宁夏海原大震区西安州—韦州剖面大地电磁探测与研究

对穿过宁夏海原大震区西安州(N36.5°,E105.5°)北至同心县韦州(N37.28°,E106.48°)的大地电磁测深剖面，采用远参考道大地电磁方法进行测量和资料处理，得到高精度的数据如视电阻率、阻抗相位、二维偏离度、最佳主轴方位角等. 依据这些数据，对测区的电性结构进行了定性分析和二维定量反演解释. 结果表明，沿剖面可以分成5个电性区块，与西、南华山隆起(Ⅰ)、兴仁堡—海原盆地(Ⅱ)、中卫—清水河盆地(Ⅲ)、中宁—红寺堡盆地(Ⅳ)和鄂尔多斯西缘带(Ⅴ)对应，各区块的边界由大断裂构成. 地表到深度10km左右，西、南华山隆起和鄂尔多斯西缘带呈高阻特性，兴仁堡—海原、中卫—清水河、中宁—红寺堡三个盆地的电阻率较低且呈盆地凹陷形状，盆地基底显示为西南深东北浅的簸箕状起伏形态，基底最深约为8km. 西、南华山隆起、中卫—清水河盆地和鄂尔多斯西缘带的下地壳为“正常”电阻率结构. 兴仁堡—海原和中宁—红寺堡盆地的下地壳上部为“异常”低电阻率带. 1920年的海原大震区存在明显的电性结构差异，震区西南侧和上部区域为相对高阻，东北侧和下部区域为相对低阻.

Crustal structure from magnetotelluric profiling in the Haiyuan earthquake area, Ningxia Hui Autonomous Region, China.

Magnetotelluric (MT) data have recently been obtained at 82 sites along a profile of about 134km long and approximately N E strike(from N36.5°, E105. 5° to N37.28°, E106.48°) in the Haiyuan earthquake area of Ningxia Hui Autonomous Region in China. The high resolution apparent resistivity, impedance phase, skewness and azimuths were obtained using the remote reference technique, which is a powerful tool for improving the MT data quality. The 2 D inversion method RRI was employed for the data interpretation. The results show that the crust of this area can be divided into 5 electrical sections, which separately correspond to the uplift of Xihuanshan and Nanhuanshan Mountains (Ⅰ), Xingrenbao—Haiyuan Basin (Ⅱ), Zhongwei—Qingshuihe Basin (Ⅲ), Zhongning—Hongsibao Basin (Ⅳ) and western marginal zone of Ordos (Ⅴ) from southwest to northeast. The boundaries of these sections are coincident with the known deep faults. At the depth of 10km, the resistivity is high in sectionsⅠand Ⅴ which are located near the two ends of the profile, while it is relatively low in the other sections. The other three sections (Ⅱ，Ⅲ and Ⅳ) exhibit a dustpan shaped basin with electricalbasement which is deeper in the southwest part and shallower in the northeast part. The maximum sediment basement is about 8 km deep. The high conductivity zone reflects the discontinuous characteristics in the middle or lower crust along the profile. The conductivity structure in sections Ⅱ and Ⅳ has an anomalous low resistivity zone in the upper part of the lower crust. The resistivity distribution in the Haiyuan earthquake focus area is significantly heterogeneous, and the high conductivity zone is offset obviously near the hypocenter region.