Finite fault inversion of the September 25, 1999 (Mw=6.4) Taiwan earthquake: Implications for GPS displacements of Chi-Chi, Taiwan earthquake sequence

[1] The September 25, 1999 Chi-Chi Taiwan aftershock (Mw = 6.4) occurred on a down dip extension of the fault ruptured in the mainshock. Strong motion data were used to invert for the finite-source process and test for the causative fault plane. We performed a grid-search over a range of focal mechanisms and found a preferred model (strike = 5degrees, dip = 30degrees, slip = 100degrees) different from teleseismic studies (strike = 28 +/- 10degrees, dip = 27 +/- 5degrees, slip = 106 +/- 9degrees) but similar to the mainshock (strike = 5degrees, dip = 34degrees, slip = 65degrees). The aftershock asperity has a dimension of 10 km 10 km with a maximum slip of similar to1.8 m and a static stress drop of 8 MPa. We forward-predict the GPS displacements and found up to 0.033 m of surface horizontal displacements at some GPS sites, indicating that studies of post-seismic deformation may need to account for the effects of large aftershocks.