龙门山断裂带西南段地震空区地震危险性的三维数值模拟研究

The relocation of the after-shocks of the 2008 Wenchuan earthquake and the 2013 Lushan earthquake indicate that there are two seismic gaps in the south-west segment of the Longmen Shan fault. The debating of earthquake hazard of these seismic gaps is still on going. Based on the analysis of the in-situ stress data and the Coulomb stress change by the Wenchuan earthquake and Lushan earthquake, some researches suggested that these areas are still posing the potential of earthquake hazard. While other researches have the opposite opinion, since low Vs anomaly, suggesting of ductile rocks with low stress, is found in the gap between the Wenchuan and Lushan earthquakes. How does the Vs velocity anomaly affect the stress accumulation distribution in the Longmen Shan fault? What is the earthquake hazard in the seismic gaps in the Longmen Shan fault? This study aims to discuss these two problems through 3-D numerical modeling the stress accumulation process on the Longmen Shan fault. Based on the high resolution crustal structure revealed by seismology investigation, several finite element models are set up with boundary conditions of steady compressional deformation rate constrained by GPS observations. We calculate the long term stress accumulation distribution along the Longmen Shan fault, and analyze the earthquake hazard of the seismic gaps in the south-west segment of the Longmen Shan fault.

2008年汶川地震与2013年芦山地震余震定位研究表明，龙门山断裂带西南段芦山地震的北东端与西南端分别存在两个地震空区。针对这两个地震空区的地震危险性的讨论目前尚存有争议。一部分研究从地应力测量与地震库仑应力影响的角度出发，认为地震空区还有地震危险性的存在；另一部分研究从震源区的精细速度结构考虑，认为断裂带已破裂与未破裂的断层速度结构不同，进而否认地震空区地震危险性的存在。该断裂带速度横向不均匀性对构造应力积累分布的影响究竟有多大？龙门山断裂带西南段地震空区的地震危险性究竟如何？这是本项目拟解决的两个问题。本项目将结合地震勘探所揭示的龙门山断裂带岩石圈的精细速度结构，建立三维粘弹性岩石圈有限元模型，以跨断层的地质与地球物理观测为约束，数值模拟构造挤压作用下龙门山断裂带上的应力积累分布特征，以分析龙门山断裂带西南段地震空区的地震危险性。

为探讨龙门山断裂带的地震发生机理和断裂带西南段两个地震空区的地震危险性。我们建立了不同流变结构的龙门山断裂带三维岩石圈有限元模型，数值模拟了龙门山断裂带在数千年以上长期匀速构造挤压作用下的地壳形变与应力积累特征，探讨了地壳流变结构的横向非均匀性对龙门山断裂带应力积累的影响，分析了该区域地震空间分布与构造应力积累的关系。结果表明，东向运动的青藏高原与稳定的扬子克拉通相互挤压的过程中，龙门山断裂带的上地壳底部出现应力积累集中现象；该应力集中现象解释了龙门山断裂带的地震发生；龙门山断裂带的应力集中积累过程受控于青藏高原东缘的地壳流变结构的横向不均匀性，即：青藏高原较软弱的中、下地壳与扬子克拉通较硬的中、下壳的力学强度对比。整个龙门山断裂带的上地壳底部（包括其西南段的两个地震空区）均处于应力集中积累状态；在震间数千年的时间尺度下，地震成像结果所揭示的地震空区相对低速，并未影响地震空区的构造应力线性集中积累过程；龙门山断裂带西南段的两个地震空区仍处于应力集中积累状态，应力与能量还未得到释放；两个地震空区仍然存在较高的潜在地震危险性，应值得注意。