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 (M-L=7.3) of the 20th century in Taiwan occur red at 0147hrs local time on 21 September, 1999 near the small town of Chi- Chi, in central Taiwan, The crustal deformation pattern, caused by the Ch-C hi earthquake, appears to be unrelated to the velocity field that GPS stati ons measured in the period 1992 to 1995, I exhibits a greater horizontal an d vertical slip in the northern block between the Tachia River and the Wu R iver 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 Sh uilikeng 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 b y the Department of Interior, ROC in 1985, The results, simulated by the di ffusion model, indicate that the changes are attributable to the crustal mo vement in this area over the past 80 years. It is concluded that crustal de formation has been more evident in the block between the Tachia River and t he Wu River than that between the Wu River and the Tzoshui River. This resu lt agrees with the faulting net slip caused by the Chi-Chi earthquake. The method used in this study has been verified as a feasible tool in the resea rch of earthquake prediction and seismic hazard assessment, Further studies are necessary in such a tectonically active area as Taiwan.