Abstract:Based on the strong motion observation data of the Wenchuan M8.0 earthquake and its aftershocks, as recorded by fixed and mobile stations in the Gansu Province, in this paper, we select 87 earthquakes with complete 3-component records (magnitude range of 3.0 to 8.0), calculate their shear stress values according to the relationship between the peak acceleration of the source and the ambient shear stress, and discuss the characteristics of the shear stress field. The results show a good correlation between ambient shear stress and moment magnitude. The corresponding stress values of earthquakes 3 to 5 in magnitude range from about 3 to 6 MPa, those 5 to 6 in magnitude range from about 6 to 9 MPa, and those 6 to 7 in magnitude have stress values above 9 MPa. The stress value of the Wenchuan M8.0 earthquake is biggest at about 20.6 MPa and the stress values of some of its bigger aftershocks were above 10 MPa. Due to the differences in site conditions, the corresponding ambient shear stress values of earthquakes of the same size exhibit a large range of changes that show a certain extent of discreteness. Using the least squares method, we obtained the relationship between the ambient shear stress and moment magnitude and found the ambient shear stress to also have a strong dependance on focal depth. When the focal depth is about 15 km, it can accumulate high ambient shear stress; but when the focal depth is either smaller or larger, it cannot. In addition, we found the spatial distribution of the shear stress value of the Wenchuan earthquake sequence to be NE trending, which is consistent with the spatial distribution of its aftershocks and its isoseismal contours. The high stress area of the aftershock sequences did not occur around the mainshock but were distributed in the northeast region of the mainshock.
