Aspects of larger-sized earthquakes in the Taichung-Changhua-Nantou Area, Taiwan

Abstract In this study, we compiled historical and instrumentally-recorded larger-sized earthquakes with M s ≥5 in the Taichung-Changhua-Nantou area, Taiwan. Totally, thirty-one M s ≥5 earthquakes, including mainshocks of earthquake sequences and the larger-sized events of two swarms, occurred in the area from 1795 to 2025 are taken into account. The epicentral distribution demonstrates that the earthquakes happened mainly in Nantou. The temporal variation in earthquakes displays irregular recurrence behavior with low periodicity. For the whole area, the time series may be separated into three time intervals. Considering the inter-occurrence time between two sequent events, the largest value was 22,242 days (60.94 years) and the smallest one was 7 days (0.0193 years). The average inter-occurrence times for the whole time interval, the first, second, and the third time intervals are, respectively, 2656.70 days (7.28 years), 3604.64 days (9.88 years), 853.73 days (2.34 years), and 6811.00 days (18.66 years). In terms of inter-occurrence time, observed probabilities only slightly deviate from the theoretical Poisson probability function. The activity of earthquakes is slightly high in September, November, and December, and low in May and August. Seismic activity is about two times higher in the local night-time than in the local daytime, thus indicating that the effects on generating M s ≥5 earthquakes could be higher from lunar tides than from solar tides.