The Chichi earthquake, 1999, and its role in the Taiwan orogen

The destructive Chichi earthquake occurred in Taiwan on 21 September 1999. This earthquake had exceptional consequences in terms of both the seismic hazard concerns that it raised in the island and the new seismotectonic and geodynamic concepts that arose concerning the Taiwan collision. This earthquake is quite typical of the SE-NW plate convergence. It reactivated a thrust ramp at the front of the belt. The rupture trace shows typical tectonic patterns that control the distribution of the damage associated with fault slip. The importance of geological studies for earthquake mitigation purposes is thus highlighted. Based on two examples (Chelungpu fault for locking and earthquakes, Chihshang fault for creep), the need for a better definition of the seismic cycle in Taiwan is highlighted. The Chichi earthquake suggests that the average shortening velocities are of the same order at the belt front and in the suture zone in the back of the belt. The concept of double convergent boundary is consequently introduced in the lithospheric model of Taiwan. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.