Reassessing the complexity of the rupture of the 2010 Jia-Shian earthquake (Mw 6.2) in Southwestern Taiwan by inverting jointly teleseismic, strong-motion and CGPS data

International audienceA new interpretation is proposed for the 2010 Jia-Shian earthquake (Mw 6.2) which occurred on March 4, 2010 in southwestern Taiwan. Among the few tens of CGPS stations which recorded the mainshock at a distance less than 50 km we identified a few stations displaying horizontal displacements in disagreement with the dominant pattern of horizontal coseismic motion. Based on this observation, we find that a secondary rupture plane explains these peculiar motions and helps improving the modeling of the overall set of CGPS stations. We also model the teleseismic and strong-motion records to refine the rupture model. Finally, we obtain the slip distribution on a two-fault model by the joint inversion of all three datasets (CGPS, teleseismic, and strong-motion). The main fault plane is consistent with previous studies. It corresponds to a N311°E fault dipping 33° to the NE, with left-lateral–reverse oblique slip, likely related to the Chishan transfer fault zone (CTFZ). We find that the earthquake started with a sharp slip patch localized at the hypocenter. The secondary rupture plane is a N20°E thrust fault related to the Lungchuan anticline. Faulting on the main NW–SE plane during the Jia-Shian earthquake and stress inversions from previous studies both suggest a direction of maximum horizontal compression oriented NE–SW in the area of the earthquake, very different from the dominant NW–SE compression direction in Taiwan. We propose that such a variation can be related to a stress deviation in the immediate vicinity of the CTFZ, in a similar way as observed along the San Andreas Fault in California. On the other hand, rupture on the secondary fault plane associated with the Lungchuan anticline appears to be compatible with the dominant compression in Taiwan