Study on the absolute stress level in 2003 Dayao earthquake source region

The Ms6.2 Yunnan Dayao earthquake occurred on July 21, 2003, followed by a major Ms6.1 earthquake about 88 days later in the same region. The hypocenters are almost coincidence in the same place. Using the P wave first motion polarities of the two aftershocks sequences recorded by temporary stations and the improved version of grid points test method by Yu et al, we study the stress field in the aftershocks zone and get the two stress field directions in Dayao region. Using the stress value calculation method by Wan, we get the underground stress value in the focal area. The results show that the pre-seismic shear stress is larger than that of the post-seismic, and their difference corresponds to the stress drop approximately. The underground compressive stress is very large, but the differential stress is small. Before the main shock occurred on October 16th, the maximum principal stress is 166.27 MPa, intermediate principal stress is 158.71MPa and the minimum principal stress is 150.97 MPa. The normal stress and shear stress projected on fault plane are 157.25 MPa, 7.40 MPa respectively before the main shock occurred on October 16th. The normal and shear stress projected on fault plane are 158.81 MPa, 0.15 MPa respectively after the main shock occurred on October 16th.