We investigated the effect of changes in static stress caused by the 1999 ML 7.3 Chi-Chi earthquake on the surrounding fault systems in Taiwan. We examined this problem from two perspectives. First, we applied a statistical test for the hypothesis of static-stress triggering by comparing the distributions of the Coulomb failure stress changes on the focal mechanism nodal planes both before and after the mainshock. Since preshocks could obviously not have been affected by the mainshock, the deviation of these distributions determined the significance of static-stress triggering. Second, we calculated the Coulomb failure stress changes on the Chukou and Meishan faults in specified slip directions to evaluate the stability of these two active faults after the Chi-Chi earthquake. The static-stress changes were obtained by applying the theory of dislocation in an elastic half-space to a 3D rupture model derived from the surface displacements associated with the Chi-Chi earthquake. Our results showed that the distributions of the Coulomb failure stress changes for events preceding and following the Chi-Chi earthquake were significantly different, yet consistent with the hypothesis of static-stress triggering. We also found that both the Chukou and the Meishan faults were in stress shadows at depths shallower than 10 km. However, because a number of aftershocks fell in these stress shadows, it is possible that other mechanisms, such as dynamic stress, were responsible for triggering these events.

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