Focal mechanism and seismogenic structure of the 2019 Yellow Sea M_L4.6 earthquake sequence

The 2019 Yellow Sea M_L4.6 earthquake sequence occurred near the NW-trending Subei-Binhai fault zone in the Yellow Sea, where occurred many destructive earthquakes in history. In order to determine whether the Yellow Sea earthquake sequence induced by the Subei-Binhai fault zone, we used the digital waveforms of seismic network within 400 km of the epicenter of M_L4.6 mainshock, carried out inversion for the focal depth and mechanism of the Yellow Sea M_L4.6 earthquake on 8 December 2019 and Yellow Sea M_L4.1 earthquake on 12 December 2019 with the CAP method, and then relocated the Yellow Sea earthquake sequence using the HypoDD method. Our result shows that the focal depths of the Yellow Sea M_L4.6 and M_L4.1 were 20 km and 21 km, suggesting that the two events occurred in the brittle-ductile transition zone within the seismogenic region. For the Yellow Sea M_L4.6 event, the strike, slip and rake are 123°, 74°, and 61° for nodal plane Ⅰ , and those are 6°, 33°, 149° for nodal plane Ⅱ , respectively. For the Yellow Sea M_L4.1 earthquake, the strike, dip and rake are 135°, 77° and 32° for nodal plane Ⅰ , and those are 37°, 59° and 165° for the nodal plane Ⅱ , respectively. The nodal parameters of source mechanism for the two events are inconsistent with the geometric parameters of the Subei-Binghai fault zone, indicating that the Yellow Sea earthquake sequence has no direct relationship with the main fracture of Subei-Binhai fault zone. The relocation for Yellow Sea earthquake sequence shows that M_L4.6 earthquake sequence distributed in NW direction. According to the focal mechanisms from inversion and the relocation of the earthquake sequence, we deduced that the M_L4.6 and M_L4.1 earthquakes ruptured in NW direction, and the Yellow Sea M_L4.6 earthquake sequence is a left-lateral slip event induced by northwestward shearing rupture under tectonic stress field in the brittle-ductile transition zone.