HR: 0800h
AN: S21B-0275    [Abstracts]
TI: Tidal Triggering Effect on Earthquake Occurrence Precursory to Large Thrust Earthquakes in Subduction Zones
AU: * Tanaka, S
EM: tanaka@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AU: Ohtake, M
EM: otake-masakazu@jnes.go.jp
AF: Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AU: Sato, H
EM: sato@zisin.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AB: We observed a clear tidal triggering effect on earthquake occurrence precursory to large thrust earthquakes associated with the subduction of oceanic plates. We measured the correlation between the Earth tide and earthquake occurrence using shallow reverse-fault type earthquakes with Mw $\geq$ 5.0 in and around the focal regions of eleven interplate earthquakes with Mw $\geq$ 7.5. For each earthquake, we assigned the tidal phase angle at the origin time by theoretically calculating the tidal stress change on the fault plane. Based on the distribution of the tidal phase angles, we statistically tested whether they concentrate near some particular angle or not by using the Schuster's test. As a result of analysis, a precursory tidal effect was found for six earthquakes. For all the six cases, a high correlation appeared for several years preceding the occurrence of large earthquakes, and the high correlation disappeared after the main shocks. The frequency distribution of tidal phase angles in the pre-event period exhibited a peak at the phase angle where the tidal stress change is at its maximum to accelerate the fault slip. This indicates that the high correlation observed in the pre-seismic stage is not a stochastic chance but is a physical consequence of the tidal stress change. The optimum value of the frictional coefficient varies in a wide range from 0.2 to infinity (only the normal stress change is effective) showing a positive correlation with the age of subducting oceanic plate.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 7209 Earthquake dynamics and mechanics
DE: 1249 Tides--Earth
SC: Seismology [S]
MN: 2004 AGU Fall Meeting