Abstract

We study the stress regime associated with the 1999 Chi-Chi, Taiwan, earthquake sequence. Broadband waveforms of 115 events recorded by the Broadband Array in Taiwan for Seismology (BATS) are analyzed to determine their focal mechanisms, depths, and seismic moments. These solutions are then used to invert for the reduced stress tensor using the algorithm of Angelier (1998). We perform the stress inversion in three different ways. First, we aim at defining, in the first approximation, a uniform stress field by taking the entire dataset as a whole. The inversion result shows a stress regime with the maximum compression along the direction of N119°E, remarkably consistent with the direction of the relative motion between the Philippine Sea plate and Eurasia. This solution is stable and varies little in terms of the orientations of stress axes (within 8°) and the ratio of stress differences (between 0.26 and 0.44). In the second approximation, we aim at explaining more subtle features of our observations by considering differently built subsets. We divide the dataset according to different seismogenic structures as identified by Kao and Chen (2000). Our results yield rather similar results (axes of maximum compressive stress fall in the range of N99-138°E in terms of trends, and in the range of 1-17° in plunges) for all subsets except a small group in the footwall. Furthermore, we observe a systematic clockwise change in the direction of compression from south to north, representing the more general fan-shaped pattern of stress trajectories that characterizes the whole collision zone. Finally, the inversion is performed for the three main types of faulting, that is, reverse, strike-slip, and normal types. The axis of maximum compression for the reverse mechanisms has the same direction as that for the strike slip, whereas the normal type subset yields a N54°E extension that trends obliquely to the compression. The fact that the stress regime associated with the Chi-Chi aftershock sequence is similar to that derived from geological and seismic data in the region implies that the stress regime corresponding to the regional collision in Taiwan is responsible for the occurrence of the Chi-Chi earthquake. Moreover, systematic, secondary variation in the state of stress is observed for different types of focal mechanisms and seismogenic structures. Such variation may play an important role in producing local stress concentration that leads to the nucleation of disastrous earthquakes in central Taiwan.

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