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Abstract

In 1679,the Sanhe-Pinggu M8 earthquake was located at the transition point between Yanshan Fold and North China Plain,and its shallow and deep seismogenic dynamic environment is still unclear.Based on the high resolution Bouguer gravity anomaly data in the Sanhe-Pinggu earthquake region and the latest global gravity field model WGM2012,the Moho fluctuation and density disturbance distribution of the crust in the earthquake region are obtained by comprehensive trend analysis,wavelet multi-scale signal separation technique and spatial iterative inversion algorithm.The Sanhe-Pinggu earthquake region is located in the east of the Taiyuan-Yanqing Bouguer gravity anomaly gradient belt and the Moho abrupt alteration belt.The depth of Moho is between 31 and 36km.The epicentre of the earthquake is located in the transition area between the Moho abrupt change zone and the uplift zone.The three-dimensional density structure of the crust in the earthquake area shows the characteristics of high density in the middle and low density in the north and south.The high density bodies may be derived from mantle magma intrusion and melting activities,while the low density bodies may be restricted by the extension of the North China Plain and the equilibrium of Yanshan uplift.The northeast upwelling activity of Daxing high density body in central China is very important for the formation and development of Xiadian fault and the breeding and occurrence of Sanhe-Pinggu earthquake.For this reason,a model of the joint action of magma upwelling and fault is proposed.The results can provide important reference for source identification of giant earthquakes.