Seismogenic Structure and Tectonic Mechanism of the 2021 Mw 5.0 Yancheng Earthquake in the South Yellow Sea Basin, East Asia

Abstract On 17 November 2021, a moment magnitude (Mw) 5.0 earthquake occurred off the Yancheng city in the South Yellow Sea. This earthquake has caused ground motions and been felt by people in a few megacities in east China, for example, Shanghai, Nanjing. However, due to limited seismic data, the focal fault and mechanism of this earthquake remain unknown. Here, we combine the seismic profiles, seismicity, focal mechanism, well data with regional tectonics to investigate the seismotectonics of this event and the possible role of fluid migration in triggering the earthquake. Our new results suggest that two sets of preexisting strike-slip faults were developed near the epicenter of the Yancheng earthquake, including the north–northeast-trending fault F1, the North Wunansha fault, and the northwest-trending fault F5. These preexisting faults have been reactivated entirely or partly with dextral and sinistral motions during the Miocene to Quaternary. Fault F1 runs through the epicenter of the Yancheng earthquake and is therefore interpreted to be the seismogenic fault. Various igneous complexes and associated hydrothermal vent complexes have been identified in the vicinity of the reactivated fault F1 and other strike-slip faults. Combined with the published igneous rock ages from nearby region, the igneous and hydrothermal activities were suggested to be continue until the Miocene and Quaternary, which are coincident with the reactivation of the preexisting strike-slip faults. The reactivation of the preexisting fault, and igneous and associated hydrothermal activities were suggested to be caused by the subduction of Pacific and Philippine Sea plates and collision of India and Eurasia plate. The hydrothermal fluids may have migrated through the preexisting fault F1 and other strike-slip faults, which enhanced the reactivation of these faults and in turn promoted the fault slip, eventually triggering the Yancheng earthquake and other historical events in the study area.