Characteristics of strong ground motion across a thrust fault tip from theSeptember 21, 1999, Chi-Chi, Taiwan earthquake

Near fault tip strong motion records from the northern part of the major ea rthquake (Mw=7.6), namely the Chi-Chi earthquake on September 21, 1999 in c entral Taiwan demonstrated systematic differences on the hanging wall and f ootwall, and simulated by the finite element method. The extraordinary grou nd motion differences on either side of the northern fault tip can be expla ined by a 2-dimension kinematic source model with fault rupture breaking to surface. In this study, the earthquake faulting was considered as bilatera l from the center of a low angle thrust fault which is 30 km in length with a dip angle of 31 degrees. Based on waveform modeling, the source rupture velocity, rise-time and dislocation of 2.0 km/sec, 5 sec and 6 meters, resp ectively are suggested. The results of this study show that on the northern part of the Chi-Chi earthquake fault there was lower rupture velocity and longer rise-time of the fault slip than that previously reported. Furthermo re, the effects of surface breaking from the fault movement contributed lar ge ground deformations near the fault tip and, consequently, induced extens ive damage.