Earthquake Focal Mechanisms and Stress Environment in Southern Taiwan

We conduct a systematic examination of the stress field environment surrounding the 4 March 2010 Jiahsian earthquake (MW 6.3) in southern Taiwan. First, we perform waveform inversion to determine the focal mechanisms of 318 earthquakes of magnitudes 3.5 ≤ ML ≤ 6.4 that occurred between January 1997 and December 2010 within 50 km of the Jiahsian epicenter. These focal mechanisms are then used to derive the orientations of the principle stress axes using a damped regional-scale stress inversion algorithm. Our results show that southern Taiwan is dominated by two types of stress environments: the overall northwest–southeast compression in the Western Foothills and the Longitudinal Valley resulting from the convergence of the Philippine Sea and Eurasian plates and the northeast–southwest extension due to the exhumation of the Central Range. Moderation by local conditions led to areas with distinct types of stress conditions: thrust in the Western Foothills and the Longitudinal Valley, strike-slip in the Central Range north of 23° N, normal in the southeastern Central Range, and mixed-type across the Chaochou Fault. Significant earthquakes, such as the 1999 Chi-Chi and the 2010 Jiahsian earthquakes, have a detectable influence on the regional stress field.