Two-dimensional numerical modeling for near source strong ground motions of the Chelungpu fault during the 1999 Chi-Chi Taiwan earthquake

Two-dimensional numerical modeling based on the finite element method is employed to simulate near fault tip Strong motion records of the Chi-Chi Earthquake on September 21, 1999, in order to examine the kinematics of earthquake faulting and near field wave propagation properties, The modified 'split-node' technique of Melosh and Raefsky is employed to simulate the equivalent body forces system for a double couple without moment. Different source parameters are examined to synthesize ground motions to simulate the observed near source strong motion records. Based on waveform modeling, a low angle thrust fault 30 kin in length with a dip angle of 30degrees is suggested for the ruptures near its northern end. The rupture is interpreted as bilateral from the center of the fault. The determined source rupture velocity and rise-time are 2.0 km/sec and 5 sec, respectively, while the dislocation is about 6 meters. The estimated peak fault slip velocity is about 3.5 km/ see, showing a largest value than previously reported. Results of this study show that there was lower rupture velocity and longer rise-time of the fault slip on the northern part of the Chi-Chi Earthquake fault. Numerical modeling of this study indicates that the directivity of source rupture does not show significant low frequency ground motion near the earthquake fault tip. However, the effects of surface breaking of the fault movement have contributed large ground deformations near the fault tip, consequently, inducing large damage.