龙门山断裂带南段地壳电性特征: 来自速度结构约束下大地电磁反演的证据

龙门山断裂带区域内大震频发、构造运动活跃, 其地史演化与隆升动力学机制一直未有定论.2013年4月20日雅安大地震后, 于小金至雅安段布设了一条垂直于龙门山构造带长约200 km的大地电磁剖面.为了克服龙门山地形起伏影响, 同时增强大地电磁反演的垂向分辨率, 本文采用宽角折/反射地震走时反演获得的速度模型作为结构约束, 通过交叉梯度项引入大地电磁非线性共轭梯度二维反演, 实现了考虑地形的速度结构约束大地电磁二维算法.设计地堑-地垒模型进行合成数据反演试算, 结果显示基于速度结构约束的带地形二维大地电磁反演算法能够减少由地形影响引起的假异常, 同时对异常体边界轮廓的刻画更加清晰.实测数据未加地震资料约束的大地电磁二维反演结果, 具有明显"低-高-低"的宏观电性特征, 与前人结果基本吻合.本文将新算法应用于实测数据反演的结果表明: 松潘—甘孜地块下方包含两部分低阻异常带, 并有相互连通趋势.西侧低阻体埋深更深, 分布于15~45 km范围内, 顶部有通道延伸至地表; 东侧低阻体相对变浅, 向东逐渐延伸至高阻异常体之上, 具有向扬子地体逆冲推覆的趋势.龙门山构造带薄皮盖层下方大规模高阻异常体厚约50 km, 以Moho面为底界, 且呈现北西倾向的特征, 与人工地震资料推测的龙门山三条主断裂的深部延伸的产状一致.

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-Garzê 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 15~45 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.