The 2010 ML 6.4 Jiashian earthquake was a significant earthquake in the fold-and-thrust belt region in southwestern Taiwan, rupturing along the Chihshan Transfer Fault zone. In this study, we collected data from a temporary seismic network deployed after the mainshock to construct an aftershock catalog containing 2506 events, surpassing the event counts of previous studies, using deep-learning-based tools. The aftershocks are then relocated using the double-difference method and used as the input for a 3D seismic tomography inversion combined with regional travel time data. The alignment of the relocated earthquake sequence reveals the structure of the seismogenic fault, trending in the northwest direction and gently dipping to the northeast at a depth of 15 km to 25 km, with three clusters: Cluster 1 aligns with the mainshock's northwest propagation, while Clusters 2 and 3 emerge southwest and south, indicating subsidiary faults. The seismic tomography images also reveal the structure of the major Chaozhou Fault in the area as the boundary between contrasting velocity zones and the seismogenic zone characterized by low velocity and a high Vp/Vs ratio structure. We determined 329 new focal mechanism solutions by first-motion P-wave polarity inversion. The P-axes of the aftershocks are predominantly oriented northeast-southwest, with greater variability in the north-south and northwest-southeast directions for earthquakes with magnitudes less than 2. The results of this study reveal the complexity of the Chihshan Transfer Fault zone and southwestern Taiwan's seismotectonics.
