中国大陆背景地应力场下汶川地震引起地应力变化的模拟研究

发布时间：2018-09-13 09:52

【摘要】：地应力是影响地震发生的重要因素之一。大地震前后地应力状态的研究对于探索地震的孕育和发生机制具有重要的意义和作用。本文首先采用中国大陆三维球壳有限元蠕变模型,以实际观测成果为约束,获得中国大陆现今背景应力场。然后,通过给定发震单元的临界应力值,模拟5.12汶川大地震断层脆性失效引起的地应力变化。模拟结果与前人研究得到的汶川地震后的应力场特征较为吻合。通过浅部实测的水平最大主应力大小、方位和深部震源机制所反映的应力类型作为模型的目标约束条件,确定了较为合理的边界条件,模拟得到了中国大陆现今背景应力场,其特征为：(1)受印度块体、欧亚块体、菲律宾块体和太平洋块体的相对运动的作用,中国大陆地壳浅部水平最大主应力方位由南北方位绕青藏高原东构造结顺时针旋转,水平最大主应力方位在上地壳30km范围内具有一致性。(2)断裂带附近区域的最大水平主应力方位变化具有不连续性,最大水平主应力大小受到断层性质影响。(3)深度2000 m的水平最大主应力量值在10 MPa-80 MPa之间,青藏块体、西域块体最高,东南块体最低,其它块体中等。(4)深度2000 m处,龙门山断裂带附近区域的水平最大主应力在南部方位为SE-E,北部方位为NE-E。(5)龙门山断裂带在竖直剖面上的差应力从上地壳到中地壳逐渐增大。深度4000 m以下到中地壳的差应力比下地壳的大。由此推断断层浅部差应力较大,可能容易造成断层破坏。在中国大陆现今背景应力场下,以5.12汶川地震为例,分析断层脆性失效引起的应力场变化。建立破裂面上局部坐标系和断层面上最大剪切失效准则,降低剪切模量模拟研究地震破裂面上脆性断裂。汶川地震发生后,地壳应力场显示：(1)龙门山中央断裂带中段和北段浅部的水平最大主应力有较大的下降,断裂带周围应力受到地震影响较大,下降量值随着距断裂带的距离增加而减小。差应力下降较为明显,下降大小随着距断裂带距离的增加而减小。最大水平主应力计算变化量值与浅表应力震后实测量变化的量值相当。(2)龙门山南段应力在整个断裂带中依然高于其它地区,断裂带南段依然处于高应力积累之中。(3)断裂带上水平最大主应力的总体方位较一致,在震中位置深度2000m处水平最大主应力方位290°。与汶川地震前的计算结果对比,地震发生前的水平最大主应力为2800,说明地震前后水平最大主应力发生了顺时针旋转,旋转角度10°左右。由于受到此模型误差影响,此结果仅能作为应力变化的参考值,精度还有待提高。本模型可作为解释强震远距触发、迁移的基础。对于断层摩擦滑动失效和压碎破坏等断层失效机制的研究以及模型精度改进有待进一步工作。
[Abstract]:Ground stress is one of the important factors that affect the occurrence of earthquake. The study of in-situ stress state before and after a large earthquake is of great significance and function in exploring the mechanism of earthquake preparation and occurrence. In this paper, the 3-D spherical shell finite element creep model is used to obtain the present background stress field in the mainland of China. Then, the variation of in-situ stress caused by brittle failure of the 5.12 Wenchuan earthquake fault is simulated by the critical stress value of the seismogenic unit. The simulation results are in good agreement with the characteristics of the stress field after Wenchuan earthquake. By using the maximum horizontal principal stress, azimuth and the stress type reflected by the deep focal mechanism as the target constraint conditions of the model, the reasonable boundary conditions are determined, and the present background stress field in the mainland of China is simulated. Its characteristics are as follows: (1) due to the relative movement of the Indian block, Eurasian block, Philippine block and Pacific block, the maximum horizontal principal stress orientation of the shallow crust of the Chinese mainland rotates clockwise around the eastern tectonic junction of the Qinghai-Xizang Plateau from the position of the south to the north. The azimuth of horizontal maximum principal stress is consistent in the 30km range of upper crust. (2) the variation of maximum horizontal principal stress azimuth in the area near the fault zone is discontinuous. The maximum horizontal principal stress is affected by the fault properties. (3) the maximum horizontal principal stress value of 2000 m depth is between 10 MPa-80 MPa, the Qinghai-Tibet block, the western region block is the highest, the southeast block is the lowest, and the other blocks are medium. (4) the depth is 2000 m. The maximum horizontal principal stress in the area near the Longmenshan fault zone is in the southern direction: the northern direction of SE-E, is NE-E. (5) the differential stress in the vertical section of the Longmenshan fault zone increases gradually from the upper crust to the middle crust. The differential stress from the depth below 4000 m to the middle crust is greater than that from the lower crust. It can be concluded that the shallow differential stress of the fault is large, which may easily lead to the failure of the fault. Taking the 5.12 Wenchuan earthquake as an example, the variation of stress field caused by fault brittleness failure is analyzed under the background stress field of mainland China. The local coordinate system on the fracture surface and the maximum shear failure criterion on the fault plane are established, and the brittle fracture on the seismic fracture surface is simulated by reducing the shear modulus. After the Wenchuan earthquake occurred, the crustal stress field showed: (1) the horizontal maximum principal stress in the middle and north segment of the central fault zone of Longmen Mountain decreased greatly, and the stress around the fault zone was greatly affected by the earthquake. The drop value decreases with the increase of the distance from the fault zone. The difference stress decreases obviously and decreases with the increase of the distance from the fault zone. The calculated variation value of maximum horizontal principal stress is equivalent to that measured after shallow surface stress earthquake. (2) the stress in the southern section of Longmen Mountain is still higher than that in other regions in the whole fault zone. The southern segment of the fault zone is still in the process of high stress accumulation. (3) the overall orientation of the maximum horizontal principal stress in the fault zone is consistent, and the maximum horizontal principal stress direction is 290 掳at the depth of 2000m in the epicenter. Compared with the calculated results before the Wenchuan earthquake, the maximum horizontal principal stress before the earthquake is 2800, which indicates that the maximum horizontal principal stress rotates clockwise before and after the earthquake, and the rotation angle is about 10 掳. Because of the influence of the model error, the result can only be used as the reference value of the stress change, and the accuracy needs to be improved. This model can be used as the basis for explaining the remote triggering and migration of strong earthquakes. The research of fault failure mechanism such as fault friction sliding failure and crushing failure and the improvement of model precision need further work.
【学位授予单位】：中国地震局地壳应力研究所
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P315.727

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