The M S 6.2 Dayao,Yunnan,earthquake occurred on July 21,2003,followed by a major M S 6.1 earthquake about 88 days later in the same region.Hypocenters of the two earthquakes are almost in the same place.Based on the P wave first motion polarities of the two aftershock sequences recorded by temporary stations,we have studied the stress field in the aftershock zone and obtained the two stress field directions in Dayao region using the new version of PKU Grid Test Software provided by Chunquan Yu.Assuming that the rotation of the stress field is caused by the second main shock,we estimated the crustal stress value in the focal region by using the stress value calculation method proposed by Yongge Wan.The estimated maximum,intermediate and minimum principal stresses are 166.3 MPa,158.7 MPa and 151 MPa,respectively,before the second main shock.The normal and shear stresses projected on the fault plane of the second main shock before it occurred are 157.3 MPa,7.4 MPa,and are 158.8 MPa,0.2 MPa after it occurred,respectively.The perturbed input parameters experiments attest the stability of the solution.The result shows that the preseismic shear stress is larger than the post-seismic one,and their difference corresponds to the stress drop approximately.The estimated compressive stress level is very high,but the differential stress is low.The result is helpful for friction coefficient estimation,plate motion simulation and related studies.
