Resolving the 1906 Mw 7.1 Meishan, Taiwan, earthquake from historical seismic records

The 1906 Mw 7.1 Meishan earthquake, named after the village near where it occurred, was one of the most damaging earthquakes in Taiwan in the early twentieth century. The historical literature and former studies claimed to show that this earthquake was related to the Meishan fault, which had about 12.5 km right-lateral surface rupture orientated in an east-west direction. However, the surface rupture is short with respect to the magnitude of the earthquake. The north-south pattern of damage is not consistent with the strike of the surface rupture. These apparent inconsistencies raised severe doubts regarding the exact mechanism of the earthquake. Using the original Omori records of the 1906 Meishan earthquake from the historical Taipei,Taichung, and Tainan stations, we carried out an effective waveform simulation to evaluate several different geological fault models. Synthetic Omori records from these geological fault models clearly show a discrepancy in the first motions of P and S waves at the Tainan station, which suggest that the focal mechanism may not be a pure strike slip. We used observed P- and S-wave first motions in the historical Omori records as a constraint and performed a grid search to find possible focal mechanisms. The preferred focal mechanism is an oblique thrust fault oriented in the northeast-southwest direction, with a small right-lateral component. Such a focal mechanism not only reproduces the general feature shown in the observed intensity map, it is also more consistent with the spatial distribution of significant aftershocks, which are predominantly located toward the north and south of the epicenter. This result highlights the importance of historical recordings in resolving the earthquake mechanism in a complex fault system.