Study On The Crustal Stress Field And The Focal Fault Parameter Of Large Earthquake Of North China Plain Block

The North China Plain block is an active block with many large earthquakes in history. In this paper, we present a detailed study on the crustal stress field and focal fault of large earthquake in and around the North China Plain block, so as to provide background tectonic reference for the earthquake predication of this area. The main contents are as follows: First, the relocation of the medium and small earthquakes with the minimum one dimensional (1-D) velocity model of different tectonic zone. Second, based on the relocations, the crustal stress field of the North China Plain block is calculated by applying the composite focal mechanism method on a large number of P-wave initial polarities. Third, on the basis of the relocation results and the crustal stress field, the geometrical parameters and the motion modes of the 1679 Sanhe-Pinggu Earthquake,the 1830 Cixian, Hebei, Earthquake and the 1303 Hongdong, Shanxi, Earthquake are studied. The details are presented respectively as follows.First of all, we use the virtual network technology to integrate the phase data of 11 provincial seismic network and experimental mobile seismic array from January 1, 2001 to December 31,2013 in and around the North China Plain block, and carry out the absolute location by using Hypo2000 method. After selecting 6504 earthquakes with the location accuracy A and B, the minimum 1-D P-wave velocity model of 8 different tectonic units in the North China Plain was obtained by using VELEST program. Based on the 8 minimum 1-D P-wave velocity models, the Hypo2000 method is used to relocate the earthquakes of the North China Plain block. We get the following understanding on the relocations: 1, the horizontal position of the relocated epicenter changes little, earthquake clustering and striping become more obvious. 2, the relocated earthquake distributes along the vertical range of 0-30km, the focal depth gradually becomes shallow from south to north in Shanxi fault subsidence zone, the vast majority of earthquakes linearly distribute along the control boundary fault of rift basin; the lower reaches of the basin is delineated by the focal depth in Shanxi fault subsidence zone. 3, after the relocation of the focal depth profile reveals some more meaningful phenomenas, such as beneath the Tancheng earthquake source area the vertical earthquake distribution, the northeastern dipping earthquake belt in Huoshan area, the northwestern dipping earthquake dense belt in Xingtai source area.Secondly,based on the absolute location and the minimum 1-D velocity model,the fine 0.5°×0.5° ×20km stress field were obtained by using the composite focal mechanism method with the 116571 P-wave first-motion polarities of the 22069 earthquakes in the North China Plain block.The crustal stress field of the North China Plain block has the following characteristics: 1. The type of the composite focal mechanism solutions is dominated by the strike-slip and normal fault.The strike of one nodal plane of the composite focal mechanism solution is consistent with the strike of the main fault, which is in accordance with the current state of the shear tension in and around the North China Plain block. 2, the P-axis orientation shows the NE-NEE-near EW rotation image from west to east, and the T-axis orientation is perpendicular to the main control boundary fault in the Shanxi fault subsidence zone and show the NW-NNW-near NS counterclockwise rotation, T-axis azimuth consistency is better than the P-axis orientation,indicating that the North China Plain block is currently subject to the NW-NNW principal tensile stress. 3, the stress state of the Shanxi fault subsidence zone is mainly normal fault, and the local characteristics of the strike-slip type appear in the north of Linfen basin and the Xinding basin.Zhang-Bo zone is divided into normal fault, strike-slip and normal fault, P-axis orientation show the NE-NEE-EW clockwise rotation from west to east. The stress structure of the Tan-Lu fault zone is dominated by strike-slip and normal faults. The direction of principal compressive stress shows the NEE-EW-SEE from north to south, which also gradually the ENE direction rotated to the near EW direction from west to east. 4, Qinling-Dabie Mountain tectonic belt is weakly controlled by the stress field of the North China Plain block, and gradually turned to the stress field of the South China block. The stress field of the North China Plain block is mainly controlled by the subduction of the Pacific plate and the compression of the Qinghai-Tibet Plateau, and the northwestward subduction of the Pacific plate is stronger than that of the Qinghai-Tibet Plateau.Finally, based on the assumption that the long-term aftershocks activity and the aftershocks occurr on the fault plane and its surrounding area in the large earthquake source area, the geometrical parameters and the motion modes of the 1679 Sanhe-Pinggu Earthquake, the 1830 Cixian, Hebei, Earthquake and the 1303 Hongdong, Shanxi, Earthquake was determined by the present earthquake locations and regional stress field. The fault plane parameters of 1679 Sanhe-Pinggu earthquake are as follows: the strike is 38°, dipping to southeast, the dip angle is 82°, the slip angle was -156°, which suggest the fault type was the dextral strike-slip with normal fault component. The fault plane parameters of the 1830 Cixian, Hebei, earthquake are as follow:the strike of focal fault is 283°, dipping to northeast, 74° of dip angle, the slip angle is -26°, the fault type is the left-lateral strike-slip with normal fault component; the strike of the focal fault of the 1303 Hongdong, Shanxi, earthquake is 19°, the dip angle is 88°, the slip angle is -179°, which indicate the sliding type is dextral strike-slip.