HR: 14:55h
AN: S23C-06 INVITED     [Abstracts]
TI: The Decay of Aftershocks with Distance Implies Dynamic Triggering
AU: * Felzer, K R
EM: kfelzer@moho.ess.ucla.edu
AF: University of California, Los Angeles, 3806 Geology Building, Los Angeles, CA 90095-1567
AU: Brodsky, E E
EM: brodsky@moho.ess.ucla.edu
AF: University of California, Los Angeles, 3806 Geology Building, Los Angeles, CA 90095-1567
AB: With relocated earthquake catalogs it has become possible to look carefully at how aftershock density decays with distance from the mainshock. It has also become possible to more accurately investigate how far away aftershocks may be triggered, especially after small mainshocks. We focus on small mainshocks $2\leq M \leq 5$, which we treat as point sources. We use standard and relocated earthquake catalogs for Southern California, and the Japanese JMA catalog. In the Southern California relocated catalog of Shearer et al.(2003), we find that aftershock triggering extends to at least 16 km following small mainshocks, even those as small as $M$ 2-3, for which 16 km represents more than 44 fault lengths. The decay of aftershock density ($\rho$) with distance from the mainshock hypocenter ($r$), at distances of 1 fault length to 16 km (where we can see a clear relationship between distance and aftershock density) is well described by $\rho = r^{-1.2}$. If we make the assumption that the number of aftershocks triggered varies linearly with stress amplitude our relationship for distance vs. density agrees with triggering by dynamic stresses, as the amplitude of seismic waves decays somewhat faster than $r^{-1}$. In contrast, the amplitude of static stress change decays as $r^{-3}$. In our data set the percentage of aftershocks occuring at distances between one fault length and 16 km from the mainshock hypocenter is 43% for M 2-3 mainshocks, 31% for M 3-4 mainshocks, and 23% for M 4-5 mainshocks, respectively. We also calculate distances between aftershocks and the mainshock fault of the M 7.3 Landers, California earthquake, using the Shearer et al. (2003) catalog and the mapped fault trace of the mainshock, assuming a vertical dip. We find that the decay of aftershocks with distance is best fit by $r^-1.39 \pm 0.13$ at 98% confidence (for the first five days of data), between 1 and 16 km from the fault. This is similar to the results for the smaller mainshocks, although more difficult to interpret physically since near field stress changes may be complex.
DE: 7223 Seismic hazard assessment and prediction
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting