Is a Large Earthquake Coming Soon due to High Strain Rate in SW Taiwan?

Abstract:

A high shortening rate of 0.5-1.7 μstrain/yr from the previous GPS observations across the frontal thrust belt in SW Taiwan has implied high probability of future earthquakes than in other areas of Taiwan. However, no significant earthquake has occurred since 1964 M6.3 Baihe earthquake on this fault system. In this study, we therefore adopted the geodetic data from 109 campaign-mode GPS stations, 61 continuous GPS stations and 388 leveling benchmarks from 2002 to 2011 to comprehend the characteristics of modern crustal deformation in SW Taiwan. Horizontal and vertical velocities inferred from GPS and leveling data are relative to the Chinese continental margin. Horizontal velocities decrease, from SE to NW, from ~54 mm/yr to ~1 mm/yr. A significant shortening of ~15 mm/yr has been noticed between the Liuchia (west) and Chukou (East) faults. The vertical velocity field represents ~10 mm/yr east of the Liuchia fault with the maximum rate of ~12.5 mm/yr. The subsidence rates larger than 10 mm/yr in the westernmost coastal area, reflecting the effect of artificial groundwater withdrawal. To estimate the slip rates on faults, we inverted the geodetic data using a 2D elastic kinematic fault model. We assumed a fault system comprising a creeping décollement and several branched faults locked near the surface in an elastic half space. The fault slip rates are estimated by least squares, while the optimized fault geometries and locking depths are searched by the Monte-Carlo inversion. Based on our optimal model, the probability to generate the next large earthquake on the branched active faults in SW Taiwan is low. However, this high strain accumulation may be released by the future earthquakes on the deep buried faults or by the transient slip events on the décollement.