Two-dimensional interpretation of audio-magnetotelluric data around the epicenter distribution of the 2016 Gyeongju earthquake (ML 5.8), Korea

On September 12, 2016, an M L 5.8 earthquake occurred near Gyeongju City in the southeastern part of the Korean peninsula. Two representative major faults, the Yangsan Fault (YSF) and Ulsan Fault (USF), are close to the earthquake hypocenter. However, the actual location of the hypocenter was reported to be outside of these fault zones. To identify an earthquake-generating fault and analyze its precise mechanism, it is necessary to understand the deep geological structure of the fault system. In this study, an audio-magnetotelluric (AMT) survey was conducted to analyze the subsurface structure of the fault system and geological characteristics in the Gyeongju area of Korea. AMT data were acquired from 301 stations around the earthquake epicenter and processed using robust processing with the remote reference technique. Inversion profiles were designated as either parallel (N-S) or perpendicular (E-W) to the regional fault strikes. The geoelectrical structure was obtained using two-dimensional (2-D) inversions of the joint set of transverse electric (TE) and transverse magnetic (TM) modes. The vertical sections along the E-W and N-S profiles that passed through the epicenter distribution contain a low resistivity zone at a depth of 2–3 km. The horizontal section of the resistivity distribution confirms that a high resistivity zone is located around the epicenter distribution. In contrast, a low resistivity zone is located at the center of the epicenter distribution. The high resistivity zone is located in the Yucheon Group (pyroclastic rocks) and granites, which are located on the western part of the YSF. In contrast, the low resistivity zone was observed in the alluvium to the northwest. The Hayang Group (clastic sedimentary rocks) and granites are located between the YSF and USF, and the observed high resistivity zone is coincident with the location of the granites. Paleogene and Neogene sediments distributed on the eastern part of the USF contained low resistivity zones. Compared with a magnetic anomaly map, the pattern of the magnetic anomalies is similar to that of the resistivity. This indicates that the resistivity model presented in this study is consistent with the geological characteristics of the study area.