基于Monte Carlo方法数值反演区域初始构造应力场——以巴颜喀拉块体为例

Numerical inversion of regional initial tectonic stress based on Monte Carlo method——A case study of Bayan Har block

构造应力场往往对地震活动性具有控制作用,应力快速集中的地方常常是地震频繁发生的地方.本文以巴颜喀拉块体及其边界断裂带近20年来的7次中强震为例,结合区域历史地震震源信息、地质背景及GPS等观测数据,利用MonteCarlo方法和库仑-摩尔破裂准则为计算依据,反演该块体的震前初始构造应力场.通过将初始应力场反演中不确定部分限定在一个合理的上下限范围内进行独立的重复性随机试验,并运用统计学方法得到了巴颜喀拉块体1997年玛尼MW7.5地震震前区域初始应力场.计算结果显示:(1)巴颜喀拉块体10km深度处最大水平主应力方向自西向东呈顺时针旋转趋势,由NS向转变为近EW向,与浅部实测地应力数据、历史地震类型和板块运动方向吻合较好.(2)最大/最小水平主应力和二者差值自西向东均逐渐增加,最大水平主压应力值~400MPa,最小水平主压应力值~250MPa.差应力在昆仑山断裂带与阿尔金断裂带交汇处及甘孜—玉树断裂带西段较低(~150MPa);在昆仑山断裂带东端和甘孜—玉树断裂带的东南段局部地区较高(~220MPa).

The regional tectonic stress controls the seismic activity,so earthquakes usually occurred where stress is rapidly concentrated.We took 7 moderate-strong earthquakes occurring in the Bayan Har block and the boundary fault zone as a case study to calculate the regional initial tectonic stress of the block.Combined with the focal mechanism of historical earthquakes,geological settings and GPS velocities,we used the Monte Carlo method and the Coulomb-Mohr fracture criterion to invert the initial tectonic stress field.We took abundant repetitive independent calculations within the reasonable range,and used statistical method to get an initial tectonic stress before the 1997 Manyi M W7.5 earthquake occurred.Our results show that at the depth of 10 km,from west to east,the direction of the maximum horizontal principal stress gradually changes from NS to EW direction in a clockwise rotation trend,which is consistent with the data from the observed shallow crustal stress,historical earthquakes and plate movement.The maximum/minimum horizontal principal stress and the their difference values gradually increase from west to east,with the maximum horizontal principal compressive stress value is about 400 MPa,and the minimum value is about 250 MPa.The differential stress value is lower(about 150 MPa)at the junction of the Kunlun fault and the Altyn fault zone and the western section of the Garzê-Yushu fault zone.However,the eastern end of the Kunlun fault zone and the southeastern segment of the Garzê-Yushu fault zone is distributed with the higher differential stress values(about 220 MPa).