Discovery of new activity of Xiahe fault in Gansu：Discussion on seismogenic structure of the 2019 Xiahe M_S5.7 earthquake

On October 28, 2019, a magnitude 5.7 earthquake struck the Xiahe county, Gansu Province. This event was located between the Lintan-Dangchang fault and West Qinling fault, where surface faults were poorly mapped and studied, and there was no fault directly related to the Xiahe M_S5.7 earthquake. In this paper, through the remote sensing interpretation and field investigation, the geometric pattern and new activity of geological faults around the Xiahe earthquake epicenter were studied preliminarily. Combined with the relocated small earthquakes and the focal mechanism solutions, the seismotectonic model of the Xiahe earthquake was built up and comprehensively analyzed. The results are as follows. First, the surrounding faults of the Xiahe earthquake include two well-known but poorly-studied faults （the West Qinling fault and Lintan-Dangchang fault） and two poorly-known faults （the Xiahe fault and Damai-Hezuo fault） that were only mapped on the geological map. Second, this work has acquired more detailed geometric pattern and new activity of the Lintan-Dangchang fault, the east segment of the Xiahe fault, and the Damai-Hezuo fault, and we have discovered Holocene activity of the Xiahe fault for the first time, it is of both left-lateral strike-slip and northward reverse-slip. Third, the seismogenic fault plane （the strike, dip direction and dip angle are 312°, 42° and 48°, respectively） determined by previous studies based on earthquake sequence relocation is likely to be one hidden fault derived from the east segment of Xiahe fault, the hidden fault is at a small angle （22°） to the east segment of Xiahe fault, and merges with Xiahe fault in the deep earth, slip direction of the hidden fault （rake is 48°） agrees well with the slip senses （with both left-lateral slip and reverse slip） of the east segment of the Xiahe fault. The east segment of the Xiahe fault may be structurally part of the west segment of the Lintan-Dangchang fault and is part of the positive flower structure of the West Qinling fault, and the Xiahe M_S5.7 earthquake in 2019 represents the tectonic activity of the west segment of the Lintan-Dangchang fault.