Electrical characteristics of the crust in the south part of Longmenshan fault zone : Evidence from magnetotelluric inversion with velocity structure constraints

The Longmenshan fault zone is characterized by frequent large earthquakes and active tectonic movement, and its geohistorical evolution and dynamic uplift mechanism have not been determined. After the Ya' an earthquake on April 20, 2013, we carried out a magnetotelluric profile with a length of about 200 km perpendicular to the Longmenshan fault belt from Xiaojin to Ya'an. To overcome the drawback of obvious undulant terrain and enhance the vertical resolution of the magnetotelluric inversion, this paper developed a two-dimensional (2D) nonlinear conjugate gradient magnetotelluric inversion algorithm in consideration of both the terrain and the velocity structure constraints. The seismic velocity model was obtained from a wide-angle refraction/reflection traveltime inversion and was coupled with the magnetotelluric inversion by using the cross-gradients as a structural constraint. A graben-horst model was designed for inversions of synthetic data, and the results showed that the new algorithm was able to avoid the false anomaly caused by the influence of terrain and make the boundary contour of abnormal body more clearly. In the magnetotelluric inversion of observed data without seismic constraint, the result exhibited obvious macroscopic electrical characteristics of "low-high-low", which is basically consistent with the previous results. The new algorithm was also applied to the inversion of the measured data and the results demonstrated that there were two low-resistivity anomalies under Songpan-Garze block, which were probably connected with each other. The buried depth of the low-resistivity body on the west side was deeper (in the range of 15 similar to 45 km), with channels extending to the surface, while the conductive body in the east was relatively shallower overlaying high-resistivity anomaly bodies to the east, which showed a trend of thrusting and napping toward the Yangtze terrane. The thickness of the large-scale high-resistivity abnormal body was about 50 km beneath the thin skin caprock in the Longmenshan fault belt, with the Moho interface as the bottom boundary. It has the feature of a northwestward trend, which is consistent with the attitude of the deep extension of the three main Longmenshan faults inferred from the artificial seismic data.