Abstract: Magnetotelluric data have recently been obtained at 9 sites along a profile of about 8km long and orientated approximately E-W in the Ning'er earthquake area which lies in southwestern Yunnan province.From these data,the high-resolution apparent resistivity,impedance phase curves,skewnesses and regional strike were calculated by using the remote reference technique and robust data processing technique,and then analyzed.The 2-D inversion method NLCG was used for data interpretation.According to the 2-D electrical structure of Ning'er seismic area,there may be an extended fault because of an obvious different electrical structure between the left & right side of the site 12.On longitudinal distribution,the crust of this area can be divided into four electrical layers:1)Mesozoic and Cenozoic sediments,about 1km,2)Paleozoic sediment,about 5km,3)high conductive layers in crust,and 4)middle and lower crust.A high conductive layer exists in the crust of the area.Between the site 12 and 13,there might be a decollement structure on top of high conductive layer under them,because the buried depth of the layer is quite different between the left & right side of the site 12.Focal depth of the Ning'er earthquake is about 5km,the decollement structure presumed in this research lies in this layer,and its strike is NW,same as that of the major axis direction of the isoseismal curve of this earthquake.So we conclude that the decollement structure is the seismogenic structure.

Key words: magnetotelluric sounding, apparent resistivity, electrical structure, Ning'er earthquake, 2-D inversion

摘要： 在2007年6月3日宁洱6.4级地震区进行了宽频带(1000～1/4096Hz)大地电磁观测,采用了远参考道技术和Robust技术对观测数据进行了处理,分析了视电阻率和相位曲线、二维偏离度及区域电性结构方向,采用非线性共扼梯度(NLCG)二维反演技术对实测资料进行了反演,得到了与实测曲线拟合程度较高的反演模型。从二维解释结果可以看出:横向上,以12号测点为界,明显地划分为2个不同的电性分区,推测存在具一定延伸的断裂构造。纵向上,30km深度以上大致可分为4个电性层:1)中、新生代盆地沉积层(Mz—Cz),厚约1km。2)古生代沉积层(Pz),厚约5km。3)壳内高导层(HCL)。4)中、下地壳(MLC)。其中壳内高导层埋深在12号点左右存在较大的差异,推测在12—13号点下高导层顶部存在壳内的滑脱构造。宁洱地震震源深度为5km,文中所推测的滑脱构造位于这一层位上。认为该滑脱构造为本次地震的发震构造。

关键词: 大地电磁测深, 视电阻率, 电性结构, 发震构造, 宁洱地震, 二维反演