Deep electric structure beneath the epicentre of the 1927 Gulang M8 earthquake and its adjacent areas from magnetotelluric sounding
1927年古浪8级大震区及其周边地块的深部电性结构

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 TM. 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,and converge 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.