HR: 1340h
AN: T23B-0553    [Abstracts]
TI: Long-range triggered aftershocks in geothermal areas
AU: * Brodsky, E E
EM: brodsky@ess.ucla.edu
AF: Dept. of Earth and Space Sci, UCLA, 595 Charles Young Dr., East, Los Angeles, CA 90095
AB: Seismic waves trigger earthquakes. As surface waves passed through geothermal sites like Yellowstone and Long Valley after earthquakes like the 1992 M_w 7.3 Landers and 2002 M_w 7.9 Denali events, local earthquakes flared up. However, the increased seismicity did not end with the end of the surface waves. It continued for anywhere from an hour to 1 week. How do the transient waves of the farfield earthquakes make a sustained signal? This sustained signal has been taken as a key piece of evidence for or against various triggering mechanisms (Gomberg 2001, Brodsky et al., 1998). Others have suggested that a handful of earthquakes are triggered locally during the passage of the seismic waves and all subsequent earthquakes are aftershocks of these first, locally triggered events (Hough and Kanamori, 2002). Here I show that the sustained seismicity from long-range triggering has the same behavior as ordinary aftershock sequences. Where data is sufficient to study the time progression and spatial distribution, the sequences follow typical patterns like Omori's law. Key evidence comes from a temporary deployment of broadband seismometers in The Geysers geothermal field in Northern California. The 2005 M_w 7.2 Mendocino-triggered earthquakes are captured on both the broadbands and local networks. The sequence follows Omori's law with a p-value of 1, has a productivity that can be predicted from its largest event and has a spatial distribution that is typical of aftershock sequences with a 1/r^1.4 decay of linear density. Other, less ideally instrumented sequences show similar patterns. Even the most prolonged long-range triggering sequence may fit this pattern; the Landers-triggered earthquakes and deformation at Long Valley are consistent with a local M_w 5.6 mainshock hidden in the coda. If this result continues to hold, it implies that the sustained nature of seismicity is no more (or less) mysterious than the prolonged duration of aftershock sequences.
DE: 7209 Earthquake dynamics (1242)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Tectonophysics [T]
MN: Fall Meeting 2005