Geoelectrical structure beneath the epicentral area of the 2016 Gyeongju earthquake, southeastern Korea, from magnetotelluric data

Southeastern Korea is transected by the Yangsan Fault System, a major fault network comprising numerous faults developed around the Yangsan Fault. In September 2016, a M_L 5.8 earthquake occurred in the Gyeongju area along the Yangsan Fault, with a focal depth of ~12–16 km. This event represents the largest instrumentally recorded earthquake in South Korea, which is located in an intraplate tectonic setting. Given the complex fault geometry and stratigraphy of the Gyeongju area, imaging the deep subsurface structure is essential for understanding the seismotectonic framework.

We conducted three-dimensional (3D) inversion of magnetotelluric (MT) data from 120 sites in the epicentral area and interpreted the resulting geoelectrical structure. The resistivity model delineates high-resistivity zones corresponding to granitoids and volcaniclastic rocks, whereas low-resistivity zones are consistent with sedimentary rocks and Quaternary alluvial deposits. This correspondence indicates that the geoelectrical structure reflects regional lithologic variations. At depths corresponding to the hypocentral range, a pronounced low-resistivity anomaly is resolved, and the 2016 Gyeongju earthquake is interpreted to have occurred beneath this conductive body. These results indicate that the deep geoelectrical structure beneath the Gyeongju area is characterized by distinct conductive and resistive features. In future studies, a multidisciplinary approach is needed for a reliable, integrated interpretation of the seismotectonic framework.