有限元数值模拟龙门山断裂带地震循环的地壳变形演化
中国科学院计算地球动力学重点实验室, 中国科学院大学地球与行星科学学院, 北京 100049
国家自然科学基金（41574085，41590865）资助
中图分类号:P315;P541
Key Laboratory of Computational Geodynamics of Chinese Academy of Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
现今地壳变形数据显示横跨龙门山断裂带的地壳缩短速率低于3 mm·a-1，如此小的地壳缩短速率与龙门山断裂带附近的长期地质造山（平均高程约4.5 km）形成强烈对比.我们构建并使用了一个二维平面应变黏弹塑性有限元模型来模拟龙门山断裂带的地震循环位移变化，从而探讨了短期变形与长期变形之间的关系.模型模拟了地震循环的各个阶段（震间加载期、同震瞬间和震后黏性松弛调整期）以及多个地震循环（万年尺度）的地表变形，揭示了变形在地震循环中是如何累积、释放、调整以及最终形成永久变形导致了造山.模拟结果显示，岩石圈流变结构以及断层几何形态均对地震循环的地表位移变化有着显著的影响.经过多个地震循环，青藏高原东缘整体产生水平缩短与增厚抬升，而四川盆地基本保持稳定，区域的水平缩短主要由断层位错及青藏东缘的缩短抬升来调解，造成了青藏东部与川西盆地的差异抬升.研究结果将地震循环时间尺度的短期变形与长期地质造山联系起来，帮助我们理解青藏高原东部的隆升机制.
The present-day crustal deformation data show the shortening rate is less than 3 mm·a-1 across the Longmen Shan fault zone. Such a small shortening rate is a sharp contrast to the long-term orogeny with topographic relief of about 4.5 km. To address this issue, this work constructs a series of 2-D plane strain visco-elastic-plastic finite element models to explore the relationship between short-term and long-term deformation. We simulate the surface displacement of different stages (inter-seismic, co-seismic and post-seismic) in one seismic cycle, and the total displacement of multiple seismic cycles. The results show that permanent deformation leading to orogeny is generated after a seismic cycle. The rheological contrasts in the lithosphere and fault geometry have a significant effect on surface displacement distribution. After multiple seismic cycles, the eastern Tibetan plateau experiences overall uplift and shortening, while the Sichuan basin remains relatively stable. The shortening of the entire region is mainly accommodated by fault slip along the Longmen Shan fold-and-thrust zone, resulting in differential uplift between the eastern Tibetan plateau and the western Sichuan basin. Our model links the short-term deformation of multiple seismic cycles with long-term geologic orogeny, which helps the understanding of the uplift mechanism of the eastern Tibetan plateau.
图 1青藏高原东部区域地形图
青藏高原东部区域地形图
Figure 1.Topographic map of eastern Tibetan plateau
Topographic map of eastern Tibetan plateau
图 2二维黏弹塑性有限元模型; TP代表青藏高原东缘，SB代表四川盆地
二维黏弹塑性有限元模型; TP代表青藏高原东缘，SB代表四川盆地
Figure 2.Two-dimensional visco-elastic-plastic finite element model. TP represents eastern Tibetan plateau. SB represents Sichuan basin
Two-dimensional visco-elastic-plastic finite element model. TP represents eastern Tibetan plateau. SB represents Sichuan basin
图 4断层上盘深度为11.2 km处节点上的应力随时间变化曲线上图是下图矩形框区域的放大, 本文压应力为正.
Figure 4.Stress-time curve of a node on fault at depth of 11.2 km Upper is the magnification of the rectangular box in lower. Compressive stress is positive.
Stress-time curve of a node on fault at depth of 11.2 km Upper is the magnification of the rectangular box in lower. Compressive stress is positive.
图 5一个地震循环的地表位移
一个地震循环的地表位移
Figure 5.Surface displacement of one seismic cycle
Surface displacement of one seismic cycle
图 6多个地震循环后的地表总位移
多个地震循环后的地表总位移
Figure 6.Total surface displacement after multiple seismic cycles
Total surface displacement after multiple seismic cycles
图 7震间地表变形速率对比
震间地表变形速率对比
Figure 7.Velocity contrast between different models at inter-seismic stage
Velocity contrast between different models at inter-seismic stage
图 8同震地表位移对比
同震地表位移对比
Figure 8.Displacement contrast between different models at co-seismic stage
Displacement contrast between different models at co-seismic stage
图 9震后5年地表平均变形速率对比
震后5年地表平均变形速率对比
Figure 9.Average velocity contrast between different models for first 5 years after an earthquake at post-seismic stage
Average velocity contrast between different models for first 5 years after an earthquake at post-seismic stage
图 10计算同震位移矢量图
计算同震位移矢量图
Figure 10.Computed co-seismic slip vectors
Computed co-seismic slip vectors
图 11不同断层几何形态的同震地表位移对比
不同断层几何形态的同震地表位移对比
Figure 11.Co-seismic displacement contrast between different models which have different fault geometries
Co-seismic displacement contrast between different models which have different fault geometries
图 12模拟地表变形和地表观测数据的对比
模拟地表变形和地表观测数据的对比
Figure 12.Contrast between modeling results and observation data
Contrast between modeling results and observation data
图 13龙门山断裂带的一个地震循环地表总位移
龙门山断裂带的一个地震循环地表总位移
Figure 13.Total surface displacement of one seismic circle in Longmen Shan fault zone
Total surface displacement of one seismic circle in Longmen Shan fault zone
表 1参考模型岩石圈各地层物质参数设置
参考模型岩石圈各地层物质参数设置
Table 1.Material parameter of the reference model
Material parameter of the reference model
表 2黏度η及断层角度φ设置
黏度η及断层角度φ设置
Table 2.Angle φ and viscosity η settings
Angle φ and viscosity η settings
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Topographic map of eastern Tibetan plateau
Two-dimensional visco-elastic-plastic finite element model. TP represents eastern Tibetan plateau. SB represents Sichuan basin
Stress-time curve of a node on fault at depth of 11.2 km Upper is the magnification of the rectangular box in lower. Compressive stress is positive.
Surface displacement of one seismic cycle
Total surface displacement after multiple seismic cycles
Velocity contrast between different models at inter-seismic stage
Displacement contrast between different models at co-seismic stage
Average velocity contrast between different models for first 5 years after an earthquake at post-seismic stage
Computed co-seismic slip vectors
Co-seismic displacement contrast between different models which have different fault geometries
Contrast between modeling results and observation data
Total surface displacement of one seismic circle in Longmen Shan fault zone