汶川8.0级地震环境剪应力特征研究

基于甘肃强震固定台和流动观测记录的汶川8.0级地震主震及余震加速度资料,选用三分向记录均完整的87次地震(震级范围为3.0~8.0级),根据用位错理论二维断裂模式推导的震源峰值加速度与环境剪应力关系式计算汶川地震序列的环境剪应力值,探讨其环境剪应力场特征。结果表明,环境剪应力和矩震级有较好的相关性,3~5级地震对应的应力值多在3~6 MPa,5~6级多在6~9 MPa,6~7级多在9MPa以上,且环境剪应力对震源深度也有较强的依赖性。

Study of Ambient Shear Stress Characteristics of Wenchuan M8.0 Earthquake

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.