The variation of the deformation-strain field before MS7.1 Yushu earthquake

Base on the velocity field calculated from the GPS data from 1999 to 2007 and from 2007 to 2009, the movement characteristics of Qinghai-Tibet subplates were studied. The strain rate of Hoh Xil-Bayan Har block and the Qiangtang block, the velocity and the strain rate of Hoh Xil-Yushu fault zone were calculated by the block (rotation and linear stain) elastic movement formula. The deformation strain before M(s)7. 1 Yushu earthquake has been studied. The result shows that between 1999 and 2007 the activity in Yushu fault zone was mostly left-lateral strike slip. Between 2007 and 2009, the strain rate of Hoh Xil-Yushu fault zone in the western part of Yushu has increased remarkably, which is favorable to stress accumulation. The Hoh Xil-Yushu fault zone in the eastern part of Yushu showed mainly right-lateral strike slip activity. Yushu was a transitional area between different kinds of active strain and became an unstable zone. The activity in Hoh Xil-Yushu fault zone became a condition of the M(s)7. 1 earthquake, in the meantime,accumulated energy for the earthquake.