The seismogenic structures and migration characteristics of the 2021 Yangbi M 6.4 Earthquake sequence in Yunnan, China

We constructed a more complete earthquake catalog in the 2021 Yangbi M 6.4 focal area by re-scanning the continuous waveforms integrated with deep learning and template matching techniques, to explore the seismogenic structures of the Yangbi mainshock and its nucleation process. The new catalog has three times the number of earthquakes than the CENC catalog, and the magnitude completeness has dropped from 1.1 to 0.5. The distribution of earthquakes indicates a broom-shaped structure consisting of several oblique secondary faults and a strike-slip main fault which strikes to 315° with 80° dipping to NE. The earthquakes extend along the fault strike about 27 km in width and 2–13 km at depth and have noticeable variations on seismicity in the mainshock’s north and south. Compared with the north, the south has denser and higher magnitude aftershocks and also has a seismic gap probably weakened by the fluid at the depth range of about 5–6 km. The foreshocks were mainly active in the 8-kilometer-long fault zone south of the mainshock, which show a steady drop in b -values over time and a migration pattern toward the epicenter of two steep jumps, stagnation, and then acceleration which finally triggered the mainshock. While in the north, seldom foreshock occurred, and the aftershocks were delayed triggered 3 hours after the mainshock, and sparsely scattered shallow at depth and small in magnitude. To summarize, the northern part of the Yangbi seismogenic fault is thought to be relatively locked, whereas the southern part has a weakening zone and promotes pre-slip. The nucleation mechanism of the mainshock and its onset at the junction of the locked and pre-slip zones may be a combination of pre-slip and cascade triggering.