The Chi-Chi earthquake sequence: Active, out-of-sequence thrust faulting in Taiwan

We combined precise focal depths and fault plane solutions of more than 40 events from the 20 September 1999 Chi-Chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central Taiwan is a moderately dipping (20 degrees to 30 degrees) thrust fault away from the deformation front. A second, subparallel seismic zone Lies about 15 kilometers below the main thrust. These seismic zones differ from previous models, indicating that both the basal decollement and relic normal faults are aseismic.