The aftershock sequences and seismogenic structure of the 2018 Taiwan Shoal M_W5.7 earthquake

On Nov 26(th), 2018, a M(W)5.7 earthquake occurred on the northern edge of the Taiwan Shoal. There was no any known large fault across the epicenter, and the rupture directivity was doubtful due to the lack of near-station record. Based on the waveform data recorded from the seismic network of Fujian, Guangdong and Taiwan, we obtained a more complete aftershock sequence using microseismic detection method. The number of detected aftershocks is 4 times that of the catalogue from the Fujian seismic network. These aftershocks were distributed in a 2 kmx8 km east-west trending strip. Furthermore, we obtained the focal mechanisms of the main shock and five strongest aftershocks using the GCAP method. The results show that the main shock and the strongest aftershocks were strike-slip earthquakes with high dip angles, and the principal compressive stress direction was NW-SE in the epicenter area. The focal depth of the main shock is about 14 km, while the ones of the strong aftershocks above M(W)3.9 distributed between 12 and 17 km. From east to west, the aftershock activities were quite different on the time scale. After the main event, the aftershocks on the east were mainly concentrated in the first month while the aftershocks on the west continued to be active in the next six months. This indicates that the stress on the east was relatively fully released after the main event. Moreover, the multi-channel seismic profiles across the epicenter reveal that the shallow active faults in the epicenter are EW-trending, with characteristics of strike-slip. The location, trending, and properties of these shallow faults are consistent with the distribution of aftershocks and the focal mechanisms. Based on the temporal-spatial distribution of aftershocks, focal mechanisms and the features of shallow active faults, we inferred that the seismogenic fault of the 2018 M(W)5.7 earthquake is a near east-west trending Taiwan Shoal Fault, which is on the extension of the B Fault on the island of Taiwan. The lateral variation in the subduction rate of Eurasian Plate resulted in the strong dextral shear stress in the upper crust, which is the kinetic cause of the 2018 Taiwan Shoal M(W)5.7 earthquake.