地壳深部稳定性——震源体破裂机制和摩擦滑动机制的综合研究

本文通过地壳应力测量结果和地震资料的综合分析,对深部应力状态与断层运动的关系作了讨论:进而应用库仑准则,推导了三维应力作用下完整岩体和已有的任意空间方向断层面的失稳条件及其滑动方式的解析表达式.通过建立描述岩体和已有断层稳定性的两个函数————破裂函数 Fm 和摩擦函数 Ff,给出了应用破裂机制和摩擦滑动机制综合分析地壳稳定性和失稳性态的方法:根据这种分析方法并结合华北平原区的水压致裂应力测量资料,以及孔隙压力、大地热流等观测结果,定量研究了本区地壳的稳定问题,计算并图示了地壳内破裂函数沿深度的分布,以及各种走向和倾向断层面上的摩擦函数和剪应力分布;计算中以 Byerlee 定律作为断层运动的约束条件,并考虑了地壳密度纵向非均匀性导致的垂直应力沿深度的非线性增长以及深部超静水压力的异常孔隙压力作用.结果表明,华北平原区地壳失稳性态主要表现为已有断层的滑动;伴有高剪应力降的断层运动的深度范围在8至2.0多公里之间:陡断层稳定性低于缓断层,其运动方式以走滑为主;本区 NNE-NE 走向的陡断层是一组易震断层,其震时错动为右旋走滑;孔隙压力的增长对地壳稳定性有显著的影响;华北平原区深部高异常孔隙压力是地震活动的一种重要背景. 

COMPREHENSIVE STUDY ON STABILITY OF DEEP CRUST AND UNSTABLE BEHAVIOR OF EARTHQUAKE SOURCE BY BOTH FAILURE MECHANISM AND FRICTIONAL SLIDING MECHANISM

In this paper the relation between fault movement and stress state in deep crust is discussed, based on synthetic analysis of the crustal stresses measured over the world and the concerned data of focal mechanism. Using Coulomb criterion for shear failure and frictional slip, an analytical expressions for estimating stabilities of intact rock and existing fault in the crust and for identifying the type of faulting (normal, strike-slip or thrust fault) are derived. By defining the Failure Function Fm and the Friction Function Ff, which may describe steadinesses of crustal rock and existing fault, respectively, a synthetic model is set up to consider both fracturing mechanism and the sliding mechanism. By this model, a method to study stability and unstable behavior of crustal rock and fault at different depths is given.According to the above model, quantitative study on the crustal stability in the North China Plain is made in terms of the measured data of hydraulic fracturing stress, pore-fluid pressure, terrestrical heat flow in this region. The functions Fm and Ff and the shear stresses on faults with different strike directions and dip angles at various depths in this region are calculated. In the calculation the constraint condition of fault movement obeys Byerlee s law, and the depth-dependent nonlinear change in the vertical stress due to inhomogeneity of crustal density and the high anomalous pore-fluid pressure in deep crust of this region are considered.The conclusions are: the unstable behavior of the crust in the North China Plain is not failure of crustal rock but slip on existing fault; the depth range where stick-slip of fault may happen is about from 8 to 20km or more; stability of steep fault is lower than that of gentle sloping fault; the shear stresses in the range where stick-slip may occur are nearly horizontal; the steep faults trending from NNE to NE in this region are liable to produce strong earthquakes, whose co-seismic faultings are, for the most part, right lateral slip; the change in pore fluid pressure in depth remarkably affects the stability of the crust and the increase in pore-fluid pressure, therefore, would be an important factor exciting strong earthquakes in this region. The above theoretical inferences are consistent with the data measured in this region.