HR: 16:45h
AN: S24A-04    [Abstracts]
TI: A Global Search for Stress Shadows
AU: * Mallman, E P
EM: emallman@pangea.stanford.edu
AF: Stanford University Department of Geophysics, 397 Panama Mall, Stanford, CA 94305 United States
AU: Parsons, T
EM: tparsons@usgs.gov
AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: For years scientists have recognized a decrease in seismicity in the regions surrounding the 1857 Fort Tejon and 1906 San Francisco earthquakes in California. This decrease in seismicity has since been correlated with calculated Coulomb stress decrease and termed a stress shadow. Earthquake hazard assessments increasingly incorporate probability perturbations resulting from calculated static stress changes. However, some researchers have questioned the existence of stress shadows when detailed studies of seismicity rates following other earthquakes failed to show correlations between calculated stress decreases and seismicity rate decreases. Regions of decreased seismicity rate are difficult to find compared with rate-increased regions; we thus look for evidence of post-seismic stress rotations, which are easier to detect, require none of the assumptions inherent to static stress calculations, and are a necessary condition for the occurrence of a stress shadow zone. A stress rotation, as determined by comparison of focal mechanisms before and after the mainshock, indicates a changed regional stress state that suppresses events of a certain mechanism and enhances events of a differing mechanism. To independently test the shadow hypothesis, we examined a global catalog of 119 M$>$7 earthquakes and the events within a 2-degree radius around them that were recorded by the Harvard CMT catalog. We first examined the average mechanism before and after the mainshock, and then compared this with the changes in rate of each of the mechanisms. Of the 119 mainshocks that we identified, 15 showed a rotation of average mechanism following the mainshock that was significant at the 1-sigma level relative to the observed pre-mainshock variability. Of these 15 events, only 2 did not show increases in rate across all mechanisms. That is, only 2 of the 119 show a decrease in at least one mechanism following the mainshock, and also show a significant rotation in mean mechanism. It is only 2 of 119 events worldwide that dynamic stress effects cannot explain the changes in seismicity, suggesting that dynamic triggering may be a much more significant effect globally than static stress triggering.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting