Abstract
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.
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