1927年古浪8级大震区及其周边地块的深部电性结构

Deep electric structure beneath the epicentre of the 1927 Gulang M8 earthquake and its adjacent areas from magnetotelluric sounding

沿穿过古浪大震震中区乐都—武威—白马岗北北东方向约340km长的剖面,进行了16个测点的大地电磁探测.使用Robust技术处理观测数据,分析了视电阻率、阻抗相位、Swift二维偏离度、最佳电性主轴方位角等参数,并采用NLCG二维反演技术对TE和TM两种模式的数据进行了二维反演模拟.结果表明:大通山—大坂山、西海原、皇城—双塔、龙首山和北大山5条断裂为明显的电性边界,其中西海原、皇城—双塔、龙首山和北大山断裂由西南向东北依次变浅变缓并在深部收敛于壳内低阻层上.沿剖面上地壳的电性构造较中下地壳的复杂,上地壳自西南向东北可分为中祁连隆起、北祁连褶皱带、冷龙岭隆起、武威盆地、潮水盆地和北大山隆起6个构造单元样式,显示与地表地质调查一致的构造格局;而在中、下地壳,武威盆地、潮水盆地和北大山隆起为一体,都属于阿拉善地块.中祁连和阿拉善地块电性成层性好,存在西南深、东北浅的壳内低阻层,北祁连褶皱带和冷龙岭隆起带电性结构复杂,高、低电阻体相间.1927年古浪地震震中区西南侧和上方区域的电阻率为高电阻率区,下方和东北侧区域为低电阻率区,处于电性结构明显呈台阶状陡变的地带,表明古浪地震是一次与断坡作用有关的地震.

A magnetotelluric sounding has been carried out at 16 sites along a 340 km long Yuedu-Wuwei- Baimagang profile, trending NNE across the epicentre of the 1927 M8 Gulang earthquake. The observational data of this survey are processed by using the Robust method, yielding a series of parameters of the deep electric structure, including apparent resistivity, impedance phase, Swift skewness, and azimuth of the principal electric axis. By means of the NLCG technique,2D inversion modelling is performed with the data of two modes TE and TIM. The resulted 2D electric structure shows that there are distinct boundaries indicative of five faults along the profile from southwest to northeast, which are the Datongshan- Dabanshan, west Haiyuan, Huangcheng-Shuangta, Longshoushan, and Beidashan faults. These faults become shallower and gentler from southwest to northeast, andconverge into a detachment zone of low resistivity at depth. Along the profile, the electric structure is relatively simple in the middle and lower crust and complex in the upper crust. The upper crust can be divided into six electric blocks, geologically corresponding to the middle Qilian swell, north Qilian fold zone, Lenglong swell, Wuwei basin, Chaoshui basin, and Beidashan swell, respectively. Below the Wuwei basin, Chaoshui basin, and Beidashan swell, there exist low-resistivity layers in the middle and lower crust at different depths. It is inferred that the upper crust of the Qilian block overthrusts toward northeast while the middle and lower crust of the Alxa block underthrusts to southwest. The epicentre of the 1927 Gulang M8 earthquake is located at a steep step- shape zone in the electric structure. South to and above the seismic source there is high resistivity, while north to and below the source there is low resistivity. Combining these features of deep electric structure with the seismotectonics of this area, it is suggested that the 1927 M8 Gulang earthquake was resulted from the overthrusting along a low-angle detachment sheet at depth on a foreland fault under the NE-SW directed compressive stress.