青藏高原东北缘活动断裂剪切模量及应力状态数值模拟

Numerical modeling of the shear modulus and stress state of active faults in the northeastern margin of the Tibetan plateau

作为控制断层两盘相对运动的重要因素,断裂带介质力学性能与断层面上的滑动速率及应力状态、区域地壳运动速度场等密切相关.受印度板块北东向推挤以及阿拉善地块和鄂尔多斯地块的阻挡作用,青藏高原东北缘构造变形复杂.本文在综合区域动力学环境、活动断裂空间展布以及下地壳黏滞性结构的基础上构建了青藏高原东北缘三维有限元动力学模型;以GPS速度场为约束模拟研究了断层剪切力学性能对区域地壳运动速度场图像的控制作用,进而在最优模型基础上分析了当前青藏高原东北缘不同断裂的应力状态.结果显示:阿尔金断裂东段和广义海原断裂对区域地壳运动速度场控制作用强烈,但二者剪切力学性能相反,阿尔金断裂东段断层剪切模量与周边地壳介质相当,而广义海原断裂断层剪切模量可低至周边地壳介质剪切模量的1/10000;六盘山断裂和西秦岭北缘断裂对区域地壳运动速度场的控制作用较弱,模拟结果显示二者均具有较强的剪切力学性能.基于最佳模型的应力状态分析指出:阿尔金断裂东段,广义海原断裂西段的木里—江仓断裂、中段的金强河—毛毛山—老虎山断裂、东段的六盘山断裂,以及西秦岭北缘断裂中西段当前应力率水平较高,且与前人给出的青藏高原东北缘高闭锁区域吻合.动力学上的高应力率与运动学上的强闭锁良好吻合,预示着这些断裂是地震危险分析值得关注的区域.

As an important factor that controls the relative movement of two walls of a fault,the mechanical properties of rock are closely related to the fault slip,stress state and the regional crustal velocity field.Subjected to the northward motion of the India plate,and blocked by the Ordos and Alashan blocks,the northeastern margin of the Tibetan plateau exhibits a complex pattern of tectonic deformation.In this paper,a 3-D finite element geodynamic model is established based on the synthetic investigation of the geodynamics environment,spatial distribution of active faults and the crustal rheological structure.Constrained with GPS data,this model is employed to investigate the fault shear modulus and its effect on the regional crustal velocity field.Furthermore,the stress state of the major active faults is determined with the best fitted model.The simulation results indicate that the regional crustal motion is closely related to the mechanical properties of eastern Altyn fault(AEJ)and the Generalized Haiyuan fault(G_HY),and the shear properties of these two faults are opposite to each other.The shear modulus of AEJ is as strong as the surrounding crustal media while that of the G_HY could be as low as 1/10000 of the surrounding intact rocks.Moreover,the Liupanshan fault(LPS)and the north-edge fault of the West Qinling(XQL)are not sensitivity to the crustal velocity field,but both exhibit relative strong shear property.In addition,the stress state analysis points out that AEJ,three segments of G_HY(Muli-Jiangcang Fault(MJF),Jinqianghe-Maomaoshan-Laohushan fault(JMLD)and LPS that are located in the western,central and eastern of G_HY respectively),and the middle to western section of the XQL are all at high shear stress rates currently.The region of high stress rate is consistent with the high fault locking area in previous studies.That the high stress rate in dynamics agrees well with high fault locking in kinemics manifests that these regions are at high seismic risk that deserve more attention in the further analysis.