The Stress Field Changes Near The Boundary Fault Zones In The Bayanhar Block And Their Effect On The Large Earthquake

Bayanhar block, located in the mid-eastern of Tibetan plateau, is mainly bounded by three fault zones. They are the East Kunlun fault zone, Fenghuoshan-ganzi-yushu-xianshui River fault zone and Longmenshan fault zone.7large earthquakes have shocked around/near the block since1997. It seems that this block is now in the strong earthquake active perod. So some questions arise:such as what about is the stress state near these fault zones, what is the relationship among the fault segments, and so on. These are always the hot questions about this active structure area recently. In this thesis, the author have calculated the stress differences applied by the long term tectonics activation and the Coulomb stress changes caused by large earthquakes around the block after analyzing the segmentation of the boundary fault zones of the Bayanhar block, including the faults long term slip velocity, the earthquake recurrence and the main region tectonic stress orientation and analyzed the effect on the large earthquake by these two factor. In the last part, the primarily seismic hazard assessment has been done on the main boundary fault zones of the Bayanhar block.(1) Based on the fact that the India plate has pushed the Tibet plateau with a nearly constant velocity since the late Quaternary Period and presuming that the velocity in the depth about10km is the same as that on the earth surface, the tectonic stress differences have been caculated applied in a a large earthquake recurrence period about1000-4000years focus on the main boundary fault zone areas using the2D limited element techniquke.The tectonic stress differences are different along the main faults segments. In a period of a large earthquake recurrence about1000-4000years, the stress differences are as follow along the8fault segments,①3.15～11.7Mpa along the Maergaichaka fault segment, equal to a tectonic loading rate of about2.83～13.15kpa/yr;②5.65～119.2Mpa along the west t segment of East Kunlun fault, equal to a tectonic loading rate of about4.8～15.65kpa/yr;③The mid-segment of East Kunlun fault,3.23～110.8Mpa, equal to2.73～13.23kpa/yr;④The east t segment of East Kunlun fault,1.69～16.0Mpa, equal to1.49～11.69kpa/yr;⑤1.68～14.7Mpa among the Fenghuoshan fault, equal to1.16～11.68kpa/yr;⑥Ganzi-Yushu fault,2.44～17.7Mpa, equal to1.93～12.44kpa/yr;ianshui River fault,2.34～16.7Mpa, equal to1.68-2.34kpa/yr; and⑧Longmenshan fault,1.14～14.8Mpa, equal to1.2～11.14kpa/yr.(2)Using the PSGRN/PSCMP, a program for calculating co-and post-seismic deformation based on the viscoelasticity semi-infinite space dislocation theory, the author has calculated the additional stress field caused by the16large earthquakes around the Bayanhar block. The calculated Coulomb stress changes caused by the previous large earthquake are much different revealing in11key structural locations along8boundary fault segments of the Bayanhar block. Some previous earthquake has the triggering Coulomb stress changes, but some others have the delaying Coulomb stress changes.The large previous earthquakes occurred on some faults around the Bayanhar block which can trigger the coming large earthquake on special fault are as follow:①Those large earthquakes occurred on the Maergaichaka fault, the mid-segmentof the east kunlun fault and the fault in the inner fault near south Dari can trigger the earthquake shocked on the west segment of the east kunlun fault, such as the2001west of the Kunlun Moutain pass Ms 8.1earthquake.②Those large earthquakes occurred on the Maergaichaka fault, the west segment of the east kunlun fault and the east boundary fault zones of the block can trigger the earthquake shocked on the mid-segment of the east kunlun fault, such as the1963Dulan earthquake.③the coming large earthquake will be triggered by those earthquakes shocked on the Maergaichaka fault, the west and the mid segment of the east kunlun fault, Ganzi-Yushu fault, Huya fault and Longmenshan fault.④Those large earthquakes occurred on the Xianshui River fault, the mid-segment of the east kunlun fault trigger the occurrence of the earthquake on the Ganzi-Yushu fault, such as the2010Yushu Ms7.1earthquake.⑤Those large earthquakes occurred both on the north and south segment of Xianshui River fault can trigger the occurrence of the earthquake on the north segment of Xianshui River fault.⑥Earthquakes occurred on the north segment of Xianshui River fault can trigger the occurrence of the earthquake on the south segment of Xianshui River fault.⑦Earthquakes occurred on the Ganzi-Yushu fault, Xianshui River fault and on the inner fault can trigger the occurrence of the earthquake on Fenghuoshan fault.⑧Earthquakes occurred on theMinjiang fault, the Huya fault, the south segment of SXianshui River fault, the west and the mid segments of the east Kunlun fault and the Maergachaka fault can trigger the occurrence of the earthquake on the Longmenshan fault, such as the2008Wenchuan earthquake.Among10large earthquakes shocked sinced1955,9occurred in the areas where the calculated coulomb stress changes are positive. Of these9earthquakes,7ones have the coulomb stress changes larger than the threshold value0.01Mpa. Detailed values as follows:①The calculated coulomb stress change△σc was about56kpa just before the1955Kangding earthquake occurred;②The△σc was about50kpa just before the1963Dulan earthquake occurred;③before the1973Luhuo earthquake, the△σc was about138kp;④before the1973Mani earthquake, the△σc was about0.4kpa;⑤before the Songpan earthquake,shocked on August16th,1976, the△σc was about220kpa;⑥before the Songpan earthquake shocked on August23th,1976, the△σc was about437kpa;⑦before2001west of Kunlun Moutain pass earthquake, the△σc was about4.2kpa;⑧before2008Wenchuan earthquake, the△σc was about4.2kpa;⑨before2010Yushu earthquake, the△σc was about2kpa;⑩before2013Lushan earthquake, the△σc was about-7.0kpa, accumulating109years. It was the only earthquake which had the negative△σc(3) At the depth of about10km, the tectonic stress difference plays a major role and the the clulum stress change caused by the previous large earthquake can trigger a next large earhthquake for the most boundary fault zone areas. The total stress difference of these two is usually about0.5-19.2Mpa, the maximum stress difference on some special location along the fault segment is not larger than the value about32Mpa in a large earthquake recurrence period about1000-4000years. At the10km depth, the minimus stress difference at the crital state of rock fracture is about108Mpa in a lateral slip fault system and about336Mpa in the thrust fault system. That means the stress state along the main boundary fault zones is near the critical state of rock fracture, with a stress difference about76Mpa in the lateral slip fault system and～306Mpa in the thrust, or the stress state around the Bayanhar block is near the crital state of rock fracture with a gap about32Mpa stress difference supposing that the rock friction coefficient is0.6and in a hydrostatic pressure.(4)The large earthquake danger assessment by the tectonic loading and the additional coulomb stress shows that, after the2013Lushan earthquake, in four earthquake locked segment faults since1320A.D., three segments are more danger, and one less danger. The three comparably more danger segments are Dong-xidatan-xiugou segment, the east segment of the east Kunlun fault including the Maqin segment and the Maqu segment, and the Qingchuan segment. The less danger one is Fenghuoshan fault.