Genesis of Seismic Gap of the 2008 Wenchuan and 2022&2013 Lushan Earthquakes: Insight from 3d Electrical Structure

Abstract

The Wenchuan Ms 8.0 earthquake on May 12, 2008 (WCEQ), and the Lushan earthquakes (LSEQ) on April 20, 2013, and June 1, 2022 (Ms 7.0 and 6.1, respectively), occurred in the middle and southern segments of the Longmenshan (LMS). The asymmetric spatial distribution of aftershocks following these events resulted in the formation of a seismically quiescent zone—designated as the “seismic gap” (SG)—between their rupture areas. Magnetotelluric sounding is employed to map the three-dimensional electrical structure of the middle-southern LMS region to elucidate the geophysical mechanisms underlying SG formation. Two fragile zones (A and B) act as a buffer zone are identified along the fault system: Zone A restricting northeastward propagation of LSEQ aftershocks, while Zone B constrains southwestward expansion of WCEQ aftershocks, collectively shaping the current SG configuration. Zone A exhibits a shallow surface fracture structure combined with partial melts derived from crust and the upper mantle, forming a low-resistivity anomaly zone (C2). Significant spatial variations in principal electrical axis orientation, maximum compressive stress direction, fault kinematics, and crustal block movement are observed across the SG boundary. These differences indicate the presence of tensile stress components along SG-margin faults, promoting crustal tearing processes. Consequently, upper crustal materials within Zone B undergo fragmentation and fluid infiltration, generating additional low-resistivity anomalies (C1 + C3) associated with enhanced crustal hydration.