I have a three-pronged research program, which is typically funded by NSF and Department of Defense.
My research in earthquake seismology has roots in my Ph.D. work at Caltech, where I was introduced to analysis of large earthquake ruptures using body and surface wave inversion methods developed there by Don Helmberger and Hiroo Kanamori. I have pursued many analyses of large earthquake rupture processes for the purpose of characterizing their spatio-temporal rupture process. The primary motivation for such work is to establish the nature of ruptures and to seek controlling factors in the mode and attributes of large earthquake failures. This work has played a major role in the development of the Asperity Model for earthquake failure, in which faults are viewed as involving regions of concentrated stress/strength that pin the fault from sliding as stress builds up prior to failure. I have also pursued development of rapid analysis capabilities that allow fault geometries and rupture attributes to be determined very rapidly after an earthquake, including some of the most rapid characterizations of finite faulting that have been achieved. Recently, my interests have focused on using variations in earthquake rupture process to characterize depth-varying properties of major interplate thrust zones. I have alsow worked on all of the major earthquakes of the past decade addressing both their specific seismotectonics and methodologies by which we can determine robust slip distributions on the faults. This includes new methods of back-projection of remote observations, new strategies for finite-fault inversions, and use of surface wave source time functions.
My earth-structure interests are broad, and I have been engaged in research on gross crustal properties, upper mantle structure, lower mantle structure, outer core structure, and structure of subducting slabs. Of particular note has been my work on structures near the core-mantle boundary, which commenced with the discovery in 1983 of strong gradients in shear velocity that produce an extra arrival, or triplication, of the wavefield near depths of 2600 km. This has led to extensive investigation of laterally varying layering near the core-mantle boundary by my students and postdocs, and remains one of the primary observations of lower mantle complexity. I have worked extensively on anisotropic and structural heterogeneity of this lowermost mantle region, and have collaborated with mineral physicists on the interpretation of a perovskite-to-post-perovskite phase transition as one interpretation of the structures that we observe. I have worked on discontinuities in the upper mantle, using waveform stacking methods, along with engaging in global surface wave tomography. My students have pursued three-dimensional wavefield modeling of amplitude and travel time anomalies produced by velocity gradients in deep slabs.
The third prong of my research program has involved analysis of signals from underground nuclear explosions for the purpose of characterizing the yields, burial depths, source processes and triggered tectonic release of such events. With the current Comprehensive Nuclear Test Ban Treaty being globally complied with by most nations through a moratorium on testing, this research is primarily directed at monitoring efforts that involve development of methods for identifying any violations of the treaty (by differences in their seismic waveforms from natural sources such as earthquakes). This research area has also been the arena for development of complex wave propagation codes for 2D and 3D media. I collaborate with Researcher Ru-Shan Wu in his program on elastic screen methods which are of growing importance in both crustal studies and in oil exploration processing.