Seismogenic Structure of the 1975 Haicheng Ms 7.3 Earthquake (NE China) Inferred from 3D Magnetotelluric Imaging

On 4 February 1975, the Haicheng Ms 7.3 earthquake occurred in the Liaodong Uplift, northeastern China. To investigate its seismogenic structure and deep geological environment, we acquired broadband magnetotelluric data along two intersecting profiles across the epicentral region and performed three-dimensional inversion. Two orthogonal electrical sections were then extracted from the resulting 3D resistivity model to image the crustal structure beneath the Haicheng earthquake area. The model reveals that the northern segment of the Tanlu fault corresponds to a major electrical discontinuity between the Xialiaohe Basin and the Liaodong Uplift, suggesting that it may represent a deep-seated fault zone extending into the lithosphere. Beneath the Liaodong Uplift, a prominent mid-crustal low-resistivity layer is developed, and a synform conductive body is resolved beneath the source region. The Haicheng mainshock and relocated aftershocks are mainly distributed along the interface between this conductive body and the overlying high-resistivity upper crust. In addition, the Haichenghe–Dayanghe fault is imaged as a conductive zone that connects the mid-crustal conductor with shallower crustal levels. These electrical features suggest that deep crustal fluids, possibly related to Pacific Plate subduction and craton destruction, may have migrated upward along fault zones, weakened the seismogenic fault system, and promoted earthquake nucleation. Compared with the volcanic regions of the Jilin–Heilongjiang orogenic belt, where conductive anomalies extend into the upper mantle, the Haicheng region is characterized mainly by intracrustal conductors. This contrast highlights the role of crustal-scale conductive structures in the seismogenic environment of the Haicheng earthquake and provides geophysical constraints for comparing earthquake- and volcano-related deep processes in northeastern China.