Three-Dimensional CSRMT Imaging for Buried Active-Fault Investigation in Urban Covered Areas: A Paleoseismic Trench Case Study

The Nankou–Sunhe Fault is an important NW-trending buried active fault in the Beijing Plain. Its near-surface structure is difficult to identify because of thick Quaternary cover and weak geomorphic expression. In this study, controlled-source radio-magnetotelluric (CSRMT) data were acquired around the Baishan paleoseismic trench, where fault-related deformation has been directly exposed. A total of 61 sites were measured at 17 frequencies between 15 and 450 kHz. After quality screening, 58 sites were retained for 3D inversion. A terrain-constrained three-dimensional resistivity model was obtained after data quality control, apparent resistivity–phase analysis, and 3D inversion. The model shows an overall high–low–high resistivity pattern from shallow to deeper levels. Fault-related markers are directly exposed on both trench walls and were positioned by RTK. At those independently measured positions, the vertical sections of the 3D resistivity model show an electrical gradient zone, supporting the use of CSRMT to locate the electrical expression of the known fault. A broader conductive anomaly occurs near Site 011. The conductive feature may reflect fault-related deformation superimposed on water-bearing and fine-grained Quaternary sediments; the electrical data alone do not distinguish these contributions uniquely. These results demonstrate that CSRMT can delineate the electrical expression of a buried active fault and provide continuous subsurface constraints beyond trench-scale geological observations, although fault activity and kinematics should still be constrained mainly by trenching, drilling, seismic profiling, and chronological evidence.