Resolving the 1906 Mw 7.1 Meishan, Taiwan, Earthquake from Historical Seismic Records

The 1906Mw 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 Pand 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. Electronic Supplement: Figures of Pand S-wave polarities, simulated intensity maps, and simulated and observed seismograms, and table of parameters of the instrument responses from Omori’s and Gray–Milne seismographs for the ancient stations. INTRODUCTION The 1906 Meishan earthquake, one of the most damaging earthquakes in Taiwan’s history, occurred in the southwestern Taiwan at local time 6:43 a.m. on 17 March. It is located at 23.550° latitude, 120.450° longitude, and 6 km depth (Yeh et al., 1998; Fig. 1). The magnitude of this earthquake was MH 7.1, first determined by Hsu (1980a,b), and M s 6.8, subsequently determined by Abe and Noguchi (1983). This earthquake caused 1258 deaths, 2385 injuries, and leveled 6769 houses. According to the Omori (1907) field investigation and the historical literature (Taiwan Governor-General Office of Civil Affairs, 1907; Kondo, 1906), a right-lateral surface rupture with a length of about 12.5 km, identified as the Meishan fault, and liquefactions were observed along the east– west direction (Fig. 2). Thus, the 1906 Meishan earthquake had long been considered as a significant event from the Meishan fault with an east–west strike-slip focal mechanism. However, the length of the fault is disproportionally short compared to the damage caused by this Mw 7.1 earthquake. The intensity maps compiled by government agencies (Fig. 3a,b,d) show a general north–south pattern, rather than the east–west intensity pattern reported by Omori (1907; see also Fig. 3c). Significant aftershocks were also located to the north and south of the epicenter (Fig. 1). These inconsistent observations had raised long-standing debates regarding the understanding of this devastating historical earthquake. In 1906, Taiwan was administered as colony by the Japanese government which installed four seismic stations (Fig. 1) over the main island of Taiwan (TAP, TCU, and TAN) and a small island of Penghu (PNG) with the Omori instrument and Gray–Milne seismograph. It was fortunate that valuable seismograms recorded by these stations were discovered in the earthquake data archive project conducted by the Earthquake Research Institute of Tokyo University (Noguchi et al., 2001). These well-preserved historical waveforms allow us to use doi: 10.1785/0220170285 Seismological Research Letters Volume XX, Number XX – 2018 1 SRL Early Edition