Tidal triggering of earthquakes in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, Japan

Abstract

We found a characteristic space–time pattern of the tidal triggering effect on earthquake occurrence in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, central Japan, where a large interplate earthquake may be impending. We measured the correlation between the Earth tide and earthquake occurrence using microearthquakes that took place in the Philippine Sea plate for about two decades. For each event, 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 Schuster's test. In this test, the result is evaluated by p-value, which represents the significance level to reject the null hypothesis that earthquakes occur randomly irrespective of the tidal phase angle. As a result of analysis, no correlation was found for the data set including all the earthquakes. However, we found a systematic pattern in the temporal variation of the tidal effect; the p-value significantly decreased preceding the occurrence of M ≥ 4.5 earthquakes, and it recovered a high level afterwards. We note that those M ≥ 4.5 earthquakes were considerably larger than the normal background seismicity in the study area. The frequency distribution of tidal phase angles in the pre-event period exhibited a peak at the phase angle where the tidal shear stress is at its maximum to accelerate the fault slip. This indicates that the observed small p-value is a physical consequence of the tidal effect. We also found a distinctive feature in the spatial distribution of p-values. The small p-values appeared just beneath the strongly coupled portion of the plate interface, as inferred from the seismicity rate change in the past few years.