Crustal Seismology
Seismic Imaging
Tomography
Volcano Seismology
Signal analysis
Jonathan M. Lees

Jonathan M. Lees

Professor, Geophysics

Department of Geological Sciences

University of North Carolina at Chapel Hill

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

Research Program:


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My main research program is directed at geophysical characterization of active fault zones, active volcanoes and active geothermal systems. Primary focus is given to regions where the dynamics of the subsurface can be explored structurally, geologically and physically. These geologic areas have immense impact on our environment, from the perspective of geologic hazards and associated social implications and in terms of long term global change. My main contribution has been in the area of seismic tomographic imaging, the three-dimensional distribution of velocity, attenuation, and anisotropy of seismic waves in complex media. These investigations are aimed at delineating three-dimensional structures which control the evolution of geologic systems: how do rocks interact, what roles do fluids play, how do they move through the system and how can we observe these variations. I am especially interested in coordinating joint analyses of diverse data sets including gravity, aeromagnetic, heat flow, stress distribution and geology to quantitatively constrain dynamic models of the subsurface. To this end I have made considerable contributions to the methodology of seismic inverse theory, developing and applying new techniques for extracting and interpreting information from complex data sets.

The San Andreas Fault system is a target of great importance because of its social impact, high rates of seismicity and dense monitoring systems. Tomographic analysis along the strike slip faults of Southern California has provided spatial mappings of fault asperities and their control on seismicity at Parkfield, Loma Prieta, Joshua Tree-Landers, and North Palm Springs. I was the first to associate high velocity and dynamic rupture at San Andreas asperities in regions where large main-shock events occurred. These patterns can be used to estimate future sites of rupture along the fault.

Besides being objects of great curiosity, volcanoes are important because they represent geologic windows on processes that occur deep in the crust and upper mantle. Volcanoes I have investigated include Mount St. Helens, Washington, Mount Rainier, Unzen volcano, Japan, Mount Fuji, and, more recently, Karymsky and Kliuchevskoi Volcanoes in Kamchatka, Russia and Sangay Volcano, Ecuador. In these studies I have delineated detailed structures of the conduit and magma systems that feed explosive, potentially destructive, andesitic volcanoes. The tomographic images provide unique constraints on the magma storage systems, which are essential for forecasting future eruptions. In some cases I have expanded the scale of my analyses to illuminate the association of volcanism and structure in the Cascades of western Washington and the collision tectonics of the South Fossa Magna, Japan. In 1998-1999 I extended this work in Kamchatka and Ecuador, where I am currently investigating the relationship of volcanism, slab geometry, and upper mantle fluid flow.

The Coso Geothermal field of southeastern California provides an exceptional laboratory for determining the relationship between hydrothermal fluid flow, tectonic and induced seismicity, and heat sources in the extensional regime of the Basin and Range. Our detailed, three-dimensional analysis of seismic velocity, attenuation, anisotropy and stress is being used as an exploration tool for locating new sources of geothermal energy. Derivation of these models will allow us to propagate waves on a local scale so that we can perform full waveform inversions at high frequency. Our present method of waveform propagation is called the pseudo-spectral method and we have derived optimal boundary conditions for this purpose. In Iceland we have extended our research program at Krafla and Hengil to deepen our understanding of geothermal structures.

My most recent project is a major field effort in Kamchatka, Russia, to study upper mantle flow as it relates to the edge of the subducting Pacific Plate (see book on Kamchatka). Flow around the edge of the Pacific Plate may have a significant influence on the nature of seismicity and volcanism in Kamchatka, the location of the most intense magmatism in the Pacific Rim. Our working hypothesis, that the exposed edge of the Pacific Plate has significant implications for tectonics and volcanism in the western Aleutians and northern Kamchatka Arc, is currently being tested by a large-scale seismic array spanning the Bering Kamchatka collision zone. The project is multidisciplinary, including associated investigations into geochemistry, fluid dynamics, and geodesy. Preliminary results suggest that our model is correct. To study the dynamics of exploding volcanoes, we have installed short term seismo-acoustic arrays on Karymsky Volcano, Russia, monitoring harmonic tremor and its association with active explosions and magma extrusion. Karymsky caldera is one of the most active volcanic regions of the world. Unusual seismo-acoustic signals at Karymsky provide constraints on the explosion dynamics in the volcano conduit that ultimately control periodic eruptions. In 1999 I organized a multidisciplinary field effort to observe seismo-acoustic, gravity, GPS, tilt, COSPEC, and audio-visual data associated with the Strombolian activity. Results from these experiments have recently been used to develop a new model describing this kind of harmonic tremor based on sequences of discrete explosions near the volcano vent. The long-term goal of my program is heading towards true, four-dimensional analysis, where temporal variations are included, potentially in near real time. By incorporating time into active processes we will be able to observe physical changes in earth processes while they occur. This will allow us to connect the dynamic evolution of earth systems to static observations made in the past and to predictions of systemic evolution. Sometime in the (not too distant) future, we will be able to monitor volcano and earthquake zones, view tomographic images as temporal snapshots and relate changes in seismicity and surface manifestations to subsurface processes deep in the earth's crust.

Concentration on Volcano Studies

Since 1996 I have focused a considerable effort on investigation of explosive volcanism. Volcanoes offer seismologists and geophysicists considerable challenges, as they combine solid earth geophysics and mix phase fluid dynamics. This combination of difficult issues provides an ideal multi-disciplinary platform to investigate earth processes. My emphasis will be on observations that cut across disciplines, involving collaborations bridging fields such as geology, geophysics, fluid dynamics and volcanology. I have already begun this process in my recent expeditions at Karymsky, Tungurahua, Sangay, Reventador, Stromboli and Santiaguito Volcanoes as well as extensive work done at Coso Geothermal field. The prospect of gathering together young and energetic researchers to attack problems of great significance in cooperation is one of my most exciting and important goals.

Teaching Program

I have taught numerous courses at the graduate and undergraduate levels. These include: Introduction to Seismology, Global Plate Tectonics, Introduction to Geophysics, Active Tectonics, Inverse Theory, Signal Analysis, and Geophysical Data Analysis. I am especially concerned that students develop a strong quantitative background for analysis of data in the earth sciences. To this end, my courses emphasize connective themes that show how methods and techniques are related by a common analytic approach. I believe that students learn the most by struggling with difficult problems and I challenge them to think independently, particularly when simple solutions are not available from an answer sheet. In addition to problem solving, I stress writing and scientific exposition in all my courses. I require students to write research term papers in all of my classes. I believe that students should get a substantial amount of experience communicating by writing and presenting material orally.

Field Program

I have been involved in numerous field efforts over my career. Before graduate school I worked for five years at Shell Oil Company as an exploration geophysicist involved in seismic processing, interpretation and acquisition in active reflection seismology. I worked on the Southern Sierra Seismic Transect for two years scouting the line and participating in the planning and deployment of the stations during acquisition. I was funded to participate in the Western Washington Seismic Transect near Mount Rainier, Washington. This data was incorporated in a tomographic inversion of the Mount Rainier region. I was involved in the initial analysis of the borehole seismic network of the Parkfield Experiment when I worked as a Post-Doc at U. C. Santa Barbara and I have been a primary consultant on the borehole array established at Coso, California. In 1997 I was funded to head up the Side Edge of Kamchatka experiment, an NSF funded IRIS/PASSCAL project. We deployed 15 broadband instruments along the length of Kamchatka. During each summer of this project we deployed stations around Karymsky volcano to investigate the dynamics of Strombolian volcano explosions. Simultaneous acoustic and seismic recordings of the periodic Karymsky explosions provide a way to examine the dynamics of the volcano vent and conduit. In September, 1999, I organized an extensive expedition, in collaboration with researchers from University of Washington, U.C. Berkeley, New Mexico Inst. Tech., Arizona State University and CalTech to observe Karymsky volcano with a variety of instrumentation. We made simultaneous observations of seismo-acoustic, gravity, COSPEC, GPS, tilt, weather, video signals associated with the eruptions. To enrich the Karymsky data set, I organized in 1998 the first broad band expedition to Sangay Volcano, Ecuador. Sangay, like Karymsky, has periodic Strombolian explosive activity. For the further study of seismo-acoustics on active volcanoes I have established a mobile volcano observatory consisting of infrasonic microphones, weather stations, video cameras and laptop field computers. These, together with PASSCAL seismic sensors, are ready for deployment at any time anywhere in the world.


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Address:

  
Prof. Jonathan M. Lees
Department of Geological Sciences
CB #3315, Mitchell Hall
University of North Carolina
Chapel Hill, NC  27599-3315
(919) 962-1562
FAX (919) 966-4519