Analysis of strong earthquake activity characteristics in Taiwan

Based on qualitative analysis and Morlet wavelet method, the distribution characteristics of the main tectonic zones and strong earthquakes in Taiwan, China are studied. The Taiwan, China can be divided into the Taidong seismic zone and the Taixi seismic zone, bounded by the Central Mountains. The former is mainly composed of the Huadong longitudinal valley fault, the coastal mountains and the sea area to the east of the coastal mountains. It is the region with the strongest seismic activity in Taiwan. The latter seismic zone mainly includes the western foothills and the western coastal plain. In addition, due to the influence of the Philippine Sea Plate pushing the Eurasian Plate towards the northwest, most earthquakes in Taiwan and surrounding waters are thrust-type earthquakes. 　　There have been three active periods of M_S≥7.0 earthquakes in Taiwan since 1900. The first active period was from 1902 to 1925, which lasted nearly 23 years; the second was from 1935 to 1978, about 43 years; the third was from 1986 to 2006, lasting 20 years. Since the Hengchun sea area M_S7.2 earthquake in Taiwan on December 26, 2006, Taiwan earthquakes above M_S7.0 have been quiet for nearly 16 years, which is the longest quiet time in history, and there is a possibility of a new active period. 　　From the regional distribution perspective, M_S≥6.9 earthquakes in the eastern Taiwan had six active cycles, most of which lasted about 16 years on average. Each active cycle included active and quiet periods, and all M_S≥6.9 earthquakes occurred in active periods. Statistics show that the activity intensity of the eastern Taiwan had gradually weakened since 2002, when it entered the sixth active cycle. A new active period may have started since the Hualian M_S6.9 earthquake occurred in the eastern Taiwan on September of 2022. At the same time, the Morlet wavelet method was used to calculate the period spectrum of seismic activity of the Taidong seismic zone and the significance testing. The results showed that the 3-year and 16-year periods in the region passed the 80% confidence testing, and the 16-year period could better reflect the average duration of most seismic periodic activities in the Taidong seismic zone. The 3-year period was consistent with the average occurrence interval of M_S≥7.0 earthquakes with every three years in the Taidong seismic belt from 1900 to 2006. As for the western Taiwan, M_S≥6.0 earthquakes have cycles of about 92 years and 14 years which passed the 80% confidence testing. 1901−1993 was a large active-quiet cycle （about 92 years） and a new round of large cycle activity began from 1994. At the same time, the large cycle also included some small cycles with an average period of 14 years. At present, the Taixi seismic belt is in the quiet period of the small cycle of 2010 to 2022. Based on the average duration of the small cycle of about 14 years, the Taixi seismic belt maybe enter a new round of the small cycle activity of M_S≥6.0 earthquake in the future. 　　From the perspective of focal depth, the focal depth of earthquakes in Taiwan has the characteristic of gradually deepening from west to east. Among them, earthquakes in the northeast of Taiwan and nearby waters are mainly of medium to deep source earthquakes, while earthquakes in the waters of the central Taiwan are mostly distributed within the range of 20−40 kilometers, mainly located in the waters near Hualien. The distribution characteristics of source depth are consistent with the eastward dipping characteristic of the Huadong longitudinal valley fault in the region. The accumulation level of small earthquakes calculated by the Nowcasting method also shows that the M_S≥7.0 earthquakes in the eastern zone of Taiwan and M_S≥6.0 earthquakes in the western zone of Taiwan have a high background probability of earthquake occurrence, and the strong earthquakes in Taiwan have a certain corresponding relationship with the moderate-strong earthquakes in South China on an annual scale.