地壳流变结构控制作用下的龙门山断裂带地震发生机理  

Crustal rheology control on the mechanism of the earthquake generation at the Longmen Shan fault

作　　者：柳畅[1,2,3] 石耀霖[3] 朱伯靖[3] 程惠红[3] 杨小林[4] 

机构地区：[1]华中科技大学物理学院地球物理研究所,武汉430074 [2]Laboratoire de Gfiologie, CNRS-UMR8538, ecole Normale Superieure, Paris 75231 [3]中国科学院大学中国科学院计算地球动力学重点实验室,北京100049 [4]陕西省地震局,西安710068

出　　处：《地球物理学报》2014年第2期404-418,共15页Chinese Journal of Geophysics

基　　金：国家科技支撑计划《地震分析预测若干实用技术研究》项目(2012BAK19B03-5);国土资源部深部探测项目(SinoProbe-07)资助

摘　　要：青藏高原东缘低地形变速率的龙门山断裂带上相继发生了2008汶川Mw7.9级地震和2013芦山Mw6.6级地震.地震勘探与震源定位结果揭示了龙门山区域地震空间分布特征:纵向上,龙门山断裂带这两次地震主震均发生在龙门山断裂带上地壳的底部(14～19km),绝大部分余震均发生在上地壳范围(5～25km),而在其中、下地壳深度范围内鲜见余震发生;横向上,地震(Mw>3)在龙门山断裂带青藏高原一侧密集分布且曾有大震发生,而四川盆地地震稀少(Mw>3).为探讨龙门山断裂带地震发生机理,并解释以上龙门山区域地震空间分布特征,本文建立了龙门山断裂带西南段跨芦山地震震中区域的四种不同流变结构的龙门山断裂带三维岩石圈模型,以地表GPS观测资料为约束边界条件,数值模拟龙门山断裂带岩石圈在数千年以上长期匀速构造挤压作用下的应力积累特征,探讨了地壳分层流变性质对地壳应力积累的影响,分析了该区域地震空间分布与构造应力积累速率的关系.计算结果表明:该区域在数千年的应力积累过程中,脆性上地壳中应力表现近于恒定值的线性增长趋势,龙门山断裂带上地壳底部出现应力集中积累现象,这一应力集中现象可以解释龙门山断裂带汶川地震与芦山地震主震的发生,及其大部分余震在脆性上地壳中的触发;青藏高原一侧上地壳应力积累速率远远高于四川盆地的应力积累速率,这一应力积累分布现象可以解释龙门山区域青藏高原一侧地震密集而四川盆地地震稀少的地震空间分布特征;通过比较不同流变结构模型中的应力积累状态,认为导致这一应力积累空间分布状态的重要控制因素在于青藏高原中、下地壳较低的黏滞系数与四川盆地中、下地壳较高的黏滞系数的差异.在柔性的中、下地壳内,应力增长近于指数形式,稳定状态之后其应力增长速率近于零,构造应力积累In the recent 5years the 2008 Mw7.9 Wenchuan earthquake and 2013 Mw6.6Lushan earthquake occurred in the Longmen Shan fault at the eastern margin of the Tibetan Plateau,where the crust convergence rate is low（～3mm/a）.In previous researches it is shown the main shocks of these two earthquakes are located at the bottom of the upper crust of the Longmen Shan fault,and aftershocks mainly are located at the depth ranging from 5to 25km in the upper crust.It is also noted that earthquakes（Mw＆gt;3）are densely recorded in the Tibetan Plateau side,while rarely in the Sichuan Basin.This study aims to explain the mechanism of the earthquake generation in the Longmen Shan fault,and the earthquakes spatial distribution in the Longmen Shan area through 3-D numerical modelling.Several finite element models based on different crustal rheology are set up with boundary conditions of steady compressional deformation rate constrained by GPS observations.We calculate the long term stress accumulation process in the different lithosphere models of the Longmen Shan area.Our results show that stress increases almost linearly with time in the brittle upper crust of the Longmen Shan fault during the interseismic period of several thousand years due to the crust shortening.Stress concentrates at the bottom of the brittle upper crust of the Longmen Shan fault.This stress concentration process is responsible for the earthquake generation at the Longmen Shan fault.The primary reason for the stress concentration is the large viscosity difference of the middle and lower crusts between the Tibetan Plateau and the Sichuan Basin.Spatially the stress accumulation rate in the upper crust of the eastern Tibetan Plateau is much higher than that of the Sichuan Basin.This stress rate spatial distribution helps to explain the earthquake spatial distribution that earthquakes（Mw＆gt;3）are densely recorded in the eastern Tibetan Plateau,while rarely in the Sichuan Basin.Stress increases exponentially to a steady level in the ductile middle an

关 键 词：龙门山断裂带 汶川地震 芦山地震 应力集中 黏性差异 Moho面突变 

分 类 号：P315[天文地球—地震学] P313[天文地球—固体地球物理学]