Discussion of Relationship between the Wenchuan Earthquake and Lushan Earthquake from the Viewpoint of Coulomb Failure Stress Change and Spatial Distribution of Aftershocks
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摘要: 本文以龙门山及周边地区为研究对象，考虑区域地质构造差异、主要活动断裂带、地表附加重力影响，建立能反映地表起伏和岩石圈分层结构的龙门山地区三维粘弹性有限元模型。以地壳水平运动速率观测值为约束条件重建研究区现今构造背景应力场，在此基础上分别模拟了汶川地震和芦山地震的发生机理。通过分析同震库仑破裂应力变化与余震空间分布的关系，探讨了2次地震主震对余震的触发作用以及汶川地震对芦山地震的影响。研究表明，汶川地震和芦山地震的余震大部分由其主震触发，汶川地震对芦山地震的余震有约6.78%的触发作用。汶川地震的同震库仑破裂应力在芦山地震主震位置的增加值约为0.016MPa，如果龙门山断裂带南段库仑破裂应力年累积速率按照0.4×10-3-0.6×10-3MPa·a-1计算，汶川地震使芦山地震提前了约27-40年。计算还表明汶川地震和芦山地震的发生使鲜水河断裂带南段和虎牙断裂的库仑破裂应力增加，这些断裂带在未来发生地震的可能性增加。Abstract: Taking the Longmenshan and its surrounding area as the research area, with consideration of the difference of regional geological structure, the main active fault zone, additional surface gravity, we constructed the 3-D viscoelastic finite element model of the Longmenshan area with irregular topography and the layered lithosphere structure. Using the observed values of crustal horizontal movement as constraint condition, the present tectonic background stress field in the research area is reconstructed. Based on it, the occurrence of Wenchuan earthquake and Lushan earthquake are simulated respectively. Through the analysis the relationship between coseismic Coulomb stress change and the spatial distribution of aftershocks, the trigger action of main shock to aftershock and the influence of Wenchuan earthquake on Lushan earthquake is investigated. The results show that the most aftershocks of Wenchuan earthquake and Lushan earthquake are triggered by their main shock, and only 6.78% aftershock of Lushan earthquake is triggered by Wenchuan earthquake. The coseismic Coulomb stress of the location of Lushan earthquake caused by Wenchuan earthquake is about 0.016MPa. Assuming tha the annual cumulative rate of Coulomb stress in the southern segment of the Longmenshan fault zone is 0.4-0.6×10-3MPa·a-1, the occurrence of Wenchuan earthquake causes the occurrence of Lushan earthquake nearly 27-40 years earlier than it should. Our results also show that the Wenchuan earthquake and Lushan earthquake cause the Coulomb stress increase in the southern segment of the Xianshuihe fault zone and Huya fault, which increases the possibility of earthquake in this area in the future.
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图 1 研究区域 (根据邓起东等，2011)
Figure 1. Research area of this study (after Deng et al., 2011)
图 4 地壳水平运动速率模拟值与观测值的对比
Figure 4. Comparison of the simulated and the observed values of the crustal horizontal movement velocity
图 5 龙门山及周边地区主要断裂带上的库仑破裂应力年累积速率
Figure 5. Cumulative Coulomb failure stress of year in the main fault zone in Longmenshan and its surrounding area
图 6 汶川地震的同震库仑破裂应力变化与余震的空间分布
红色实心圆圈表示汶川地震主震，白色圆圈表示余震
Figure 6. Coseismic Coulomb stress change of the Wenchuan earthquake and the spatial distribution of aftershock
图 7 芦山地震的同震库仑破裂应力变化与余震的空间分布
红色实心五角星表示芦山地震主震，白色圆圈表示余震
Figure 7. Coseismic Coulomb stress change of the Lushan earthquake and the spatial distribution of aftershock (Solid red star represents the main shock of Lushan earthquake, the white circle represents the aftershock)
图 8 汶川地震的同震库仑破裂应力在芦山地震破裂面上的投影
红色实心圆圈表示汶川地震，红色实心五角星表示芦山地震
Figure 8. The projection of the coseismic Coulomb stress of the Wenchuan earthquake on the fracture plane of the Lushan earthquake
图 9 汶川地震和芦山地震共同产生的同震库仑破裂应力变化与余震的空间分布
红色实心五角星表示芦山地震，白色圆圈表示余震
Figure 9. Coseismic Coulomb stress change of the Wenchuan earthquake and Lushan earthquake as well as the spatial distribution of aftershock
图 10 汶川地震和芦山地震共同引起的龙门山地区主要活动断层的库仑破裂应力变化
Figure 10. Coulomb stress change of the main active faults in Longmenshan and its surrounding area caused by the Wenchuan earthquake and Lushan earthquake
表 1 研究区介质分层材料参数
Table 1. Material parameters of the layered medium in the research area
分层 深度/km 四川盆地 青藏高原东缘 E/1010·Pa υ η/1022·Pa·s E/1010·Pa υ η/1022·Pa·s 地表 0-0.5 3.75 0.21 0.8 3.75 0.21 0.8 上地壳 0.5-20 7.2 0.246 9.96 7.39 0.25 10.3 下地壳 20-Moho面 12.5 0.253 0.925 11.9 0.254 2.02 岩石圈 Moho面-100 17.5 0.265 0.05 17.50 0.265 0.05 软流圈 100-200 17.5 0.265 0.5 17.50 0.265 0.5 
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表 2 汶川地震和芦山地震参数
Table 2. Earthquake parameters of the Wenchuan earthquake and Lushan earthquake
事件 经度 纬度 走向 倾角 滑动角 最大水平滑动量/m 破裂长度/km 数据来源 汶川 103.4°E 31.0°N 231° 35° 138° 4.9 > 300 中国地震台网中心；Global CMT；徐锡伟等，2008 芦山 102.89°E 30.31°N 212° 44° 92° 1.3 46.7 USGS；曾祥方等，2013；徐锡伟，2013；张勇等，2013
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表 3 研究区内主要活动断层参数及汶川地震和芦山地震共同引起的库仑破裂应力变化
Table 3. Parameters of the main active faults in research area and the Coulomb stress change caused by the Wenchuan earthquake and Lushan earthquake
断裂名称 走向/° 倾角/° 滑动角/° 库仑破裂应力变化/MPa 龙门山断裂带东北段 225 60 180 -4.2-3.2 龙门山断裂带南段 220 60 90 -2-1.8 鲜水河断裂带南段 142-159.5 90 0-45 0.005-0.15 龙日坝断裂 205-229 60 135 -0.067-0 岷江断裂 180 45-60 45-70 -2.96-0 虎牙断裂 150 75 45 0~0.08
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