HR: 13:40h
AN: S23C-01 INVITED     [Abstracts]
TI: The Evidence that Long-Range Earthquake Triggering Involves Fluids
AU: * Brodsky, E E
EM: brodsky@ess.ucla.edu
AF: Dept. of Earth and Space Sci. UCLA, 595 Charles Young Dr., Los Angeles, CA 90064
AU: Prejean, S G
EM: sprejean@usgs.gov
AF: US Geological Survey Alaska Volcano Observatory, 4200 University Ave., Anchorage, ak 99508
AB: Long-range triggering of earthquakes by other earthquakes has been documented for over a decade. We now know that it is a common, perhaps even predictable, occurrence immediately following a large earthquake. We also know that the seismic waves are the transmitters of the stress that start earthquakes within minutes of the mainshock. It is commonly believed that fluids play a crucial role in the triggering mechanism. Here we review the evidence for fluid involvement based on studies in the Western U.S., Greece and Japan. The first line of evidence is that geothermal areas are more prone to triggering than elsewhere. Careful comparison of the stress fields from the 1992 Landers earthquake at well-monitored sites like Parkfield, CA with the stress fields in triggered areas in the Basin and Range show that a comparable level of stress in both sites resulted in triggering in only the Basin and Range (Spudich et al., 1995). The 2002 Denali earthquake triggered areas with hot, deep circulating hydrological systems in the Western U.S., but did not necessarily trigger areas with only high heat flow. However, the lack of triggering in Japan following very large earthquakes like the 2003 $M_w$=8.3 Tokachi-Oki event suggests that geothermal activity by itself is not sufficient for triggering. A few cases of triggering in non-geothermal areas suggest that while hot fluids promote triggering, they may not be necessary. A mechanism which predicts geothermal areas are easily triggered sites, but still allows triggering elsewhere, would explain the data. The second line of evidence comes from determining which aspect of the wavefield determines whether or not a given site triggers. Is the energy, overall amplitude or amplitude in a particular frequency band most effective? There are two observations that suggest that long-period waves are the most important part of the wavefield for long-range triggering. First, triggering after the Denali earthquake occured early in the wavepacket of dispersed Rayleigh waves suggesting that the first-arriving, longest period waves are effective triggers even though they have relatively low-amplitudes. Second, a threshold of $\sim$0.05 cm/s local vertical shaking from long-period waves ($>$30 s) separates earthquakes that trigger local seismicity from those that do not at Long Valley Caldera. Triggering at the Geysers geothermal field is also consistent with a threshold determined by the long-period waves. There may also be some evidence that perhaps the number of events triggered is linearly dependent on the amplitude of the long-period waves. Combining these two sets of observations strongly suggests that long-range triggering involves fluid movement. The increased effectiveness of long-period waves relative to short-period ones cannot be explained by rate and state, stress corrosion or bubble mechanisms. Any mechanism that involves fluid movement through a porous medium is consistent with the long-period threshold since the diffusion of pressure acts as a low-pass filter. One such mechanism, unclogging of fractures temporarily blocked by sediment or precipitate, would also explain the increased efficacy of triggering at geothermal areas. References Spudich, P. et al., JGR, 675-690, 1995.
DE: 7230 Seismicity and seismotectonics
DE: 7280 Volcano seismology (8419)
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting