| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 94 === Investigating geometry of the Chelungpu fault, Central Taiwan, by the Seismic Reflection Method
Chien-Li Lee
Abstract
The Chelungpu fault, reactivated by the 1999 Chi-Chi earthquake (M=7.6) in central Taiwan, is a bedding thrust slipping along a weak shale layer. The fault trends in the NS direction for 90 km with the eastward average dip angle about 30 degrees. The rupture behaviors of the Chelungpu fault were quite peculiar at its northern end, 50 km north of the epicenter, including: 1) turning 90o to the east (an extra EW branch 12 km); 2) inducing huge slippages (9.8 m vietically); 3) creating high displacement and low acceleration seismic waves. These special rupture behaviors make the Chi-Chi earthquake relatively ‘unique’.
More than 300 shallow seismic lines (each 500 m long approximately) and 7 deep seismic lines (one is NS orientation, the others are EW orientation and 25 km long for each) were collected to investigate the Chelungpu fault. The deep reflection profiles provide the background structural distribution and the shallow seismic sections fulfill the gaps to build up the whole structure pattern. A 3D structure was thus established after compiling all seismic images.
Three significant findings are proposed:
1. At the northern end, the fault structure has become relatively shallow (less than 1 km). In addition, the fault surface here raises quite fast (25o up-dip) and forms a lateral ramp structure trending in the EW direction, called the TT-boundary (along the Tachia river and the Tongshih anticline). It is believed that this TT-boundary acts as a barrier to interrupt the rupture from the southern epicenter and cause abnormally large displacements at the northern end of the fault;
2. The Chelungpu fault surface (100 km x 20 km) dips 30o-45o to the east as well as 10o-15o to the south. This subsurface is interestingly in an undulation shape. There are four convex at FengYuan, Chelungpu, Tsaotun and Mingjian. These up-rising parts corresponded to abnormal peak of surface rupture.
3. The surface traces between the Changhua fault and the Chelungpu fault are similar and sub-undulated structures correspond to each other. It implies that a deep E-W trending structure controls regional geomorphology.
4. The epicenter of ChiChi earthquake may trap by lateral ramp (dipping 20 degrees to south approximately) and frontal ramp (dipping 50 degrees to east) where is 8~9 km deep at north bank of the Choshuihsi river.
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