Presumed Asperities for the Anticipated Tokai Earthquake (Seismic Activity Change in the Tokai Region: Part 4)

Abstract

The slow-slip event in the area under Lake Hamana, which may have been triggered by the 2000 Miyake seismo-volcanic activities, lasted for over five years and has finally ceased at the end of 2005. Within the same period, changes in seismic activity were detected in central Shizuoka prefecture, which is assumed to be the main locked zone for the anticipated Tokai earthquake. It is considered that the slow-slip released part of the accumulated stress around the western edge of the locked zone, and that, once released, the stress was loaded on the rest of the locked zone, producing these seismicity changes. This study is concerned with locating the essentially locked part within the focal zone of the future Tokai earthquake, that is, distinguishing the asperities from the seismicity changes. Knowing the position of the asperity prior to the final breakage of the anticipated earthquake is beneficial for both hazard estimation and earthquake prediction. Analysis of the spatial pattern of the seismicity changes was based on the following assumption. The seismicity change was caused by a quasi-static slip that progress on the weakly locked zone of the focal area and eventually result in stress concentration on the asperities. The spatial pattern of the seismicity change is classified into two categories, activation and quiescence, and the asperities could be assigned to the former. As a result, three activated zones identified in central Shizuoka prefecture could be considered candidates for the asperities. Five evidences reported by several researchers were used to verify that the assumed asperities are actually stress-concentrated. These evidences are b-value change, tidal dependence of seismic activity, large stress drop, radiation of short-period seismic waves, and transition aspects of the slow-slip distribution. All these evidences supported our basic assumption.