Preliminary study of the tectonic structure and seismogenic environment of the M4.7 Feidong earthquake sequence on September 18, 2024 in Hefei

At 20:08, on September 18, 2024, an M4.7 earthquake occurred along the Tanlu fault zone in the Feidong County of Hefei, Anhui Province. This earthquake is the largest event in the modern history of Hefei, which caused substantial social impact. To reveal the seismogenic structure of the M4.7 Feidong earthquake sequence and assess seismic risks, we use data from both the permanent seismic network and a temporary dense nodal array deployed in the epicentral region prior to the mainshock for: (1) accurate location of the earthquake sequence and determination of the focal mechanisms; (2) obtaining the spatiotemporal distribution, b-value, and half-day occurrence frequency of the earthquake sequence. The Sentinel-1 satellite data are used to analyze the coseismic displacement. Additionally, velocity models from regional tomography and local high-resolution 2D active- and passive-source surveys across the Tanlu fault zone in the epicentral area are also used to reveal the detailed geometry of the seismogenic fault. The results indicate: (1) the M4.7 Feidong earthquake sequence is concentrated around 10.5 km in depth along a NW-dipping, subvertical fault which trends NE and is approximately 5 km in length; the focal mechanism solution also reveals that the fault hosting the mainshock is a subvertical strike-slip fault, driven by the regional compressional stress in ENE-WSW; the coseismic horizontal displacement on the surface caused by the M4.7 mainshock has a maximum value close to 1 mm; (2) the regional velocity model shows significant lateral variation in v_S in the source region, with the mainshock occurring in the area with higher velocity; high-resolution P-wave velocity structures obtained by full waveform inversion from active sources, and S-wave velocity structures from passive-source ambient noise tomography indicate that the mainshock occurred along the boundary between high- and low-velocity bodies, and the seismogenic fault dips NW; the deep seismic reflection profiling shows that the mainshock occurred within the Jurassic strata; (3) based on these results, we suggest the seismogenic fault for the M4.7 Feidong earthquake is either the Zhuding-Shimenshan fault, one of the major faults in the Tanlu fault zone, or a hidden fault to the east; the intersection of the NE-trending Tanlu fault zone and the WNW-trending Feizhong fault, along with significant velocity variations, likely create local stress concentrations which could have triggered the M4.7 Feidong earthquake sequence; (4) the strong aftershocks following the M4.7 Feidong mainshock did not further extend the fault rupture zone; the active period of the Zhuding-Shimenshan fault was the late Early Pleistocene to Middle Pleistocene, and the imaging results indicate that this fault does not cut through the shallow Feidong depression. In conjunction with the small coseismic rupture area, it is inferred that the probability of surface-rupturing earthquakes in the future is relatively low.