ComprehensiveanalysesofseismicsourcelayerinXingtaiandTangshanseismicregionsandtheconditionsofmediaab

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Comprehensive analyses of seismic source layer in Xingtai and Tangshan seismic regions and the conditions of media above and below this Layer

Statistic analyses of sectional focal depth-frequency relation for 1739M _S⩾4.0 events in North China, 541M _S⩾4.0 events in Xingtai seismic region and 999M _S⩾4.0 events in Tangshan seismic region have been carried out for the first step of the study. Then the sectional focal depth-frequency diagrams (H-N diagram) for each region were obtained. The study shows that: (1) The intraplate earthquakes in North China occur within the continental crust, and are mainly concentrated within a certain depth range of the middle and upper crust, making up a seismic source layer; (2) The seismic source layer consists of a main layer and an auxiliary layer adjacent to it. The tops and bottoms of the main and auxiliary layers correspond to the G and C, as well as to B and G geologic boundaries respectively; (3) The H-N diagrams for Xingtai and Tangshan seismic regions are significantly different from each other, indicating the regionality and localization of the seismic source layers; (4) DSS and MTS observations show that the regionality and localization of the seismic source layer depend mainly on the environment of the deep structures of the region; (5) The seismic source layer is a substance of medium layer, which can be related to the brittle-ductile transition zone within the crust. The seismic source layer is also a stress-conducting layer. As the media above and below this layer are subjected to failure or flowage, then the stress will concentrate in the stress-conducting layer, in which the earthquake occurs. Contribution No. 95A0073, Institute of Geophysics, SSB, China. This subject is supported by the Chinese Joint Seismological Science Foundation.