Research on the three-dimensional electrical structure of the shallow portion of the south segment of the Red River Fault (Dazhai Village)

The southern segment of the Red River Fault (RRF) has historically not experienced earthquakes with a magnitude of six or greater. This absence constitutes a significant seismic gap, indicating a potential risk of strong earthquakes in the future. The China Earthquake Science Experimental Site intends to conduct drilling exploration in this area, which necessitates information regarding the fault zone’s geometric distribution characteristics and deep structure. One of the drilling alternative stations, audio and broadband magnetotelluric (MT) sites were collected to obtain a continuous image of the subsurface to a depth of 5 km in the Dazhai Village of Honghe County. This study investigated the distribution of electrical structures through impedance tensor statistical imaging analysis. The results indicated that the RRF acts as a boundary between high and low phases, with a deep inclination to the northeast. We used 3D magnetotelluric inversion technology in conjunction with terrain data, to conduct a high-resolution 3D resistivity model in the region of the RRF. The results show that the electrical structure from the surface to 0.5 km is relatively complex, characterized by alternating high and low resistivity. The electrical structure becomes more simplified below 0.5 km, with the RRF displaying a northwest-southeast orientation in the shallow region along the high and low resistivity boundary and a northeastward at greater depths, consistent with qualitative analysis results. The electrical structure of the Red River Valley, which the fault zone traverses, reveals low resistivity characteristics with a lateral width of up to 2 km. It is probable that seismic activity from the Holocene epoch originated within this low resistivity zone. This study offers critical electromagnetic constraints that enhance our understanding of the tectonic characteristics and inform the design of drilling plans for the southern segment of the RRF.