A finite element analysis method for the numerical simulation of characteristic earthquakes: Case study based on the Longmenshan fault zone
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摘要: 本文构建一种应用有限元开展特征地震数值模拟的新方法， 并以龙门山断裂带中段的浅层构造和动力学机制为背景， 研究了平行逆冲断层分布格局对区域地震活动性的影响. 结果表明， 从断层活动相互影响的角度看， 包含3条平行逆断层的断裂带的整体地震活动性并不适用严格周期的特征地震模型， 当断层间距在20 km以下时， 随着断层间距的缩短， 对单条断层应用特征地震模型的适用性会逐渐降低. 龙门山断裂带中段的模拟计算结果显示， 后山断裂的地震活动相对独立， 区域活动性和中央断裂的断层活动很可能不适用严格周期的特征地震模型.Abstract: This paper constructs a finite element analysis (FEA) method for the numerical simulation of characteristic earthquakes, based on the wide-accepted knowledge of the shallow crustal structure and dynamic boundary conditions at the middle part of the Longmenshan fault zone. The influence of parallel thrust faults on regional seismic activity is focused on and simulated with this FEA model. The results show that the characteristic earthquake model with strict recurrence interval is not suitable for the entire fault zone which contains three parallel faults. When the distance between the faults is less than 20 km, the applicability of the characteristic earthquake model to each single fault is attenuated with the distance between the faults decreasing. The results based on the distribution pattern of the profile in the middle of the Longmenshan fault zone reveal that seismic activity of the back range fault is relatively independent, and the characteristic earthquake model with strict recurrence interval is not suitable for the entire fault zone and the central fault.
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图 1 龙门山断裂带中段主干断裂展布平面示意图. 图中虚线方框给出了本文有限元模型研究区域
Figure 1. The ground layout of the main faults in the middle of Longmenshan fault zone The dashed rectangular indicates the study area of the finite element analysis in this paper
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图 2 龙门山断裂带中段深部构造剖面有限元几何模型 D1，D2分别为后山断裂与中央断裂、 中央断裂与前山断裂之间的距离； ∠a为前山断裂的近地表倾角． 模型为龙门山断裂带中段垂直剖面，模型长度为120 km，自地表至底部40 km．模型南东侧固定水平自由度，40 km深度下边界固定垂直自由度
Figure 2. Geometry of the profile in the middle of the Longmenshan fault zone used for the finite element model D1 is the distance between back range fault and central fault，D2 is that between central fault and range front fault，∠a is the dip of the front fault at the crustal surface. The model is for a vertical profile cross Longmenshan fault zone with 120 km long and 40 km depth. The SE end is fixed in horizontal direction and the depth end is fixed in vertical direction
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图 4 模拟破裂的特征地震矩和破裂复发周期
Figure 4. Seismic moment and rupture recurrence intervals of the simulated characteristic earthquakes
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图 5 破裂的地震矩和孕震时间与挤压加载量的线性相关系数
Figure 5. The linear correlation coefficient of seismic moment and rupture interval with the extrusion load
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图 6 破裂地震矩和孕震时间的线性拟合残差标准差
Figure 6. St and ard deviation of linear regression of seismic moment and rupture interval
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