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

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.