Ground Motion Simulation of Three-dimensional Sedimentary Basin Based on Directly-beneath Fault Dynamic Source Model
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摘要: 沉积盆地与近断层地震共同作用会增加地震破坏的风险水平，尤其是盆地下方直下型断层发震情况。采用动力学震源模型刻画断层破裂发震过程，开展沉积盆地直下型断层谱元法地震动模拟研究，探讨不同断层面初始剪应力和成核区位置下三维沉积盆地地表响应规律。研究结果表明，断层面应力降对盆地地表地震动的影响显著，在断层面强度一定的情况下，随着初始剪应力的增大，即应力降增大，盆地地表峰值响应增大，原因在于应力降的改变影响了断层破裂释放能量，进而引起断层破裂速度改变，最终导致盆地地表响应发生变化；改变断层面成核区位置会对盆地内部地震动分布规律产生影响，当成核区位置从断层中间向断层左侧移动时，盆地左侧地震动逐渐减小，而右侧地震动逐渐增大，最终表现为盆地右侧地震动显著高于盆地左侧，原因在于改变成核区位置后，导致近断层地震动的方向性效应发生变化。Abstract: The combined action of sedimentary basin and near-fault earthquake will greatly increase the risk level of seismic damage, especially directly-beneath fault earthquake occurs under the basin. In this paper, the dynamic source model is used to depict the process of fault rupture and earthquake generation. This paper studies on the seismic simulation of sedimentary basin with directly-beneath fault by using the spectral element method, and discusses the surface response of the three-dimensional sedimentary basin under different initial shear stresses of fault and locations of nucleation area. The results show that the stress drop of the fault has a significant influence on ground motion in the sedimentary basin, when the initial shear stress increases, the stress drop increases, and the rupture velocity of fault increases obviously, which lead to amplification of the surface response inside the basin. Meanwhile, the locations of nucleation area also has an impact on its ground motion response, when the nucleation area moves from the middle of the fault to the left, the ground motion on the left side of the basin decreases gradually, and the ground motion on the right side increases gradually. The reason is that changing the location of the nucleation area leads to the change of the directivity effect of near-fault ground motion.1) 2
https://github.com/geodynamics/specfem3d.git -
图 5 各观测点位错随时间变化曲线
Figure 5. The curves of displacement versus time corresponding to observation points
图 6 各观测点对应加速度时程曲线
Figure 6. The time-histories of acceleration corresponding to observation points
图 7 不同初始剪应力下地表P4观测点加速度时程曲线
Figure 7. The time-histories of acceleration at surface observation P4 with different initial shear stress
图 8 不同初始剪应力下地表观测点PGA及其放大系数曲线
Figure 8. The curves of PGA and amplification factor at surface observations with different initial shear stress
图 9 不同初始剪应力地表观测点P4加速度反应谱
Figure 9. The response spectrum of acceleration at surface observation P4 with different initial shear stress
图 10 不同初始剪应力断层破裂时间等值线图
Figure 10. The contour of fault rupture time with different initial shear stress
图 11 不同初始剪应力下地表速度波场快照图
Figure 11. The snapshots of velocity wavefield at surface with different initial shear stress
图 12 不同成核区位置断层破裂时间等值线图
Figure 12. The contour of fault rupture time with different location of nucleation area
图 13 不同成核区位置地表观测点PGA变化曲线
Figure 13. The curves of PGA at surface observations with different location of nucleation
图 14 不同成核区位置地表速度波场快照
Figure 14. Snapshots of velocity wave field at surface with different location of nucleation area
图 15 不同成核区位置地表水平峰值速度场及位移场分布
Figure 15. Distribution of peak ground velocity and displacement with different location of nucleation area
表 1 计算模型介质参数
Table 1. Parameters of the simulation model
介质密度/kg·m−3 剪切波速vS/m·s−1 压缩波速vP/m·s−1 厚度/km 深度/km 基岩 沉积盆地 基岩 沉积盆地 基岩 沉积盆地 3 000 2 400 3 200 300 5 500 500 0.3 0.3 3 000 2 400 3 200 500 5 500 800 0.3 0.6 3 000 2 400 3 200 800 5 500 1 500 0.4 1.0 3 000 — 3 200 — 5 500 — 29.0 30.0 
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表 2 观测点位置
Table 2. Location of observation points
观测点编号 坐标（x，y，z） P1（盆地外） （10 km，0.5 km，0） P2（盆地右边缘） （5 km，0.5 km，0） P3（盆地右1/4处） （2.5 km，0.5 km，0） P4（盆地中心） （0 km，0.5 km，0） P5（盆地左1/4处） （−2.5 km，0.5 km，0） P6（盆地左边缘） （−5 km，0.5 km，0）
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表 3 断层面上摩擦和应力参数
Table 3. Friction and stress parameters on the fault
断层参数 成核区外 成核区内 静摩擦系数 0.675 0.675 动摩擦系数 0.475 0.475 临界滑动距离/m 0.3 0.3 初始剪应力/MPa 66.0 81.6 初始正应力/MPa 120 120
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表 4 不同初始剪应力断层平均破裂速度
Table 4. Average rupture velocity of fault with different initial shear stress
初始剪应力/MPa 初始正应力/MPa S值 平均破裂速度vR/m·s−1 剪切波速vS/m·s−1 vR/vS 64 120 2.43 2 620 3 200 0.82 65 120 2.00 2 761 3 200 0.86 66 120 1.67 2 877 3 200 0.90 67 120 1.40 2 966 3 200 0.93 70 120 0.85 4 011 3 200 1.25 71 120 0.71 4 480 3 200 1.40
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