HR: 14:25h
AN: S23C-04    [Abstracts]
TI: Earthquakes Triggered by SmS Arrivals
AU: * Hough, S E
EM: hough@gps.caltech.edu
AF: U.S. Geological Survey, 525 S. Wilson Avenue, Pasadena, CA 91030 United States
AB: Previous studies suggest that aftershocks are controlled by static stress changes, while remotely triggered earthquakes are controlled by the dynamic stress changes associated with transient seismic waves. However, other studies have concluded that aftershocks are also influenced by dynamic stress changes. To address this issue, I consider 8 well recorded magnitude 5.1-6.1 eathquakes in northeastern Canada as well as 18 magnitude 5.3-7.1 earthquakes in California. A beta-statistic analysis does not reveal evidence of widespread, statistically significant triggering following events smaller than M7. However, a weak seismicity increase is commonly observed beyond the extent of the traditional aftershock zone. The beta statistic increases slightly at a distance of 70-100 km. For the northeastern events, the strongest signal is associated with the M6.1 Saguenay, Quebec earthquake, the largest of the 8 events. The signal is especially persistent following even moderate events in southern and central California. Although the signal from each event is small, its persistent appearance suggests that ``aftershocks'' do occur more frequently than expected at a distance of approximately 100 km, the distance at which post-critical Moho reflections are known to significantly increase ground motions. I show that a persistent ``molehill'' signal is highly unlikely to arise as an artifact. Because static stress change is not influenced by SmS arrivals, these observations suggest that dynamic stress change plays an important role in the triggering of events in the intermediate distance range between conventional aftershocks and remotely triggered earthquakes. The results further suggest that remote triggering is a ubiquitous phenomenon but also one that is difficult to identify with standard statistical approaches. However, if SmS triggering does occur, it would provide a unique opportunity to stack signals from multiple events--perhaps as small as M5--and thus to further explore the prevalence and source properties of remotely triggered earthquakes.
UR: http://pasadena.wr.usgs.gov/office/hough/
DE: 7230 Seismicity and seismotectonics
DE: 7299 General or miscellaneous
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting