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.