Numerical Simulation On Tectonic Stress Field,Active Fault And Regional Seismicity

The earthquake is the result of the dynamic lithospheric stress field evolvement and is generally thought as sudden rock split in response to function of tectonic stress to rock body in the earth.Though the frequency of strong earthquake is low, it's destructiveness is sizeable and it may cause other secondary disasters . Once it occurs,serious loss of life and property of people will be brought.So we must strength theorical research of earthquake formation law to improve monitoring and forecast level in order to ruduce the catastrophic loss utmostly .Tectonic stress field is the breakthrough point of research as being the main factor of earthquake.In this thesis, I summarize the research development,study method and some research production of tectonic stress field systemically and introduce the finite element method comprehensively.Domestic tectonic stress field research began from 1970s and much outcome have been got later by many scholars.Now the methods to study tectonic stress state include Geological data analysis,stress field measurement,focal mechanism solution,fault scrape reversion,experiment imitation,numerical simulation etc.Because of inherent characteristic of earth medium,numenrical simulation offsets the other methods'shortcoming and becomes an important quantitative means to study tectonic stress field.With the development of computer and applitation of GPS observation, the feasibility and reliability of numerical simulation,to which is paid more attention,have been improve greatly.In addition to stress field,earthquake is also relative to regional active construct, especially the active fault which has some dominant function. Here from the aspect of couple function by tectonic stress and active fault,we discuss the evolvtion feature and analyse the motion mode of active fault caused by earthquake sequence.Coulomb rupture mechanism is introduced to supply information for regional feature seismicity.Take Guangzhou aera as the first research example. A 3D finite element model with different stike,slope faults and intersection of old and new faults, which was been horizontal districted and vertical layered, was built up to sumulate the displacement field and stress field on the given GPS boundary condition.From the result we find active faults have large control function to them, change gradient is big along the faults and the stress concernates on the end and interaction of faults.With the obtained ratio of vertical displacement and horizontal displacement, we can know the motion mode of Guangzhou-conghua,Shougouling and West and East branches of Zhujiangkou fault are compressive thrust,right-lateral strike slip and dorminately right-lateral slip and thrust also, respectively.The Coulomb stress on active fault plane is also be calculated and the result is that,the value on Guangzhou-conghua fault and east segment of Shougoulin fault is higher than that on west segment of Shougouling fault and west and east branches of Zhujiangkou fault. The value on middle segment of Guangzhou-conghua fault and west and east branches of Zhujiangkou fault are all higher than their side segment.In general,value on northeast-oriented fault is higher than that on east-west-oriented and northwest-oriented.All is relative to regional stress field consanguineously.The distribution of stress has explained the historical earthquakes and make speculation for seismicity in the future this area。Take Sichuan and Yunnan area as the second research example. On the basis of the related data of seismogeology,geophsics, and Geodesy and etc,We establish a 3D elastic contacted frictional finite element model of Sichuan and Yunnan. The earthquakes above Ms6.0 from 2000 to 2006 and above Ms4.5 from January to June in 2007 are used to simulate the stress of the regions where I want to discuss. From the compulated stress trendency of east,west,northwest and northeast of Yunnan which were obstained by other auspice instruments, I give the order of danger extent from high to low as east,west,norhtwest,northeast of Yunnan.According to the finding of focal mechanism solution and break process of 5·12 super earthquake,I discuss the coseismal displacement and stress field under two conditions when the dislocation of up and down part of fault is equal and unequal.At the same time,I analyse the influence to the motion mode primarily. There are different characters of surface displacement field and stress field between two conditions. The impacted area in up part is larger than down part when the ratio of dislocation of up and down is 3 to 1.Otherwise,regional active faults play an important role to displacement and stress field. The response of the east boundary of rhombic block,such as Ganzi-yushu,Xianshuihe and Xiaojiang faults to the strong shake are being unloaded while that of west boundary of it,such as Nujiang,Longling-ruili,Menglian-lancang are being loaded.