Interseismic crustal deformation of frontal thrust fault system in the Chiayi–Tainan area, Taiwan

Abstract

We derive a velocity field using GPS data between 1993 and 2007 in the Chiayi–Tainan area located in the deformation front of the Taiwan mountain belt. The crustal motion with respect to Penghu shows large velocities of about 33–44mm/yr in the west-northwest to west directions in the Western Foothills and the velocities decrease westward to 0–5mm/yr in the coastal area. Significant uplift rates of 5–20mm/yr are observed at sites to the east of the Jiuchiunken–Muchiliao–Liuchia Fault (JMLF) system. We use a block model, a buried dislocation model, and a two-dimensional fault model to invert for fault geometries and slip rates on major frontal thrust faults. Modeling results from a block model show the inferred long-term slip rate of 42mm/yr in the direction of 280° and the maximum back-slip rate of 38mm/yr on a 23° east dipping fault extending to 13km at depth. On the other hand, the buried dislocation model results in a horizontal décollement at a depth of 8km with a uniform slip rate of 41.6mm/yr. If we connect the top edge of décollement to the surface trace of JMLF as a potential future rupture, a 22° east-dipping fault is required. Results from both block model and buried dislocation model suggest the JMLF is nearly fully locked. The results of two-dimensional fault models show the frontal thrust faults have slip rates of less than 2mm/yr at shallow depths and the inferred décollement is sub-horizontal (5°–7°) at a depth of 10km with slip rates of 44–46mm/yr. Results of various approaches show general agreement on fault geometries and slip rates and reveal that the frontal thrust fault system has a high potential for large earthquakes.