台湾9.21集集地震考察兼论强震发震断层

SEISMOGENIC FAULT OF THE 1999 CHI CHI EARTHQUAKE IN TAIWAN PROVINCE AND THE FEATURES OF EARTHQUAKE DAMAGES

1999年9月21日,台湾中部山麓带发生了M7.3的大地震,震源深度为8km,财产损失及人员伤亡是百年来台湾许多地震中损失及伤亡最大的1次,其震级也是台湾本岛陆上所发生的地震级别最大的。震源机制属低角度逆冲断层成因,余震在平面上围绕着北港高基底作半圆状分布,在垂向上,则分布在逆冲断层的上盘。与此相应,地面变形及上部结构物的破坏,以车笼埔发震断层上盘最为激烈,下盘几乎不受影响。此外,地震断裂的北端,水平位移量高达9.8m,垂直抬升达10m,比主震区要大;其地面加速度峰值,亦高达水平为502gal,垂直为519gal。这些特点表明,地震是受到地下深处侏罗型叠瓦状构造的控制。此外,3个诱发地震中心均受当地的地质构造与地貌条件的控制。文中还叙述了震害及工程结构物破坏的特点,尤其是水工结构物的震害。

A large earthquake of M 7.3 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.