HR: 1340h
AN: S53B-0218    [Abstracts]
TI: Constraining crustal S-wave velocities in southern California by deformable layer tomography
AU: * Li, L
EM: lli2@uh.edu
AF: Allied Geophysical Lab, University of Houston, 4800 Calhoun Road, SR1, Rm312, Houston, TX 77204-5007 United States
AB: We are building a new crustal S-wave velocity model for southern California using more than twenty years of S-wave first arrival data from a dense distribution of local earthquakes and seismologic stations, as compiled by the Southern California Earthquake Center (SCEC) data center. Together with the P-wave model, the S-wave model plays a critical role for elastic modeling and tectonic interpretation for the crust. However, tomographic construction of crustal velocity models is challenged by the variable crustal thickness. Due to high velocity and low velocity gradient of the mantle below the Moho, the Moho-grazing first-arrival raypaths at regional distances rarely extend below the Moho. The depth-velocity ambiguity makes it difficulty to simultaneously constrain the lateral velocity variation and Moho depth variation. Despite of the dense distribution of local earthquakes and stations, the S-wave velocities in the lower crust have not been well constrained previously because there are less S-wave picks than P-wave picks, particularly with respect to the Moho-grazing S-wave. We are using a deformable layer tomography (DLT) that takes advantage of the fact that the 1-D S-wave velocity is well constrained for the crust and upper mantle at some locations from previous waveform studies. We are also using the well-constrained velocities of the shallow crust from previous studies for initial model values. Unlike most of the previous tomography methods that use fixed-in-space cells or nodes, DLT directly inverts for the depths of velocity interfaces. DLT can resolve velocity-defined units that may vary significantly in thickness and pinchout to yield robust solutions in regions of sub-parallel rays and to accurately represent basin and Moho geometries. The SCEC data center compiled over 800,000 S-wave picks from local events since 1983. The raw data were sorted according to a number of quality control thresholds to maintain the quality of hypocenters and number of picks per event, resulting in a data set of over 400,000 S-wave picks from nearly 80,000 local events. Synthetic tests of the DLT algorithm using real sources and station locations suggest that the geometry of major S-wave velocity contours can be resolved in the study area. At similar data fitness level, we expect the DLT method will achieve more reliable result of the S-wave velocity contours in the lower crust in comparison with the previous S-wave models.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting