugate gradient method is used to invert the resulting sparse system of simultaneous equations. To diminish the effects of noisy data, the Laplacian of the model parameters is constrained to be small within horizontal layers, providing smoothing of the model. The resolution of the model is estimated by calculating point spread functions at blocks of interest. Estimates of standard errors of the model parameters are calculated by the jackknife statistical procedure. The results of the inversion show correlation with some of the local geological and geophysical features. Station corrections removed the long- wavelength anomaly associated with the contrast of the Salinian block southwest of the San Andreas fault versus the Franciscan to the northeast. A velocity low located a few kilometers northwest of Parkfield (depth 2.5-3.5 km), appears to lie along the gradient of the large Bouguer gravity anomaly associated with the Parkfield syncline. The south- southeastward extension of the low velocities may relate to reflections observed on the Parkfield, Consortium for Continental Reflection Profiling (COCORP) lines. We speculate on the geological meaning of these features and interpret them either as part of the local strike slip tectonics or a shallow crustal detachment. The correlation of higher-velocity features and seismic activity may indicate that earthquakes are occurring in more competent zones while aseismic slip takes place in zones of lower-velocity, less competent rocks.
BibTeX:
@article{Lees1990b,
  author = {Lees, J. M. and Malin, P. E.},
  title = {Tomographic images of P-Wave velocity variation at Parkfield, California},
  journal = {Journal of Geophysical Research},
  year = {1990},
  volume = {95},
  number = {B13},
  pages = {21,793-21,804}
}
Lees, J.M. and Crosson, R.S., (1989) Tomographic inversion for three-dimensional velocity structure at Mount St. Helens using earthquake data, Journal of Geophysical Research. Vol. 94(B5): pp. 5716-5728.
Abstract: Tomographic inversion is applied to 17,659 P phase observations at 21 stations from 2023 earthquakes in the vicinity of Mount St. Helens to study the three-dimensional velocity structure. Block size for the inversion is 2 km horizontally and 2 km or more vertically. Locations of hypocenters are assumed known and are based on a reference one-dimensional, layered velocity structure. A conjugate gradient technique (LSQR) is used to invert the large sparse system of equations, augmented by regularization with a Laplacian roughening matrix. Resolution is estimated by computing the impulse response of the inversion for various critical locations, and uncertainties of the estimates are determined by a jackknife approach. The results of the inversion show a remarkable correlation with known geological and geophysical features. The Spirit Lake and Spud Mt. plutons are characterized by high-velocity regions extending to approximately 9 km depth. The St. Helens seismic zone, a band of diffuse seismicity extending NNW from the volcano is evident as a prominent low-velocity lineation. The change in character of the velocity anomalies south of St. Helens corresponds well with the near cessation of seismic activity there. A low-velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations.
BibTeX:
@article{Lees1989a,
  author = {Lees, J. M. and Crosson, R. S.},
  title = {Tomographic inversion for three-dimensional velocity structure at Mount St. Helens using earthquake data},
  journal = {Journal of Geophysical Research},
  year = {1989},
  volume = {94},
  number = {B5},
  pages = {5716-5728}
}