Professor of Geological Sciences
Geophysics, and especially Seismology. B.S. 1976 from M.I.T. Ph.D. 1982 from Columbia University.
My main interests are in the structure of the lithosphere, and especially in understanding the origin of its heteogeneity through the use of tomographic imaging, analysis of scattered wavefields, and modeling of waveforms. Here are several examples of my recent projects:
1. The crustal structure of Iceland. Iceland is an island formed where a hotspot interacts with the mid-Atlantic ridge. The mantle beneath iceland is upwelling and melting, producing the thick layer of crust that makes up the Island. I am interested in just how this happens - how the magma traverses the crust; whether there are large amounts of melt within the crust; the thermal state of the lower crust and the uppermost mantle; the way in which plate tectonic spreading occurs. I have approached these questions by three techniques: seismic field experiments to tomographically image the structure of the rift zones; analysis of shear wave attenuation to infer temperature; and numerical models of the crustal accretion process. I have become convinced that the structure of Iceland is quite different than what was hitertofore guessed, with a colder and thicker crust.
2. The origin of seismic heterogeneity in cratonic North America. Quite large P wave delays of up to 0.5s are observed in the older parts of North America. In tectonically active regions these would be ascribed to temperature fluctuations in the mantle, but such an explanation cannot hold in these old, cold regions. The heterogeneity must be either due to chemical variations or to differences in the degree of allignment of crystals, which lead to different degrees of anisotropy. I am trying to distinguish between these two mechanisms by searching for a correlation between P wave delay and the presence of shear wave splitting (which is diagnostic of anisotropy but not chemical variation).
3. The magmatic structure of Mid-Ocean Ridges. I am interested in how magma migrates from it source region in the upper mantle beneath ridges, how it is stored in magma chambers within the crust, and how it forms the crust of the oceans. I have used seismic imaging techniques to study magma chambers on the Juan de Fuca Ridge, off the coast of Washington State.
I am also interested in geophysical data analysis techniques, including times series analysis, inverse theory, and tomographic inversion. My most recent project in that area, completed in collaboration with Dr. Vadim Levin, was the development of a new data analysis technique for detecting seismic anisotropy in the earth's mantle. This anisotropy is created by the convective solid-state flow of the mantle, which alligns olivine crystals. Of particular interest to me is how to detect the presence of multiple layers of anisotropy, corresponding to flow patterns that vary with depth within the earth.