HR: 0830h
AN: S41C-0096    [PDF]
TI: Tidal Triggering of Earthquakes in and Around the Locked Zone of the Philippine Sea Plate in the Tokai Region, Japan
AU: * Tanaka, S
EM: tanaka@zisin.geophys.tohoku.ac.jp
AF: Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AU: Sato, H
EM: sato@zisin.geophys.tohoku.ac.jp
AF: Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AU: Matsumura, S
EM: shozo@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Tennodai, Tsukuba, 305-0006 Japan
AU: Ohtake, M
EM: ohtake@zisin.geophys.tohoku.ac.jp
AF: Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AB: We investigated statistical correlation between the earth tide and earthquake occurrence in and around the locked zone, 80 km $\times$ 60 km, of the plate interface in the Tokai region, central Japan, where a large interplate earthquake may be impending. The data we used are the origin times, hypocenter coordinates and focal mechanisms of M $\geq$ 1.5 earthquakes occurring in the subducting Philippine Sea plate that were observed by the seismic network of the National Research Institute for Earth Science and Disaster Prevention for the period from June 1986 to February 2003. Clustered events were removed from the data set. For each earthquake, we assigned the tidal phase angle at the origin time by theoretically calculating the tidal shear stress 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. In this test, the result is evaluated by p-value, which represents the significance level to reject the null hypothesis that the earthquakes occur randomly irrespective of the tidal phase angle. As a result of analysis, no correlation was observed for the data set including all the earthquakes (p = 29%). However, we found a characteristic pattern in the temporal variation of tidal effect. There were four M $\geq$ 4.5 earthquakes in the investigated period. The p-value significantly dropped before the occurrence of these earthquakes and recovered after that. The frequency distribution of tidal phase angles in the pre-event period exhibited a peak at the phase angle where the shear stress is at its maximum to accelerate the fault slip, which indicates that the observed small p-value is a physical consequence of the tidal effect. We also investigated the spatial distribution of p-value. The most remarkable low-p region was found to coincide with a strongly locked portion of the plate interface, which was inferred from seismicity change in the past few years.
DE: 1249 Tides--Earth
DE: 7200 SEISMOLOGY
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting