Wang Shanxiong, director of Fujian Earthquake Disaster Prevention Center
Li Zuning, director of Fujian Earthquake Prediction Research Center
At 7:57 a.m. on November 26, 2018, a magnitude 6.2 earthquake occurred in the southern part of the Taiwan Strait (23.28ºN, 118.60ºE) with a focal depth of 20 kilometers, which was widely felt in the coastal areas of Fujian. As of 8:00 a.m. on November 29, a total of 199 aftershocks had been recorded, including 20 magnitude 3 earthquakes and 8 magnitude 4 earthquakes, with the largest aftershock being ML4.9 (Figure 1). The Taiwan Strait, where the 6.2 magnitude earthquake is located, is located in the northern section of the southeast coastal seismic belt, and the historical strong earthquakes around the earthquake area are relatively active, with 13 magnitude 5 earthquakes, 1 magnitude 6 earthquake, and 1 magnitude 7 earthquake within 100 km of the epicenter, and the largest earthquake was the 7.3 magnitude earthquake on September 16, 1994, about 33 km from this earthquake (Figure 2). In addition, the Taiwan Strait and its surrounding areas also had a strong earthquake of magnitude 7.5 overseas in Quanzhou in 1604 and a magnitude 7.3 earthquake in the waters of Nan'ao, Guangdong in 1918, and the 6.2 earthquake was the first medium-strong earthquake in the entire southeast coastal seismic zone of magnitude 5.5 or above after nearly 24 years of calm.
The Taiwan Strait is located at the leading edge of the Taiwan orogenic belt, from north to south, gradually transitioning from the typical continental crust to the oceanic crust, that is, from the edge of the Asian continent to the marine plate of the South China Sea. Therefore, in the southwest coast of Taiwan, its geological structure has also become more complex, and it has begun to change from a relatively stable continental edge to a plot affected by the Manila Trench. Two of the most active modern fault zones in the coastal areas of Fujian (the north-east coastal fault zone and the north-west bus fault zone) are located in the Taiwan Strait. The coastal fault zone is a seismic structure of strong earthquakes in the sea area on the west side of the Taiwan Strait, which is roughly distributed along the 50m isobath line along the Fujian coast, coinciding with the turning zone of the underwater slope of the seawater, about 500 km long, the fault is generally oriented to 30 to 50 ° north and east, the overall tendency is south-east, and the inclination angle is steep. It is composed of multiple secondary fault ramps, separated by north-west faults. The bus fault zone refers to a set of north-west fault zones in the southeastern Fujian-eastern Guangdong land region that extends to the southern end of Taiwan through the Taiwan Strait, located on the south side of the Taiwan Strait, the northern boundary extends from near the Zhangzhou Basin in Fujian to Kaohsiung, Taiwan, and the southern boundary extends from the Chaoshan Basin in Guangdong to the sea south of Taiwan Island, affected by the movement of the Philippine plate, which is a shear fault zone dominated by the nature of left-handed slippage, and earthquakes in the belt are also frequent.
The solution of the source mechanism of the 6.2 magnitude earthquake shows that the earthquake has two possible fault levels near north-south and near east-west (Figure 3). There are three main sets of faults near the epicenter (Figure 4): the Dongshan Overseas Section of the North-East Coastal Fault Zone (F1-5), the North-West Penghu Uplift South Rim Fault (F2-2) (Bus Fault), and the Near-North-South Dongshan Uplift East Margin Fault (F3).
According to the energy vibration distribution map, it is distributed in a near-north direction (Figure 5), and the seismic structure of the 6.2 magnitude earthquake is preliminarily determined to be the East Rim Fault of the Dongshan Uplift. The Dongshan Uplift East Rim Fault is located between the Dongshan Uplift and the Xiamen Peng Depression. In summary, it is determined that the fault nature of the 6.2 magnitude earthquake is left-handed sliding, the fault level trend is nearly north-south, the inclination angle is nearly vertical, and the main pressure stress axis is north-west-south-east, which is consistent with the direction of the Philippine plate movement.
On September 16, 1994, a magnitude 7.3 earthquake struck the southern part of the Taiwan Strait. The source mechanism shows that the fault nature of the 7.3 magnitude earthquake is positive fault, and the main rupture direction is north-west, which is consistent with the bus fault zone. The early reason for the formation of positive faults was formed by the tension during the tension period of the South China Sea, and then due to the convergence of the Eurasian and Philippine plates, it was transformed into a fault activity dominated by slippage, which is very similar to the deep seismic seismic structure in the western region of Taiwan. The 6.2 earthquake, like the 7.3 magnitude earthquake of 1994, occurred in the boundary fault zone of the Pre-Tertiary Foreland Basin. In this area, the general first principal stress is vertical, resulting in a positive fault, due to the distance from the plate boundary, the difference between the three main stresses is not large, and the leading edge of the foreland may make the main stress exchange alignment, so that the second principal stress becomes a vertical direction, then the slip mechanism will occur, which is the reason why the earthquake is a slippery nature.