Focal mechanisms and seismotectonic stress in North Central Taiwan in relation with the Chi-Chi earthquake

The aftershock sequence of the 1999 Chi-Chi earthquake (Mw = 7.6) provided a large number of focal mechanisms recorded with BATS Centroïde Moment Tensors (CMT) which allowed us to perform reliable stress tensor inversions in North Central Taiwan. These inversions were done for three time-periods corresponding to the post-seismic phase of the 1999 Chi-Chi earthquake (Mw 7.6). In the last period the 2003 Chengkung earthquake (Mw = 6.5) occurred on the vicinity of our study area and its influence is also examined. For comparison between seismotectonic regimes of the pre- and post- Chi-Chi periods, we also carried out stress inversion using published focal mech of the pre Chi-Chi period.For each period, we delineated seismogenic domains with relatively homogeneous stress regimes in agreement with the local structures. The boundaries of these domains vary during the whole post Chi-Chi period as seismic activity decays. A 25° clockwise rotation of the main compressive stress σ1 is observed between the pre and the post Chi-Chi periods in the domain located near the northern termination of the Chi-Chi rupture. A stress perturbation related to the Chengkung earthquake could be detected in the eastern part of the Central Range. We present an interpretation of the determined stress regimes in terms of changes in mechanical behaviour of the Sanyi–Puli transfer zone.