HR: 09:45h
AN: S11D-06    [Abstracts]
TI: A Quantitative Evaluation of 3D Velocity Models in Southern California
AU: * Chen, P
EM: pochen@usc.edu
AF: Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, ZBH 117, Los Angeles, CA 90006 United States
AU: Zhao, L
EM: zhaol@usc.edu
AF: Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, ZBH 117, Los Angeles, CA 90006 United States
AU: Jordan, T H
EM: tjordan@usc.edu
AF: Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, ZBH 117, Los Angeles, CA 90006 United States
AU: Liu, Q
EM: lqy@gps.caltech.edu
AF: Seismological Laboratory, California Institude of Technology, Seismological Laboratory, California Institude of Technology, Pasadena, CA 91125 United States
AU: Tromp, J
EM: jtromp@caltech.edu
AF: Seismological Laboratory, California Institude of Technology, Seismological Laboratory, California Institude of Technology, Pasadena, CA 91125 United States
AB: We present a systematic methodology for evaluating and improving 3D seismic velocity models using waveform data from regional earthquakes. The operator that maps a synthetic waveform into an observed waveform is expressed in the Rytov form $D(\omega) = \rm{exp}[\rm{i} \omega \delta \emph{t}_{\rm{p}}(\omega) - \omega \delta \emph{t}_{\rm{q}}(\omega)]$. We measure the phase delay time $\delta t_p(\omega)$ and the amplitude reduction time $\delta t_q(\omega)$ as a function of frequency $\omega$ using Gee & Jordan's [1992] isolation-filter technique. We have applied this procedure to CISN recordings of 25 small earthquakes ($3.0 \le \rm{M_L} \le 4.8$) in the Los Angeles region. Synthetic seismograms were calculated from four types of velocity models: the 3D SCEC Community Velocity Model Version 3.0 (CVM3.0) [Magistrale et al., 2000],the Harvard 3D model (HAR3D) [S\"{u}ss and Shaw, 2003], the 1D Standard Southern California Crustal Model (SoCaL) [Dreger & Helmberger, 1993], and a set of path-averaged 1D models (A1D), which were extracted from CVM3.0 by horizontally averaging wave slownesses along source-receiver paths.(The 3D synthetics were computed using K. Olsen's finite difference code.) We measured 165 P waves, 110 SV waves, 171 SH waves and 48 other phases from 284 source-station paths, at five frequencies ranging from 0.2 Hz to 1 Hz, yielding a data set comprising 4940 $\delta t_{pq}(\omega)$ observations. Overall, the two 3D models provided a substantially better fit to the waveform data than either laterally homogeneous or path-averaged 1D models. Relative to SoCaL, CVM3.0 reduced the variance in $\delta t_p$ by 61% to 0.401 $\rm{s}^2$, and reduced the variance in $\delta t_q$ by 64% to 0.122 $\rm{s}^2$. The variance reductions of CVM3.0 relative to A1D were 55% and 53%, respectively. The variance of HAR3D is 0.362 $\rm{s}^2$ in $\delta t_p$ (65% less than Socal and 60% less than A1D) and 0.118 $\rm{s}^2$ in $\delta t_q$ (65% less than SoCaL and 55% less than A1D). The correlation coefficient between CVM3.0 and HAR3D is 0.6 in $\delta t_p$ and 0.8 in $\delta t_q$, suggesting high similarity between the two 3D models. Our measurements indicate that both CVM3.0 and HAR3D wave speeds might be too low in Los Angeles Basin. The variance reductions for S wave measurements are as large as for P wave measurements suggesting the scaling relations relating S wave velocity to P wave velocity in both 3D models are good approximations. Our analysis on 90 SV-SH pairs shows slight polarization anisotropy of about 4 $\rm{ms/km}$ in the upper crust.
DE: 7223 Seismic hazard assessment and prediction
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting