1999년 규모 7.6 치치 대지진 지역 지진유발 지하구조연구

Abstract

The destructive Mw 7.6 Chi-Chi earthquake shook the entire island of Taiwan on 21 September, 1999. The earthquake was followed by tremendous amount of aftershocks for the next a few months including ten strong aftershocks with magnitude greater than 6.0. High resolution 3-D P- and S-wave velocity models for Mw 7.6 Chi-Chi, Taiwan, earthquake source area and its vicinity have been constructed to understand the generating mechanism of the main shock and aftershock sequence. High quality P wave and S wave arrival times are selected from three sets of data: the island-wide seismic network continuously operated by the Central Weather Bureau of Taiwan and two temporary seismic networks operated by the Institute of Earth Sciences, Academia Sinica. The combined data set to invert tomographic image of the study area includes 57,263 P-wave arrivals and 38,373 S-wave arrivals from 6,440 earthquakes recorded by 70 stations. The tomography inversion and earthquake relocation package initially developed by Benz et al. (1996) was used to determine reliable 3-D P- and S-wave velocity models. Inverted velocity models show large lateral and vertical variations. The most salient feature in the reconstructed velocity model is the sudden increase of velocity anomaly and seismicity across the Shangtung fault and the Shuilikeng fault. The main event was initiated in the region of low Vp, low Vs, and slightly high Vp/Vs ratio. Resultant velocity models from this study and evidences from other studies imply the fluid in the source area might have been involved during the nucleation processes. In the mean time, most aftershocks were located in remote areas from the mainshock hypocenter. They are nucleated in the areas of the high Vp and the high Vs anomaly. Results also confirm there is a fault at deeper depths between 15 and 30 km, which divides brittle materials in the west from ductile materials in the east.