Seismology and Tectonics
My students and I are studying crustal anisotropy by modeling seismic waveforms (receiver functions). This subject is especially exciting to me because anisotropic parameters can be related to large-scale crustal fabrics and structures caused by tectonic stress or material flow. It is a growing research area where seismology, in conjunction with geologic and laboratory studies, can contribute to deciphering deformational processes in the middle to lower crust where our understanding of tectonic processes remains fuzzy. Our study of a major volcano-tectonic province (the Andean Altiplano Volcanic Complex, or APVC) is wrapping up with publications of two major papers and a submission of an interdisciplinary paper with Professor Shan DeSilva (University of North Dakota) on the APVC to GSA Today. The Tibetan Plateau crustal anisotropy study by graduate student Heather Sherrington (formerly Folsom) will appear shortly in the Journal of Geophysical Research, and graduate student Arda Ozacar has initiated a new Plateau crustal anisotropy study using the more closely spaced INDEPTH-III data. With this work, we hope to contribute to the lively continuing discussion on the role of crustal flow in the formation of the Tibetan plateau.
With students, postdoctoral researcher Hersh Gilbert, and Professor Susan Beck, I remain involved with seismological studies in the Andes of South America. In 2003, our research group has obtained some exciting preliminary results from our CHARGE experiment and presented them at international meetings as well as at the Fall AGU meeting. Graduate student Robert Fromm estimated the crustal thickness beneath the Andes and Sierra Pampeanas at 30°S from Pn apparent phase velocities and found the crust in the Sierra Pampeanas to be thicker than expected on the basis of the isostatic elevations. Graduate students Megan Anderson and Lara Wagner are investigating shear-wave splitting and seismic tomography, respectively, of the region using the CHARGE data set. In conjunction with the receiver function studies by Hersh Gilbert and the crustal earthquakes study by graduate student Patricia Alvarado, we hope to complete a broadly based seismotectonic study of this unique flat subduction environment.
I am continuing interdisciplinary studies on orogenic systems with members of our tectonics group. Discussions with Professor Mihai Ducea has reinvigorated my interest in the Southern Sierra Nevada "drip" story and in the general process of batholith formation and continental crustal evolution. Mihai, graduate student Steve Kidder, and I published a short paper on a comparison of the exposed Santa Lucia lower crustal terrane with seismic properties of the Andean Western Cordilleran lower crust. With Hersh Gilbert, Mihai Ducea, Professor Tom Owens (University of South Carolina), Professor Craig Jones (University of Colorado), and Professor Jason Saleeby (Caltech) I have been working on interpreting new receiver function images of crust-mantle interactions associated with a Rayleigh-Taylor-type instability beneath the southern Sierra Nevada. This study is tied in nicely to my crustal anisotropy work because we found evidence for a detachment shear zone at the base of the crust from an anisotropic fabric that it presumably generated. We recently published a manuscript on this work as a research article in the journal Nature.
We are continuing a collaborative effort between the University of Arizona (George Zandt, Susan Beck, and Hersh Gilbert), Arizona State University (Professors Matt Fouch and Ed Garnero) and the University of Texas in Austin (Professor Steve Grand) to deploy 9 broadband stations in southern Arizona for 1-2 years. We obtained funding for a summer undergraduate intern from IRIS and hosted an undergraduate geophysics student (Andy Frassetto) from the University of South Carolina for the summer of 2003. One of our own undergraduates (Koichi Sakaguchi) joined this summer project and continues to do research with our group. This project is continuing under the guidance of Hersh Gilbert.