Comparison Study on the Coseismic Deformation of the 1999 Chi-Chi Earthquake and Long-term Stream Gradient Changes Along the Chelungpu Fault in Central Taiwan


The largest inland earthquake (ML = 7.3) of the 20tg century in Taiwan occurred at 0147 hrs local time on 21 September, 1999 near the small town of Chi-Chi, in central Taiwan. The crustal deformation pattern, caused by the Chi-Chi earthquake, appears to be unrelated to the velocity field that GPS stations measured in the period 1992 to 1995. It exhibits a greater horizontal and vertical slip in the northern block between the Tachia River and the Wu River than in the southern block between the Wu River and the Tzoshui River. This study compares the stream profiles of three rivers in central Taiwan, namely the Tachia, Wu and Tzoshui Rivers, which run westward across the Shuilikeng fault, the Shuangtung fault , the Chelungpu fault and the Changhwa fault. Historical change of stream gradient is retrieved by a comparison of previous maps published by the Japanese authorities in 1904 and produced by the Department of Interior, ROC in 1985. The results, simulated by the diffusion model, indicate that the changes are attributable to the crustal movement in this area over the past 80 years. It is concluded that crustal deformation has been more evident in the block between the Tachia River and the Wu River than that between the Wu River and the Tzoshui River. This result agrees with the faulting net slipcaused by the Chi-Chi earthquake. The method used in this study has been verified as a feasible tool in the research of earthquake prediction and seismic hazard assessment. Further studies are necessary in such a tectonically active area as Taiwan.

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