Estimation of stress value in 2003 Dayao seismic source region

Using the P wave first motion information of two aftershocks sequence recorded by temporary stations and the new version of grid points test method, we study the stress field in the aftershock zone and get the stress field directions in Dayao region in the two occurrence time periods of the two aftershock sequences. Using the stress value calculation method, we get the underground stress value in the focal area: Before the earthquake, the maximum, intermediate and minimum principal stress is 164.07, 158.71 and 153.10 MPa respectively. The normal stress and shear stress projected on fault plane is 157.41, 5.24 MPa respectively. The normal stress and shear stress projected on fault plane after the earthquake is 158.69, 0.01 MPa respectively. The results show that the pre-seismic shear stress is larger than that of post-seismic, and their difference corresponds to the stress drop approximately. The increasing of normal stress maybe cause the termination of the rupture on the seismic fault. The underground compressive stress is very large, but the differential stress is small. It provide fundamental stress field information for the geodynamic process simulation.