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 = 5¿, dip = 30¿, slip = 100¿) different from teleseismic studies (strike = 28¿10¿, dip = 27¿5¿, slip = 106¿9¿) but similar to the mainshock (strike = 5¿, dip = 34¿, slip = 65¿). The aftershock asperity has a dimension of 10 km ¿ 10 km with a maximum slip of ~1.8 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.