Resolving the rupture directivity and seismogenic structure of the 2018 Taiwan Shoal MW5.7 earthquake

Abstract

On November 26, 2018, a Mw5.7 earthquake occurred on the northern edge of the Taiwan Shoal. The epicenter was not on the known deep fault, and the direction of the rupture was doubtful due to the lack of near-station control. Based on the broadband station recordings in Fujian, Guangdong and Taiwan, we use microseismic detection technology to obtain a more complete aftershock sequence. The number of detected aftershocks is 4 times that of the Fujian network catalogue. These aftershocks are distributed in a 2*10 km east-west trending strip. In addition, the focal mechanism solutions of the main shock and five strong aftershocks were inverted by the GCAP method. The inversion results showed that the main shock and the strong aftershocks were both strike-slip earthquakes with high dip angles, and the principal compressive stress direction was in the NW-SE direction. The obtained focal depths are slightly different, the focal depth of the main shock is 14 km, and the focal depth of strong aftershocks above Mw3.9 is between 12 and 17 km. There are significant differences in aftershock activities between the east and west of the main shock. The aftershock activities on the east are mainly concentrated within one month after the main shock, while the aftershock activities on the west continued to be active within the six months after the main shock, indicating that the stress on the east side is relatively fully released after the main earthquake. Moreover, the multi-channel seismic profiles passing through the epicenter reveal that the shallow active faults in the epicenter are EW, with significant strike-slip characteristics, and their spatial locations are consistent with the distribution of aftershocks and the focal mechanism solution. Based on the temporal-spatial distribution of aftershocks, focal mechanism solutions and the characteristics of shallow active faults, we inferred that the seismic fault of the Mw5.7 earthquake is a near east-west trending Taiwan Shoal Fault, which may be an extension of the B Fault of Taiwan Island. The strong right-handed shear stress in the upper crust generated by the lateral subduction rate difference is the dynamic cause of the 2018 Taiwan Shoal Mw5.7 earthquake.