# Jonathan M. Lees

## Professor of Geophysics, Seismology, Volcanology

### Crustal seismology, seismic imaging, tomography, volcano seismology, geophysical signal analysis

Following are abstracts of published journal articles.
Lees, J. M. (2007), Tomography of Crustal Magma Bodies: Implications For Magmatic Systems, J. Volc. Geoth. Res., 167, 37-56

Lees, J.M., SeisR & FocR: Earthquake and Seismic Analysis in R, Proceedings of the 2007 useR! Conference, August 8?10. Iowa State University, Ames, Iowa, 2007, http://user2007.org/program/presentations/lees.pdf.

Ruppert, N. A., J. M. Lees, and N. P. Kozyreva (2007), Seismicity, Earthquakes and Structure along the Alaska-Aleutian and Kamchatka-Kurile Subduction zones: A Review, in Volcanism and Subduction: The Kamchatka Region, edited by J. Eichelberger, E. Gordeev, M. Kasahara, P. Izbekov and J. M. Lees, pp. 129-144, American Geophysical Union, Washington, D.C.

Lees, J. M., J. VanDecar, E. Gordeev, A. Ozerov, M. Brandon, J. Park, and V. Levin (2007), Three Dimensional Images of the Kamchatka-Pacific Plate Cusp, in Volcanism and Subduction: The Kamchatka Region, edited by J. Eichelberger, E. Gordeev, M. Kasahara, P. Izbekov and J. M. Lees, pp. 65-75, American Geophysical Union, Washington, D.C.

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, International Conference on Intelligent Information Hiding and Multimedia, Volume iih-msp, p. 579-582.

Werner-Allen, G., Lorincz, K., Johnson, J.B., Lees, J.M. and Welsh, M., 2006. Fidelity and Yield in a Volcano Monitoring Sensor Network, Proc. of the 7th Symposium on Operating Systems Design and Implementation (OSDI '06), Seattle, WA, USA, pp. 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: IEEE Internet Computing, Special Issue on Data Driven Applications in Sensor Networks, v. 10, p. 18-25 doi:http://doi.ieeecomputersociety.org/10.1109/MIC.2006.26.

Chung, T.W., M.-H. Noh, J.-K. Kim, Y.-K. Park, H.-J. Yoo, and J.M. Lees, A Study of the Regional Variation of Low Frequency QLg-1 Around The Korean Peninsula, Bull. Seism. Soc. Am., in Press 2008.

Lees, J. M., and M. Ruiz, Non-linear Explosion Tremor at Sangay, Volcano, Ecuador, Journal of Volcanology and Geothermal Research in Press, 2008.

Lees, J. M., J. B. Johnson, M. Ruiz, L. Troncoso, M. Welsh, Reventador Volcano 2005: Eruptive Activity Inferred from Seismo-Acoustic Observation Journal of Volcanology and Geothermal Research in Press, 2008.

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, v. 33, L06308, doi:10.1029/2005GL025515.

Werner-Allen, G., K. Lorincz, M. Welsh, M. Ruiz, J. Lees, J. Johnson, O. Marcillo, (2006) Deploying a wireless sensor network on an active volcano: IEEE Internet Computing, Special Issue on Data Driven Applications in Sensor Networks, v. 10, p. 18-25 doi:http://doi.ieeecomputersociety.org/10.1109/MIC.2006.26.

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, v. 32, p. L21317 doi:10.1029/2005GL023551.

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, v. 32 doi:10.1029/2005GL022847.

Ruiz, M, J. M. Lees, and J. B. Johnson (2005), Source constraints of Tungurahua explosion events, Accepted in Bulletin of Volcanology

Werner-Allen, G., J. Johnson, M. Ruiz, J. Lees, M. Welsh Monitoring Volcanic Eruptions with a Wireless Sensor Network. in Proc. Second European Workshop on Wireless Sensor Networks (EWSN'05 #1568945184). 2005.

Tang, C., Rial, J.A., Lees, J.M., and Thompson, E., 2005, Seismic Imaging of the geothermal field at Krafla, Iceland, Proceedings, Thirteenth Workshop on Geothermal Reservoir Engineering: Stanford University, Stanford, CA, p. SGP-TR-176.

Shalev, E., and J.M. Lees (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, edited by R.E. Wells, pp. 127-142, U.S. Geological Survey, Reston, VA.

Davaille, A., and J.M. Lees, (2004) Thermal modeling of subducted plates: tear and hot spot at the Kamchatka corner, Earth Planet. Sci. Letts., 226 (3-4), 293-304, DOI: 10.1016/j.epsl.2004.07.024.

McGreger, A.D., and J.M. Lees, Vent Discrimination At Stromboli Volcano, Italy, J. Volc. Geoth. Res., 137 (1-3), 169-185, DOI: 10.1016/j.jvolgeores.2004.05.007 2004.

Lees, J. M., Gordeev, E. I. & Ripepe, M. (2004) Explosions and periodic tremor at Karymsky volcano, Kamchatka, Russia. Geophysical Journal International 0 (0), -. doi: 10.1111/j.1365-246X.2004.02239.x

Lees, J.M., 2004. Scattering from a fault interface in the Coso geothermal field. Journal of Volcanology and Geothermal Research, 130(1-2): 61-75.

Ozerov, A., I. Ispolatov, and J. Lees, Modeling Strombolian eruptions of Karymsky volcano, Kamchatka, Russia, J. Volc. Geoth. Res., 122 (3-4), 265-280, 2003.

Levin, V., J. Park, M. Brandon, J.M. Lees, V. Peyton, E. Gordeev, and A. Ozerov, Crust and upper mantle of Kamchatka from teleseismic receiver functions, in Tectonophysics, edited by I.M. Artemieva, H. Thybo, W.D. Mooney, and E. Perchuc, pp. 233-265, Elsevier, Amsterdam, 2002.

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 near the Kamchatka plate corner, in Stein, S., and Freymueller, J., eds., Plate Boundary Zones, Volume 30: AGU Geodynamics Series: Washington DC, AGU, p. 295-324.

Johnson, J.B., R.C. Aster, M.C. Ruiz, S.D. Malone, P.J. McChesney, J.M. Lees, and P.R. Kyle, Interpretation and utility of infrasonic records from erupting volcanoes, J. Volc. Geoth. Res., 121 (1-2), 15-63, 2003.

Lees, J.M., Three-Dimensional Anatomy of a Geothermal Field, in Geologic Evolution of the Central Mojave Desert and Southern Basin and Range, edited by A. Glazner, J.D. Walker, and J.M. Bartley, pp. 259-276, Geological Society of America, Boulder, CO., 2002.

Bhattacharyya, J., and J.M. Lees, Seismicity, stress and triggering in the Coso/Indian Wells Valley region, in Geologic Evolution of the Central Mojave Desert and Southern Basin and Range, edited by A.G. J., D. Walker, and J.M. Bartley, pp. 243-258, Geological Society of America, 2002.

Lees, J. M., M. Brandon, J. Park, V. Levin, A. Ozerov, E. Gordeev, (2001) Kamchatka: Edge of the Plate, Iris News Letter, 2001(1), 17-19.

Peyton, V., V. Levin, J. Park, M. Brandon, J. Lees, E. Gordeev, and A. Ozerov (2001) Mantle Flow at a Slab Edge: Seismic Anisotropy in the Kamchatka Region, Geophys. Res. Letts. 28(2), 379-382.

Yogodzinski, G.M., J.M. Lees, T.G. Churikova, F. Dorendorf, G. Woerner, and O.N. Volynets, (2001): Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges, Nature, 409, 500-504.

J. M. Lees (2000), Geotouch: Software for Three and Four-Dimensional GIS in the Earth Sciences, 26(7) 751-761. Computers & Geosciences

Johnson, J. B., and J. M. Lees (2000), Plugs and Chugs - Strombolian activity at Karymsky, Russia, and Sangay, Ecuador, J. Volc. Geotherm. Res. 101, 67-82. J. Volc. Geotherm. Res.

Lees, J. M., and H. Wu, (2000) Poisson's ratio and porosity at Coso Geothermal Area, California, J. Volc. Geotherm. Res. 95(1-4) , 157-173.

Lees, J. M., and H. Wu, (1999), P-wave anisotropy, stress, and crack distribution at Coso Geothermal Field, California, J. Geophys. Res. 104(8), 17,955-17,973.

Bhattacharyya, J., S. Grosse, J. M. Lees and M. Hasting (1999): Recent earthquake sequences at Coso: evidence for conjugate faulting and stress loading near a geothermal field, Bull. Seismol. Soc. Am. 89(3), 785-795

Hough, H. E., J. M. Lees and F. Monastero (1999)Attenuation and source properties at the Coso Geothermal Area, California, Bull. Seismol. Soc. Am.89(6), 1606-1619.

Moran,S. C., J. M. Lees, and S. D. Malone, (1999): P wave crustal velocity structure in the greater Mount Rainier area from local earthquake tomography, J. Geophys. Res. 104 (10), 10,775-10,786.

Wu, H., and J. M. Lees, Three-dimensional P and S wave velocity structures of the Coso Geothermal Area, California, from microseismic travel time data, J. Geophys. Res. 104, 13,217-13,233

Wu, H., and J. M. Lees, Cartesian Parameterization of Anisotropic Traveltime Tomography, Geophysical Journal International 137(1) 64-80

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.

Johnson, J. B., and J. M. Lees, Degassing Explosions at Karymsky Volcano, Kamchatka, Geophysical Research Letters25(21), 3999-4002

Lees, J. M., Multiplet analysis at Coso Geothermal, Bull. Seismol. Soc. Am. 88(5) 1127-1143.

Feng, Q., and J. M. Lees, (1998) Microseismicity, Stress, and fracture within the Coso geothermal Field, California. Tectonophysics, 289, 221-238.

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.

Lees, J. M., (1997): Waveform and Spatial Clustering in High Frequency Seismograms: in Inverse Problems in Geophysical Applications, Society for the Industrial Applications of Mathematics: 109-130

Shalev, E., and J. M. Lees, (1998): Cubic B-Ssplines Tomography at Loma Prieta, Bull. Seismol. Soc. Am. 88(1), 256-269 ,

Iversen, E. S., and J. M. Lees (1996), A statistical technique for validating velocity models, Bull. Seismol. Soc. Am. 86(6), 1853-1862

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.
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Wu, H. and J. M. Lees (1996). Attenuation Structure of Coso Geothermal Area, California, from P Wave Pulse Widths, Bull. Seismol. Soc. Am., 86, 1574-1590

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.
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Wu, H. and J. M. Lees (1996). Boundary Conditions on a Finite Grid: Applications with Pseudo-spectral Wave Propagation, Geophysics, 62(5), 1544-1557

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.
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Lees, J. M. and G. T. Lindley (1994): Three-dimensional Attenuation Tomography at Loma Prieta: Inverting t* for Q, J. Geophys. Res., 99(B4), 6843-6863.

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.
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Lees, J. M. and J. C. Vandecar (1991): Seismic tomography constrained by Bouguer gravity anomalies: Applications in Western Washington, Pure Appl. Geophys., 135(1), 31-52.

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.
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Lees, J. M. and C. Nicholson (1993): Three-dimensional tomography of the 1992 Southern California sequence: Constraints on dynamic earthquake ruptures?, Geology, 21(5), 385-480.

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.
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Lees, J. M. (1995): Reshaping spectrum estimates by removing periodic noise: application to seismic spectral ratios, Geophys. Res. Lett., 22(4), 513-516.

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.
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Lees, J. M. (1990): Tomographic P-wave velocity images of the Loma Prieta earthquake asperity, Geophys. Res. Lett., 17, 1433-1436.

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.
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Lees, J. M. and E. Shalev (1992): On the Stability of P-wave Tomography at Loma Prieta: A Comparison of Parameterizations, Linear and Non-liner Inversions, Bull. Seismol. Soc. Am., 82(4), 1821-1839.

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.
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Lees, J. M. (1992): The magma system of Mount St. Helens: Non-linear high resolution P-wave tomography, J. Volc. Geoth. Res., 53(1-4), 103-116.

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.
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Lees, J. M. and R. S. Crosson (1990): Tomographic imaging of local earthquake delay times for 3-D velocity variation in western Washington, J. Geophys. Res., 95(B4), 4763-4776.

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.
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Ligdas, N. and J. M. Lees (1993). Seismic velocity constraints in the Thessaloniki and Chalkidiki areas (Northern Greece) from a 3-D tomographic study: Tectonophysics: 228, 97-121.

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.
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Nicholson, C. and J. M. Lees (1992): Travel-time tomography in the northern Coachella Valley using aftershocks of the 1986 ML 5.9 North Palm Springs earthquake, Geophys. Res. Lett., 19(1), 1-4.

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.
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Ohmi, S. and J. M. Lees (1995): Three-dimensional P and S-wave velocity structure below Unzen Volcano, J. Volc. Geoth. Res., 65, 1-26.

Fischer, R. and J. M. Lees (1993): Shortest path raytracing with sparse graphs, Geophysics, 58(7), 987-996.

Lees, J. M. and R. S. Crosson (1989): Tomographic inversion for three-dimensional velocity structure at Mount St. Helens using earthquake data, J. Geophys. Res., 94(B5), 5716-5728.

Lees, J. M. and P. E. Malin (1990): Tomographic images of P-Wave velocity variation at Parkfield, California, J. Geophys. Res., 95(B13), 21793-21804.

Lees, J. M. and M. Ukawa (1992): The South Fossa Magna, Japan, Revealed by High Resolution P and S-Wave Travel Time Tomography, Tectonophysics, 207, 377-396.

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.

Lees, J. M. and R. S. Crosson (1991): Bayesian ART versus conjugate gradient methods in tomographic seismic imaging: An application at Mount St. Helens, Washington, in Spatial Statistics and Imaging, editted by A. Possolo, Inst. of Math. Statistics, 186-208.

Lees, J. M. , 1998, Multiplet analysis at Coso Geothermal, Bull. Seismol. Soc. Am., 88(5), 1127-1143

Chung, T.W., J. M. Lees, and S. Yoon, 2008, Seismic data analysis using R, Mulli-Tamsa, 11, 378-384 (Korean with English Abstract)

Chung, T.W. and J. M. Lees, 2008, Preparation of topographic maps based on the R package, Mulli-Tamsa, 11, 373-378 (Korean with English Abstract)

  Prof. Jonathan M. LeesDepartment of Geological SciencesCB #3315, Mitchell HallUniversity of North CarolinaChapel Hill, NC  27599-3315(919) 962-0678FAX (919) 966-4519