SEISMIC QUIESCENCE BEFORE THE 1999 CHI-CHI, TAIWAN, MW 7.6 EARTHQUAKE

Abstract

The Chi-Chi, Taiwan, earthquake of 20 September 1999 was preceded by a notable decrease of the regional seismicity rate. The anomalous period started in January 1999 and lasted about 9 months, up to the occurrence of the mainshock, as revealed from analyzing the Taiwan catalog data from 1 January 1994 to 20 September 1999. Our results indicate that the mean seismicity rate is 435 events per month with a standard deviation of 78 events for earthquakes with magnitude above 2.0. During the anomalous period, seismicity rates fell outside the range of one standard deviation with a mean value of 314 events per month. There was also a consistent trend of decreasing regional b-value as well as notable increases of seismic activity in areas surrounding the Chi-Chi earthquake source region via the Z-test map (ZMAP) analysis.