North China is one of the areas of strong earthquake activity on the Chinese mainland. In the 1960s~1970s, North China has undergone the 1966 Xingtai Ms7.2, 1969 Bohai Ms7.4, 1975 Haicheng Ms7.3 and 1976 Tangshan Ms7.8 earthquake, causing great losses to people's life and property, where the Tangshan earthquake caused casualties amounted to 240 thousand.

In the past 50 years, Chinese seismologists have carried on the large-scale deep geophysical exploration and research, obtained the detailed crustal and upper mantle structure, and revealed the deep tectonic environment of strong earthquake in North China.

The related research paper is entitled "Crustal and upper mantle structure and deep tectonic genesis of large earthquakes in North China" published in SCIENCE CHINA Earth Sciences, Vol.60, 2017, wrote by Professor Wang Chunyong at Institute of Geophysics, China Earthquake Administration. The research group reviewed the main achievements of the researches on the crustal and upper mantle structure as well as the seismogenic environment and tectonic patterns in North China since the 1966 Xingtai earthquake.

Based on plate tectonics theory, geoscientists have successfully explained the patterns and mechanisms of earthquake activity on the global plate boundaries. However, plate tectonics theory has not explained the occurrence of earthquakes within the continental plate.

North China is located within the Asian continent, and the geological structure, seismicity and focal mechanism show that the seismic activity is very different from that in the region of the plate boundary, indicating that the seismotectonics in North China is quite complex.

The nucleation, occurence and development of continental earthquakes, and their relationship with the deep-seated structure of the earth are one of the most important subjects in the solid Earth Science.

Seismologists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies of seismic tomography in North China, finished 42 deep seismic souding profiles with total length of ~12000 km, and seismic tomography of three-dimensional crustal and upper mantle structure based on the seismic records at ~600 stations.

Deep seismic reflection profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter.

Seismic tomography images revealed that the lithospheric structure in North China is highly inhomogeneous, where most of the large earthquakes occurred in the transition between the high- and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust.

Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault, relative lower velocities in the uppermost mantle, and local Moho uplift.

Earthquake prediction is one of the scientific problems in the Earth Science that have not yet been solved. A deep understanding of the deep tectonic environment of strong earthquakes will help to solve the problem of earthquake prediction.

Seismologists Deng Qidong (2008) presented that the surface and shallow-deep tectonic features in the large earthquake area have been releaved by the combination of deep seismic reflection profiling and the surface geology and tectonic enviorment, thus we are able to obtain more comprehensive understanding of seismogenic tectonic conditions in the active fold area (the extension zone and the compression zone) of the normal or strike-slip normal fault and the reverse fault.

He further pointed out that it is the right direction to combine the surface geological work with the deep structure and the deep material detection, and it is the key to understand the nucleation and occurrence of the large earthquake.

The study of deep-seated structure and seismogenic environment in North China is an excellent model of the combination of deep geophysical exploration and surface seismic geologic research.

Despite the deep seismic exploration and seismic tomography research has greatly enriched the knowledge of deep-seated structure and seismogenic environment, due to the complexity of continental seismotectonics, we still face some deep-seated problems, which need to further research.

Wang C Y, Wu Q J, Duan Y H, Wang Z S, Lou H. 2017. Crustal and upper mantle structure and deep tectonic genesis of large earthquakes in North China. Science China Earth Sciences, 60: 821-857, doi: 10.1007/s11430-016-9009-1

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