西秦岭与南北地震构造带交汇区深部电性结构特征

Deep electric structure beneath the intersection area of West Qinling orogenic zone with North-South Seismic tectonic zone in China

西秦岭造山带与南北地震构造带接触区是中国大陆最重要的南北向和东西向构造转化的接合部位之一.本文介绍了分别位于该区106°E东、西两侧的LMS—L3和DBS—L1两条大地电磁剖面的探测结果,两条剖面分别跨过了龙门山构造带东北段的青川段和宁强段.采用大地电磁相位张量分解技术对两条剖面上各测点的电性走向、二维偏离度等进行了计算和分析,采用NLCG二维反演方法对TE+TM模式的视电阻率和阻抗相位数据进行了二维联合反演.反演得到二维电性结构,在经度106°西侧LM-L3剖面的深部电性结构自北向南揭示出,西秦岭北缘、成县盆地北缘、康县(即勉略构造带)和平武-青川断裂带都表现为明显的电性梯度带,深部延伸可达几十公里;西秦岭造山带、碧口地块与龙门山构造带东北段3个构造单元整体表现为高电阻体、呈现往南叠合且角度逐渐变陡的趋势.在106°E西侧西秦岭造山带区域的深部存在壳内低阻层,而东侧区域表现为高电阻体,深部电性结构在106°E东、西两侧的差异与该区深部速度结构特征一致,东、西两侧深部结构差异可能是该区中强地震分布差异的深层原因.LMS-L3和DBS-L1两条剖面南段的深部电性结构图像揭示出龙门山构造带东北部的青川段和宁强段内的平武-青川断裂带具有明显不同的深部结构特征,平武青川断裂带在青川段为明显的电性梯度带,在宁强段不再表现为电性梯度带,而是完整的高电阻块体.汶川强余震向东北发展止于青川青木川附近,与平武青川断裂带延伸深度和向北东方向的延伸长度密切相关,同时高电阻块体的宁强段对汶川强余震东北发展起到了阻挡作用.

The contact area between the West Qinling orogen zone and North-South seismic tectonic zone （31°N--36°N; 104°E--108°E） represents a special conjunction between the largescale NS and EW trending tectonic zones in China mainland. Two magnetotelluric （MT） sounding profiles （LMS-L3 and DBS-L1）, located on either side of 106°E, cross through this area. They also pass through the Qingchuan section, that was ruptured by strong aftershocks of the Wenchuan M8 event in 2008, as well as the unruptured Ningqiang section of the northeastern Longmenshan fault. Using the MT data collected on these two profiles, this work attempts to reveal the deep electric structure beneath this area. Data processing and analysis includecalculation of the electric strike and 2D skewness of the area using MT phase vector decomposition, and NLCG 2D inversion on apparent resistivity and phase data of TE ＋ TM modes. The results from inversion show that the Chenxian basin northern fault and Kangxian fault, both lie in the West Qinling orogen zone, extend downward by several tens kilometers, and the Kangxian fault dips toward north at a bigger angle than that of the Chenxian basin northern fault. In the study area, mutual connection of high-resistivity bodies at depth characterizes the southern section of the West Qinling orogeny zone, Bikou block and northeastern Longmenshan fault, of which the angles becomes steeper to south, resulting in electric boundaries. The two MT profiles reveal that low-resistivity layers are present at depth west of 106°E, while no such character appears on the east side, instead a high-resistivity body exists there. The difference of deep electric structure on opposite side of 106°E is consistent with the seismic velocity structure, likely associated with differential distributions of middle-sized and major earthquakes in this region. Besides, within the Qingchuan section （ruptured） and Ningqiang section （unruptured） of the northeastern Longmenshan fault, deep electric structure beneath the Pingwu-Qingchuan fault exhibits various features, i. e. electric property changes much in the Qingchuan section while changes little in the Ningqiang section. An intact high-resistivity body is found at depth below the Ningqiang section, and the aftershocks of the 2008 Wenchuan event terminated near the Qingmuchuan town, closely related to the eastward extension depth and distance of the Pingwu- Qingchuan fault, meanwhile an intact high-resistivity body in the Ningqiang section blocks the aftershocks development of the 2008 Wenchuan event to the northeast along Longmenshan.