青藏高原东北缘海原-六盘山断裂带现今地壳应力环境的数值分析

Numerical modeling of current crustal stress state in Haiyuan-Liupanshan fault system of NE Tibet

海原-六盘山断裂是青藏高原东北缘的大型边界断裂带,是中国大陆典型的地震危险区。地壳构造加载特征的定量研究有助于分析区域孕震环境,参考青藏高原东北缘GPS形变和岩石圈精细结构等资料,本文建立海原-六盘山断裂带周缘的三维岩石圈分层模型,分析现今构造加载作用下区域地壳形变和应力演化特征。数值计算结果显示:青藏高原东北缘现今处于以北东-南西向的水平挤压为主导和北西-南东向的水平引张的变形特征。青藏高原东北缘中-下地壳流变性质影响上覆脆性地壳应力环境,中地壳较低粘滞系数对应的模型地壳应力计算值与研究区实际地壳应力场相近。海原断裂中-西段构造加载作用显著,具有相对较高的库仑应力积累和最大剪应力分布;而六盘山断裂周缘地壳应力和最大剪应力小于海原断裂带。构造应力积累的空间分布差异说明六盘山断裂具有较弱的构造孕震环境,而研究区走滑型断裂的孕震加载作用显著。尽管六盘山处于较低的应力状态,但仍不能轻易忽视其长期存在的强震空区所暗示的发震潜力。

Haiyuan-Liupanshan fault system is a huge boundary fault in Northeast Tibet,as well as the typical seismic hazard zone in China mainland,which plays an important role in accommodating the outward expansion of Tibet Plateau.Quantitative research on the features of crustal tectonic loading will contribute to analyzing the regional seismogenic environment.Based on the GPS velocities and fine lithospheric structure of NE Tibet,we set up a series of 3-D layered lithospheric models of the research area surrounding Haiyuan-Liupanshan fault system,and analyze the crustal deformation and stress evolution driven by tectonic loading.Furthermore,we analyze the influence of rheology of middle crust on stress state of upper crust in NE Tibet,via comparative numerical tests.The FEM numerical results show that:(1)The NE Tibet is experienced with dominant NE-SW horizontal compression and slight NW-SE horizontal extension,with more pronounced stress accumulation near western part of Haiyuan fault zone;(2)The rheology of mid-lower crust in NE Tibet affects the stress states in the upper crust.In the model with lower viscosity in middle crust of NE Tibet,the simulated tectonic stress state approximates the current stress states revealed by earthquake focal mechanisms;(3)Driven by tectonic loading,the mid-western segment of Haiyuan fault is distributed with high Coulomb stress changes and maximum shear stress.While the accumulations of crustal stress and maximum shear stress are relatively lower in Liupanshan fault compared with those near Haiyuan fault zone.The differences in spatial distribution of tectonic stress accumulation suggest that Liupanshan fault zone is experienced with weaker seismogenic environment,and the seismogenic loading effect on strike-slip faults in the research area is more remarkable.Despite the low crustal stress state around the Liupanshan fault zone,we still can’t ignore the potential of strong earthquakes revealed by long term seismic gap.