Geophysical analysis on the tectonic difference between northern and southern segments of Xianshuihe fault zone

Xianshuihe fault zone is one of the strongly active seismic belts in the eastern margin of Tibetan Plateau, with the features of large scale, high activity and seismic frequency. Four M≥7.0 earthquakes have occurred along Xianshuihe fault zone. It is indicated that the differences of geometry features, activity characteristics and intensity, surface rupture characteristics of earthquakes exist between northern and southern segments of Xianshuihe fault zone. The upward continuation result of gravity-magnetic anomaly in Xianshuihe fault zone is present. The optimum upward continuation height is determined by calculating correlation coefficient of anomaly at observed surface height and upward continuation height. The upward continuation result at this height is corresponding to the anomaly caused by structural setting. It shows that the bottom of lower crust is on the rise from NW to SE according to the consecutive increase of Bouguer gravity anomaly at such orientation along the fault, which probably results from eastward motion of crustal soft material in Tibetan Plateau area. Magnetic anomaly reduced to the pole reveals that there is strongly magnetic rigid base to east of Kangding-Shimian in the southern segment of Xianshuihe fault zone. The difference in physical property of basement rocks and stratum in different blocks bounded by Xianshuihe fault zone is the major reason for magnetic diversity between northern and southern segments of the fault zone. With the statistical results of focal depths of seismic events in the range of 50 kilometers on both sides of the fault zone, its three-dimensional geometry is determined by projection and linear fitting. It is indicated that the dip direction of Xianshuihe fault zone is SW and its dip angle is almost vertical in the range of 57° to 88°.