Broadband Seismological Investigations of Earth Structure and Earthquake Sources

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This is a four-year project to conduct seismological investigations of deep earth structure and earthquake sources.Using modern digital seismic waveform data, fundamental questions about dynamical structures in the mantle and rupture processes of large earthquakes will be addressed. The discovery of systematic variation of interplate earthquake rupture duration will be followed-up by detailed analysis of the source finiteness for very small thrust earthquakes on the megathrust fault in subduction zones. On average, these shallow events are found to take 2 to 3 times longer to rupture than comparable seismic moment events at larger depths. Establishing whether this is due to low stress drop or low rupture velocity requires determination of rupture dimensions, which will be pursued using directivity analysis with theoretical and empirical Green's functions. Studies of major boundary layer dynamics will also be conducted. Amplitude and arrival time anomalies for S waves from intermediate and deep focus earthquakes will be modeled by three-dimensional methods to help constrain the velocity structure and fate of deep slab downwellings. The effects of these downwellings on phase equilibria in the mantle will be examined using waveform-stacking methods that detect and characterize velocity and impedance contrasts in the mantle around subducting slabs. Complex velocity heterogeneity near the core-mantle boundary will be analyzed using new digital data sets and a hybrid 2D/3D wave propagation method that has recently been developed at UCSC.