Deep electrical structures of Qinzhou-Fangcheng Junction Zone in Guangxi and seismogenic environment of the 1936 Lingshan M 6¾ earthquake

The tectonic position of the southwest section of the Qinzhou Bay-Hangzhou Bay Tectonic Junction Zone (QHTJZ) can be determined by examining the Qinzhou-Fangcheng Junction Zone (QFJZ) in Guangxi. This zone is significant because it was the location of the largest earthquake ever recorded in the inland region of South China, specifically the 1936 Lingshan M 6¾ earthquake in Guangxi. Therefore, this region serves as an optimal location for researching the origins of intraplate earthquakes in South China. This study presents a display of a broadband magnetotelluric (MT) prospecting profile that traverses the Guangxi QFJZ and the Lingshan earthquake zone, extending from the northwest (NW) to the southeast (SE). A resistivity structure model was generated using three-dimensional (3D) inversion technology along the profile. The main faults in QFJZ were analyzed in terms of their deep extension forms and tectonic attributes. This analysis was performed by integrating the results obtained from geology, gravity, wave velocity ratio, Global Position System (GPS), and geothermal flow. The results showed that (1) the Dongzhong-Xiaodong fault (DXf), the eastern Fangcheng-Lingshan fault (FLf2), and the eastern Hepu-Beiliu fault (HBf2) were all trans-crustal deep faults, and crust-mantle ductile shear zones developed in the deep part. Two electrical boundary zones, DXf and HBf2, were identified. DXf inclined towards the northwest, while HBf2 inclined towards the southeast. The Fangcheng-Lingshan fault (FLf) exhibits a tectonic style resembling a “flower” shape in the upper crust. In the deeper section, it is characterized by an electrical boundary zone that gradually slopes towards the southeast direction. (2) The Hunan-Guangxi Passive Continental Margin (HGPCM) on the NW side of DXf had a stratified resistivity structure and relatively stable Bouguer gravity anomalies, which conformed to the quasi-craton tectonic attribute of the local failure at the southeastern margin of the Yangtze Block (YB). The southeastern side of this block is marked by the presence of the QFJZ and Yunkai Magmatic Arc (YKMA). These areas exhibit varying Bouguer gravity anomalies, indicating a combination of high and low resistivity in their electrical structures. This suggests that this zone has undergone multiple stages of structural evolution and transformation. The giant high-resistivity body under the Qinzhou-Fangcheng Remnant Ocean Basin (QFROB) might be the trace left by the extinction of the South China Ocean and the collision orogeny between YB and the Cathaysian Block (CB). The presence of sub-low-resistivity layers in the middle-lower crust between the Liuwandashan Magmatic Arc (LMA) and YKMA indicates that this particular zone is being influenced from a distance by magmatic activities originating from the Leiqiong mantle. (3) The focal area of the 1936 Lingshan earthquake was located in the brittle high-resistivity body with a low strain rate. Under the coupling action of NWW-SEE regional tectonic stress and deep thermodynamic force, the brittle high-resistivity body in the upper crust became the main body for accumulating the tectonic stress. The Lingshan earthquake occurred due to the dextral strike-slip fracture instability of FLf2, a rock layer with slightly lower strength in the sub-high-resistivity zone. This instability was triggered when the accumulated stress reached the ultimate rock strength. The unveiling of the seismogenic model of the Lingshan earthquake, as presented in this study, holds significant scientific importance in comprehending the factors contributing to intraplate earthquakes in the South China region.