有限元数值模拟龙门山断裂带地震循环的地壳变形演化

Crustal deformation across the Longmen Shan fault zone from finite element simulation of seismic cycles

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3mm·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4.5km)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 longterm 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,theeastern 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.