Integrating geologic and geodetic estimates of slip rate in southwestern Taiwan: implications of 2-D elastic dislocation modeling for seismogenic zones of fold-and-thrust belts

The Taiwan orogenic belt is proposed a basal detachment fault beneath the fold-and-thrust belt of the Western Foothills created by thrusts during crustal shortening. The southwestern Western Foothills lie within a tectonically complex area that is undergoing rapid and intense surface deformation, based on geodetic observations. In the study area, the frontal thrust belt forms an extended marine terrace and the tableland is covered by Holocene marine deposits due to postglacial sea-level rise. We first use data from 20 core logs and 14C dating data to reconstruct the stratigraphic architecture and estimate the Holocene slip rate along individual faults, and combine that with an analysis of the geologic vertical slip rate with the short-term deformation rate derived from 1999-2006 global positioning system and precise leveling observations. In this study we first use a 2-D elastic dislocation model to simulate interseismic and long-term deformation rates in order to match the three datasets, allowing us to specify patterns of locked and creeping patches and estimate slip partitioning among faults. The best-fitting model suggests that four thrust faults of the Pingshi, Lungchuan, Chungchou, and Tainan faults branching from a basal detachment have a slip rate for fault ramp of 4 mm/yr and locking depths of 6 km for the Pingshi fault, 53 mm/yr and 8 km for the Lungchuan fault, 1 mm/yr and 7 km for the Chungchou fault, and 7 mm/yr and 5-7 km for the Tainan detachment fault, respectively. Apparently, the locked patch is distributed at the shallow depth on an updip of the fault ramp. With results indicating a rapid short-term shortening and uplifting rates during the interseismic period, the model predicts a creeping detachment beneath the Western Foothills in southwestern Taiwan.