| dc.description.abstract | The 1999 Chi-Chi earthquake (M=7.6) in Taiwan was a relatively unique seismic event which activated the Chelungpu thrust fault with extraordinarily large surface ruptures. The earthquake’s hypercenter is at 8km deep and locates near the ChiChi town in Nantou. The Chelungpu thrust fault is 90km long, lying mostly in a north-south direction and with a reverse thrust dipping shallowly to east. A thin-skinned thrust model is employed to explain the behavior of this earthquake fault. In geology, the Chelungpu fault almost slides along a sedimentary layer, the Chinshui shale. It acted as a kind of detachment surface and the Chi-Chi earthquake triggered the Chelungpu fault to slide along the detachment.
From the early CPC’s (Chinese Petroleum Corporation) data about the Taichung basin, we have known that the Chinshui shale layer is not only dipping to east, but also to south with a little tilt angular. In order to further investigate the behavior of this earthquake fault, it is necessary to construct the 3D structure of the Chinshui shale. In this paper, we used the shallow reflection seismic method to detect the structures in the area between the Wuhsi river, the Choshuisi river, the Chelungpu fault and the Shuangtung fault. Over 40 seismic lines are distributed in this area, which are divided into two parts: one along the east-west direction and the other along the north-south direction. After combining the results of these shallow seismic lines and the deep seismic profile provided by CPC (supported by the NSC), we start to obtain a gross 3D structure picture of the area, which can be used to interpret the behavior of the earthquake fault
We obtain the following conclusions:
1)From the seismic profiles in east-west direction, we find that all sedimentary layers are dipping to east and the dipping angle decreases toward east. However, in the Choshuihsi area, the dipping angle becomes larger. A ‘ramp’ structure is implied in this area.
2)From the seismic profiles in north-south direction, we find that the variation of stratum almost parallels to the variation of surface topography. Up and down undulation of the stratum exists in the north-south direction. A general dipping to the south is observed.
3)The structures in the southeastern side the Hsiaomiao fault shows quick dipping to the southeast. This is interpreted as a ramp which may trap the earthquake hypercenter. | en_US |