Exploration on the deep tectonic environment of strong earthquakes in North China and relevant research findings

In the 1960--1970s,North China has undergone a series of strong earthquakes. The nucleation,development and occurrence mechanisms of strong earthquakes are closely related to the deep structure. For nearly 50 years,Chinese seismologists have carried out a great deal of researches in the fields of seismotectonics and deep environment of strong earthquakes. Deep geophysical survey and seismic tomography imaging revealed the basic characteristics of crustal structures in North China,and significant progresses have been made in the issues of the deep tectonic environment of strong earthquake occurrence. Based on the large-scale exploration of crustal and upper mantle structures in North China,this paper presents a review on the study of the seismotectonics and deep environment of 1966 Xingtai M_S7.2,1976 Tangshan M_S7.8,1975 Haicheng M_S7.3 and 1679 Sanhe-Pinggu M8.0 earthquakes. Deep seismic reflection profilings revealed that there is a high-angle deep fault in the vicinity of focal source,which passes through lower crust to the Moho. The geometry of high-angle deep fault is basically consistent with the seismic fault plane of the focal mechanism solutions,thus it is considered as the seismogenic fault. A new model of the relation between shallow and deep structures is that the shallow listric normal fault and its related tectonic system of active graben and half graben only developed above the detachment in mid-crust,while the high-angle deep active fault or ductility deformation zone developed below. The tectonic model of Xingtai earthquakes has resolved the contradiction between the typical continental extensional tectonic environment and the strike-slip focal mechanism of strong earthquakes in North China. Based on the massive broadband seismic data in North China,seismic tomography shows that most of large earthquakes occurred in the transition between the high-velocity and low-velocity zones. Comprehensive analysis of deep geophysical data reveals that the low-velocity anomaly belt beneath the source,high-angle ultracrustal deep fault,inconsis-tency of the deep and shallow structures in crust,lower-velocity in the uppermost mantle,and local Moho uplift are the seismogenic environment features held in common by strong earthquakes in the extensional tectonic region of North China. The migration and deformation of the mantle material would contribute to the energy accumulation of a large earthquake,which leads to the Tangshan earthquake occurrence. The hypothesis of the "solid body" seismo-genic model extends the ideas to address the seismogenic environment of strong earthquakes. However,despite the deep seismic reflection profiling and the seismic tomography research greatly enriched the knowledge of seismogenic environment in deep,we still face some deep-seated problems which need further research.