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

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 33-44. mm/yr in the west-northwest to west directions in the Western Foothills and the velocities decrease westward to 0-5. mm/yr in the coastal area. Significant uplift rates of 5-20. mm/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 42. mm/yr in the direction of 280° and the maximum back-slip rate of 38. mm/yr on a 23° east dipping fault extending to 13. km at depth. On the other hand, the buried dislocation model results in a horizontal décollement at a depth of 8. km with a uniform slip rate of 41.6. mm/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 2. mm/yr at shallow depths and the inferred décollement is sub-horizontal (5°-7°) at a depth of 10. km with slip rates of 44-46. mm/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.