HR: 17:00h
AN: S24A-05    [Abstracts]
TI: Inverting Peak Ground Motions for Rupture Directivity in Moderate and Large Earthquakes
AU: * Boatwright, J
EM: boat@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Seekins, L C
EM: seekins@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: We present a simple and robust inversion of peak ground motions to determine rupture direction and velocity for moderate and large earthquakes. Peak ground accelerations or velocities ({\it PGA} or {\it PGV}) are corrected for site amplification using standard station corrections, and for hypocenter-receiver distance using $exp(-\eta r)/g(r)$, where $\eta \sim 0.004$ km$^{-1}$ and $g(r)\sim r^{1.5}$ or $r$ for $r < 30$ km for moderate or large earthquakes, respectively. We fit the residual peak motions to the unilateral directivity function $(1 - (v/\beta) cos \theta_{rj})^{-1}$ where $v$ is the {\it apparent} rupture velocity and $\theta_{rj}$ is the solid angle between the rupture direction and the takeoff angle of the S-wave. Bilateral ruptures yield slower apparent rupture velocities. We apply this inversion independently to {\it PGA} and {\it PGV} recorded at $r < 50$ km from 30 large and moderate ($3.5 \leq {\bf M} \leq 6.9$) earthquakes in northern California. The rupture direction can be determined using as few as 6 peak motions if the station distribution is sufficient. The rupture velocity is unstable, however, if the rupture direction is not sampled (if all $\theta_{rj} > 45\deg$). For large earthquakes whose waveforms have been fit for slip distributions, the estimated rupture directions agree well with the rupture models, although the rupture velocity can be overestimated ($v > \beta$) for extended faults. Surprisingly, the {\it PGA} inversions perform slightly better than the {\it PGV} inversions for these large earthquakes. The inferred rupture directions for the {\bf M}5.6 and {\bf M}5.3 1980 Livermore, the {\bf M}6.0 1990 Honeydew, the {\bf M}5.1 1998 San Juan Bautista, the {\bf M}5.0 2000 Yountville, and the {\bf M}4.9 2002 Gilroy earthquakes agree remarkably well with the focal mechanisms and aftershock distributions. Aftershocks are generally located within $70\deg$ of the rupture direction, indicating that rupture directivity strongly influences aftershock occurrence. Moderate ($3.5 \leq {\bf M} \leq 5.0$) earthquakes appear to rupture updip more often than downdip or along-strike. The aftershock sequences for many of the ${\bf M} < 4.0$ earthquakes are too sparse to verify the inferred rupture directions, however.
DE: 7223 Seismic hazard assessment and prediction
DE: 7260 Theory and modeling
DE: 7209 Earthquake dynamics and mechanics
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting