Precursory change in seismicity revealed by the Epidemic-Type Aftershock-Sequences model: A case study of the 1999 Chi-Chi, Taiwan earthquake

For gaining insight into the preparatory process of the 1999 Chi-Chi, Taiwan earthquake and its related statistical feature, the Epidemic-Type Aftershock-Sequences model (ETAS model) was applied to the earthquake data in Taiwan region. By means of the ETAS analysis for Taiwanese earthquakes larger than a magnitude of 2.4, we found seismic quiescence over broader regions of Taiwan. It was also found that inland areas near the epicenter of the Chi-Chi earthquake showed seismic activation in the period from Jan. 1, 1998 to Sep. 20, 1999, which is just before the occurrence of the Chi-Chi earthquake. The assumption that this is due to precursory slip (stress drop) on the fault plane of the Chi-Chi earthquake is supported by previous researches such as a numerical simulation using rate- and state-dependent friction laws and the observation of abnormal change in crustal displacement for a station of Taiwan GPS network near southern edge of the source area of the Chi-Chi earthquake.