关键词: 9.21集集地震, 强震发震断层, 车笼埔断层, 叠瓦状构造

Abstract: A large earthquake of M7.3 took place on September 21,1999 at Chi-Chi of Western Foothill,central Taiwan region. It caused severe damage and a great number of casualties that had gone beyond the destruction and casualty caused by any one of the historical earthquakes in the past 20^th century in Taiwan region. The magnitude of the earthquake is also the largest one of the shocks occurred in the inland of Taiwan Island. The main shock was located at 23.85°N,120.82°E,with a depth of 8 km,and the focal mechanism was of a thrust type with a strike of 5°,dipping SE with a rake of 65°,which is consistent with the surface rupture along the Chelungpu earthquake fault (80km in length). The P axis is striking 293°with a plunge angle of 13°consistent with the tectonic setting in which the Philippine Sea plate collides with the Eurasian plate. Especially,the horizontal distribution of the aftershocks appeared as a semicircular form around the Peikang basement high,rather than a single line along the earthquake fault. Vertically,the aftershocks were approximately distributed on the hanging wall of the thrust fault. The other distinct features of the earthquake are the differential displacement between the hanging wall and footwall,the horizontal ground displacement of up to 9.8m,and the vertical ground elevation of up to 10m at the northern terminal of the earthquake fault,where both the surface and the subsurface data (horizontal PGA=502gal and the vertical PGA=519gal) showed accumulation of stresses in this area which caused the destruction of Shihgang Dam and other damage. Correspondingly,the deformation of the ground and the destruction of the constructions on the hanging wall of earthquake thrust were more severe than those on the footwall. All of the above characteristics show clearly the close relationship between the deep geologic structure,the surface rupture,and the seismic phenomena. Therefore,we support the“Thin-skinned-Thrust”model proposed by Taiwan seismologists for explaining the mechanism and kinematics of the Chi-Chi earthquake. The generation of the Chi-Chi earthquake can be attributed to the low-angle northwestward pushing of the Philippine Sea plate upon the imbricated structure of Jura-type on the Eurasian plate. In this paper,we describe also the features of the earthquake damages,especially the severe damage on the hanging wall and on the north terminal of the earthquake fault.

Key words: Chi-Chi earthquake, seismogenic fault, Chelongpu faults, Imbricated Structure