Rapid afterslip following the 1999 Chi‐Chi, Taiwan Earthquake

Postseismic displacements of as much as 14 cm were recorded by GPS measurements in the 3 months following the MW 7.6 1999 Chi‐Chi, Taiwan earthquake. Data from 35 continuous and 90 campaign‐surveyed stations, which show continued east over west thrusting, are analyzed to estimate the postseismic slip distribution and fault geometry. Assuming the shallow fault dips 24° E, as determined by numerous studies of the mainshock, we invert for the deeper fault structure. Our results show that the fault dip shallows with depth below the hypocenter, merging into a nearly horizontal decollement at a depth of 8–12 km. The afterslip distribution shows a maximum slip of 25 cm in the hypocentral region at 7–12 km depth as well as significant slip on the lower decollement. Afterslip is notably absent in the region of maximum coseismic slip, consistent with the afterslip being driven by the mainshock stress change.