The importance of the dynamic source effects on strong ground motion during the 1999 Chi-Chi, Taiwan, earthquake: Brief interpretation of the damage distribution on buildings

The 1999 Chi-Chi, Taiwan, earthquake, that originated on a low-angle reverse fault, showed complexity and uncommon characteristics. The records show that the hanging-wall side is characterized by larger particle motions than the footwall, and the ground motion is stronger in the northern part than in the southern part of the causative fault. Although the strongest ground motion occurred near the northern part of the trace, structural damage was heavier in the southern part. In order to get a better understanding of the complex damage distribution caused by this earthquake, the dynamic rupture process was numerically simulated. Because of the differences between the observed features of the rupture process in the northern and southern parts of the fault, each part was modeled independently by using a 2D discrete element model (DEM). The principal results of the simulation show that the velocity ground motions in the northern part, in the frequency range of 0.5-2 Hz (natural frequency range of standard structures), are small near the surface break, thus, light structural damage might be predicted near the surface rupture. Moreover, in the northern part the fault rupture propagation reaches the surface with a very slow velocity (about 1.2 km/sec); however, in the southern part the rupture propagation reaches the surface with higher velocity (about 3.0 km/sec). These differences between the models could explain why the ground motion near the surface rupture in the northern part caused less damage in structures than the ground motion in the southern part.