Seismological evidence for a deep-seated shear zone in the Langgu Depression

The focal mechanism of 2018 Yongqing M4. 3 earthquake was resolved by applying CAP method to local waveforms, showing that the best double-couple solution is as follows: the strike, dip, and rake angles of the two nodal planes are 297 degrees, 58 degrees, -32 degrees, and 45 degrees, 63 degrees, -144 degrees, respectively. The centroid depth of mainshock was resolved as about 19 km with locally converted waves, and the locations of aftershocks were also solved by the HypoDD method. The result from relocation shows that the epicenters of events are distributed along northeast, and the focal depths ranges from 17 to 19 km, distributed within a straight zone, indicating that the causative fault should be a strike-slip fault which strikes to the northeast and dips to the southeast. From the mechanism and spatial distribution, we infer that, (1) the seismogenic tectonic character of the earthquakes should not be caused by normal faults in the shallow crust, but by the deep-seated Cenozoic dextral strike-slip fault, which cuts through the whole crust vertically from the shallow to the deep portion of the crust; and that (2) the causative fault should be one of the branches of the Xiadian fault zone, which strikes from Xiadian to Wen'an with a total length of more than 150 km, and maybe related to the Baxian-Shulu-Handan fault zone; and that (3) the 2006 Wen'an M5. 1 earthquake and 2018 Yongqing M4. 3 earthquake should be both caused by the Xiadian fault zone. Furthermore, based on the comparison of the distribution of focal depth of events, regional fault zone, electrical structure and rheological model in the area under investigation, we posit that, the brittle-ductile transition depth of the newly-generated fault in the activated craton should be around 15 km.