HR: 15:10h
AN: S23C-07    [Abstracts]
TI: Anomalous aftershock decay rates in the first hundred seconds revealed from the Hi-net borehole data
AU: * Vidale, J E
EM: vidale@ucla.edu
AF: ESS, UCLA, Los Angeles, CA 90095 United States
AU: Peng, Z
EM: zpeng@ess.ucla.edu
AF: ESS, UCLA, Los Angeles, CA 90095 United States
AU: Ishii, M
EM: mishii@ucsd.edu
AF: IGPP, UCSD, La Jolla, CA 92093 United States
AU: Ishii, M
EM: mishii@ucsd.edu
AF: Dept. of Physics, Univ. of Toronto, Toronto, ON M5S 1A7 Canada
AB: Aftershocks have long been known to diminish in rate approximately as the reciprocal of the time passed in the hours to days after a mainshock. The modified Omori Law (e.g., Utsu et al., 1995) postulates that the rate is proportional to $[t+c]^{-p}$, where t is the elapse time since the mainshock, p is an exponent near one, and c is an apparent offset time, commonly a fraction of an hour. Because it is difficult to observe aftershock activity in the noisy aftermath of larger earthquakes, observations of non-zero c can be explained as an artifact of incompleteness of existing seismicity catalogs (Kagan, 2004). We have so far analyzed Hi-net borehole recordings of 39 shallow earthquakes ranging in magnitude from 3 to 4.8. By scrutinizing the high-frequency signal, we are able to distinguish mainshock coda from early aftershocks, although estimating the number of undetectable aftershocks remains our toughest challenge. We detect up to 9 times more events in the first 1000 s than are recorded in the Hi-net catalog. There is a suggestion of change in b-value from 200 s to 1000 s. We observe a fairly steady rate of aftershocks for the first 100 s, followed by a close to 1/t decay of activity. There appears to be a distinct early stage of aftershock activity that may be related to the poorly understood processes of aftershock generation. This observation may allow us to distinguish between various proposed mechanisms such as rate-and-state friction, viscous response, and fluid migration.
DE: 8164 Stresses--crust and lithosphere
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting