HR: 0830h
AN: S41C-0095    [PDF]
TI: Spatio-Temporal Distribution of Aftershocks and Stress Changes After the 1999 Chi-Chi, Taiwan, Earthquake
AU: * Nagai, S
EM: satorung@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Hirata, N
EM: hirata@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Numerous aftershocks followed by the 1999 Chi-Chi, Taiwan, Earthquake (Mw=7.6) provided us valuable data sets for studying aftershock occurrence and crustal structure in Taiwan. In this study, we focused on the role of stress changes in triggering aftershocks. This mainshock occurred on September 20, 1999 (UT) with a 100-km-long surface rupture along the Chelungpu fault with fault slips of up to about 10 m. The area that aftershocks occurred was as broad as 150 km long and 100 km wide, which was almost half of the Taiwan Island and much larger than faulted area [e.g. Nagai et al., 2003]. The aftershocks were separated into 7 clustered activities concentrated in space, each of which also had different characteristics in temporal distribution. And their distribution had changed as compared with that of past seismicity. Aftershocks have various focal mechanisms, which indicate the complex crustal structure and the regional stress distribution originated by the relative motion between the Eurasian and the Philippine Sea plates. We calculated static stress change by the mainshock on objective fault planes, inferred from each clustered aftershock distribution and their focal mechanisms. We adopted the finite source dislocation model [Okada, 1992] to calculate the static stress change in an elastic half-space by the main shock. And we assumed crust was homogeneous with a shear modulus of 3$\times$10$^{10}$ Nm$^{-2}$ and employed a Poisson's ratio of 0.25 and an effective friction coefficient of 0.4. The results showed positive correlations between aftershock areas and positive stress changes in the Chi-Chi aftershock sequence approximately. However static stress triggering is one of many factors to influence the occurrence of earthquakes, which include such as pore pressure, dynamic stress triggering by seismic waves passage, and tectonic loading. Indeed, Taiwan is located in the arc-continent collision zone between the Eurasian and the Philippine Sea plates with a convergent rate of about 7-8 cm/yr. We will discuss the relationship among the occurrence of the Chi-Chi aftershock sequence, the effect of static and dynamic stress changes by the mainshock and the larger aftershocks, and other effects such as tectonic loading and crustal structure in Taiwan.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 7230 Seismicity and seismotectonics
DE: 8015 Local crustal structure
DE: 9320 Asia
SC: Seismology [S]
MN: 2003 Fall Meeting