【摘要】 用三维流变非连续变形与有限元相结合(DDA+FEM)的方法,在青藏高原及邻近地区三维构造块体相互制约的大背景中,考虑了龙门山断裂带东西两侧地势、地壳厚度和分层的明显变化,及断裂带东侧四川盆地及鄂尔多斯块体坚硬地壳阻挡的影响,通过用GPS资料做位移速率边界约束和震源机制约束,计算得到研究区的速度场和应力场与该地区GPS测量结果和震源机制分布结果基本一致.在此基础上,模拟计算现今构造块体边界断层上表征剪应力及法向应力等综合影响的危险度分布.结果表明,上、中地壳层危险度分布中危险度较高的地段多数与近几十年来发生的七级以上大震区域基本一致.包括2008年汶川8.0级等大震的发震断层.通过分别对龙门山断裂带东西两侧的两种不同构造格局进行试算表明,龙门山断裂带东西两侧地势、地壳厚度、分层与物性明显变化对汶川大震的孕育发生均起了关键性作用.计算得到的应变率强度分布图可见,高原东部整个边缘地带均接近应变率强度的陡变带.其中以龙门山断裂带上的陡变最为明显,西侧应变率强度是东侧的近4倍,而且断裂带东侧应变率强度等值线衰减比西侧快.反映了汶川大震逆冲型发震断层地区独特的特征.此外,由计算得到的应变能密度分布图可见,龙门山断裂带在上、中地壳层中均位于宽度相同、其走向与龙门山断裂带走向一致的高应变能密度带中,在上地壳层这个带的东西两侧则是应变能密度较低的地区,而在中地壳层,其强度在断裂带东侧逐渐向东衰减,西侧应变能密度高,而东侧应变能密度较低.表明在印度板块强烈推挤作用和高原各构造块体相互制约及龙门山断裂带东西两侧特殊构造环境中,高原地壳物质向东水平运动,受到龙门山断裂带东侧介质刚性强度较大的四川盆地阻挡,使得汶川大震发震断层在大震前已积累了相当水平的应变能,并同时处于力学上的不稳定状态.更多还原

【Abstract】 In this paper, 3-D Finite Element Method combined with Discontinous Deformation Analysis (DDA + FEM) is used with the constraints of 3-D tectonic blocks of Qinghai-Xizang Plateau and neighboring areas to study the pregnant mechanism of Wenchuan earthquake. The topography on both sides of Longmenshan fracture zone, the marked change in thickness of the crust and its layers, and the influence of a stronger crust in the Sichuan Basin and Ordos block on the eastern side of the fracture zone are taken into consideration. With the GPS data constrained on the boundary, the calculated velocity and stress fields agree with the observed GPS data and the focal mechanism distribution results. On this basis, the distribution of risk factors on the boundary faults of tectonic blocks, which characterize overall influences of shear stress and normal stress on fault face, etc, is calculated numerically. The results show that the sections with high risk factors coincide with the areas where most of the M_S>7 earthquakes have occurred in recent decades, including M_S=8.0 Wenchuan earthquake in 2008. Trial computation of situations of two different structures on both east and west side of Longmenshan fracture zone show that significant changes of the topography on both sides, the thickness and the layers of the crust, and the physical properties played important roles in the pregnant mechanism of the Wenchuan earthquake. The calculated distribution maps of strain intensity show that the eastern edges of Qinghai-Xizang Plateau are close to the strain intensity steep change zone. Of the zones with steep change in strain intensity in eastern edge of the plateau, Longmenshan fracture zone shows the steepest change. The intensity contour on the east side of the fracture zone decay faster than the west side. In addition, the calculated strain energy density contour shows that the width and the strike direction of the upper and middle layers of the Longmenshan fracture zone is located in the same direction of the high strain energy density band. In the upper layer of the crust, low strain energy density is found on both sides. In the middle layer of the crust, the strain energy density decreases from west to east; the western side has high strain energy density, the eastern side has low strain energy density. Our results show that under strong subduction of Indian Plate, constraints of 3-D tectonic blocks in Qinghai-Xizang Plateau and influence of special tectonic environment on the west and east side of the Longmenshan fracture zone, the seismic fault area, where Wenchuan earthquake occured, had accumulated sufficiant strain energy. It indicates that the area is in mechanical instable state.更多还原