Numerical Modeling Of Earthquake Breeding & Triggering And Present Tectonic Stress Controlled By The NE- And NW- Trending Fault System Of The Central North China Block

The North China Block is located at the central of the East China, The had undergone a very complex process of tectonic evolution. Until now, it is still a intensively active block driven by the combined effect of the Indian Plate, the Eurasian Plate and the Pacific Oceanic Plate. Since the Mesozoic, The North China block had been cracking and formed the Bohai Bay Basin and the Shanxi Graben system gradually, making up the today’s landscape pattern. Faults development, Earthquakes frequent and strong in its internal and the most typical area is located in the central of North China Block, called the Central North China Block. Including Yanshan uplift, Shanxi uplift and Depression Basin of North China. The Central North China Block developed three tectonic zones named the Shanxi Graben Zone, the Taihangshan Fault Zone and Zhangjiakou-Penglai Fault Zone. The three tectonic zones can divided into two active fault systems:the NE-trend tectonic zones consist of a series of discontiguous active right-lateral strike-slip faults, and NW-trend tectonic zones consist of a series of discontiguous active left-lateral strike-slip faults. The seismic zonation distribution also can be divided into NE and NW direction in the area. Notably, almost all of the stronger earthquakes are located at the junction of two groups of NE-and NW-trending tectonic zones. Therefore, the Central North China Block was chose as the study area in this paper, by using the method of numerical simulation, Through the simulation of present tectonic stress field distribution characteristics and disturbing force transmission in fault system instantaneously caused by Xingtai and Tangshan earthquake, Comparing the characteristics of earthquake distribution, combined with focal mechanism solutions and geological data, focus on analysis of NE and NW faults in the study area of seismogenic and trigger regulatory role. We gain some insight through the study:The simulation result of present tectonic stress field distribution characteristics that under the control of NE- and NW-trending faults shows:the strongest stress area are located at the transition zone between NE-trending and NW-trending faults in active fault zone of Shanxi Graben, the Taihangshan Fault Zone and Zhangjiakou-Penglai Fault Zone, and correspond to seismically active area, the higher the tectonic stress value, the more earthquakes occur. It indicates that the tectonic stress is easy to concentrate in the fracture intersection area, the higher the stress value, the more earthquakes prone to happen.Comparing present tectonic stress field distribution characteristics that under the control of NE-and NW-trending faults respectively with the stress characteristics which in the interaction of two groups of faults, we found that when controlled by NW-trending faults only, the tectonic stress distribution is similar; when controlled by NW-trending faults only, the distribution is quite different. It shows that the NE-trending faults are the main controlling faults and dominant position in the control role of present tectonic stress field distribution characteristics.According to simulation result, the intensity stress values differ greatly in the eastern and western regional of Taihangshan Fault Zone. In Shanxi Graben which locates at the western of Taihangshan Fault Zone, the intensity stress value is higher; But the intensity stress value in Depression Basin of North China locates at the eastern of Taihangshan Fault Zone is lower. In the simulation of the Xingtai Earthquake, we found that the disturbance stress caused by the earthquake stress is diffcult to penetrate Taihangshan Fault Zone passed to the Shanxi Graben. It indicates that the Taihangshan Fault Zone has limit stress transfer function in the geological tectonic movement, can stop and unload partial geo-stress, protect the Depression Basin of North China.The simulation of transient disturbance stress transfer caused by Xingtai and Tangshan earthquakes results show that when the stress reach to the NE-and NW-trending faults, or stress concentration occurred, or stress unloading, it was difficult to cross faults to move on; From the two earthquakes aftershock distribution characteristics, we can see that most aftershocks occurred in NE-and NW-trending which near seismic source, It indicates that the NE-and NW-trending faults not only control the distribution of tectonic stress field, but also control the distribution of aftershocks.By the characteristics of tectonic stress field distribution and characteristics of earthquake distribution contrast, the earthquake prone zone is generally high tectonic stress fields. According to the focal mechanism and stress analysis of simulation results, it shows that the NE-and NW-trending faults in the stress environment of WNW-NW-trending principal tension stress and ENE-NE-trending principal compression stress. According to the pure shear model, this kind of stress status indicates that the NE-trending faults prone to dextral strike slip, are the seismogenic structures;the NW-trending faults prone to left-lateral slip, may breed and trigger earthquakes in the Central North China Block. The types and distribution characteristics of earthquakes and tectonic stress distribution are not only related with different fault patterns, but also geometry of the main faults.