Your query was: felzer

HR: 08:45h
AN: S31A-04
TI: Testing the stress shadow hypothesis
AU: * Felzer, K R
EM: felzer@seismology.harvard.edu
AF: Department of Earth and Space Science, Univ. Calif. Los Angeles, 1708 Geology Building, 595 Charles Young Dr. East, Los Angeles, CA 90095-1567 United States
AU: Abercrombie, R E
EM: rea@bu.edu
AF: Department of Earth Sciences, Boston University, 685 Commonwealth Ave., Boston, MA 02215
AU: Brodsky, E E
EM: brodsky@ess.ucla.edu
AF: Department of Earth and Space Science, Univ. Calif. Los Angeles, 1708 Geology Building, 595 Charles Young Dr. East, Los Angeles, CA 90095-1567 United States
AB: According to current theoretical understanding an earthquake may decrease the rate of other earthquakes in a particular area (create a stress shadow) if earthquakes influence each other via static stresses, but not if they primarily interact via dynamic stresses (Toda and Stein, 2003). Using the California ANSS catalog from 1978 to 2001, we find that the occurrence of stress shadows is difficult to prove, indicating that dynamic earthquake interaction may be a viable alternative. Stress shadows are often identified by measuring seismicity rates before and after a large earthquake and then noting where rate decreases are larger than those expected from a random Poissonian process (Matthews and Reasenberg, 1988). These counts by themselves offer no proof that the mainshock is the cause of the observed rate decreases, however. Marsan (2003) projected the decay rates of pre-existing aftershock sequences to demonstrate that in many cases measured "shadows" were actually caused by ongoing localized aftershock sequence decay. We use an alternative method to verify the results of Marsan (2003) to test whether rate decreases after large earthquakes can be attributed in general to independent causes. We compare the degree and extent of seismicity rate changes around the times of large earthquakes and around other points in time. Like the method of Marsan (2003) this method does not require declustering, often a somewhat subjective process. Preliminary results indicate that while the amount of localized rate increases after large earthquakes is much larger than at other times, the amount of rate decreases may not be significantly different. In Southern California, for example, preliminary results indicate that 31% of 8 by 8km cells experienced a significant (one sigma) increase in seismicity rate during the first year after the 1992 $M_W$ 7.3 Landers earthquake (in comparison with the preceding six years), while the number of cells experiencing significant rate increases in other one year increments, from 1984 to 1991, ranged from 16% to 20%. For rate decreases, 8% of cells experienced a significant downturn in the year after Landers, while a similar percentage of cells (ranging from 7% to 10% and averaging 8.5%) experienced significant rate decreases from 1984 to 1991.
DE: 7209 Earthquake dynamics and mechanics
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting