Broadband Seismological Investigations of Earth Structure and Earthquake Sources

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This is a four-year project supporting the seismological research program conducted by Thorne Lay at the University of California, Santa Cruz. The unifying theme of the research effort is the use of broadband digital seismic data to address fundamental questions about earth structure and earthquake rupture processes. The research is focused on the following areas: (1) seismic wave stacking and migration is being used to image lowermost mantle structure, testing the hypothesis that the post-perovskite phase transition occurs near the top of the D" region, and seeking evidence of a deeper reverse transformation to perovskite above the core-mantle boundary that can constrain thermal gradients in the CMB boundary layer; (2) analysis of large data sets to separate source, receiver and deep propagation complexity is being pursued, seeking to better understand near-source (slab) contributions to waveform complexity and shear wave splitting; (3) stacking and migration of seismograms from deep focus earthquakes is being performed to develop images of upper mantle reflectivity in the vicinity of subduction zones; and (4) analysis of moderate-size earthquake source properties along the Sumatra-Andaman rupture zone and other regions of oblique subduction will be conducted to assess variations in frictional properties. Current advances associated with this research include the first direct determination of heat flux through the core-mantle boundary based on seismological and mineral physics constraints, detection of strong scattering in localized patches at the core-mantle boundary, and three-dimensional imaging of multiple seismic discontinuities in the upper mantle beneath Tonga. This overall research effort involves the PI and two graduate students, with continuing collaborations with colleagues at other institutions. We anticipate continued progress in characterizing dynamical processes associated with the Earth's major boundary layers near the surface and near the core-mantle boundary.