流体饱和孔隙介质中裂纹扩展理论研究

For natural earthquakes, cracks could run as fast as the Rayleigh-wave speed or faster than the transverse-wave speed of the rocks at the sites. Understanding such a fast rupture phenomenon is very important in seismicity mechanics. It is found that fluid transport has a great effect on the stress fields near the surface of the aseismic slip plane. To understand the influences of underground water on coseismic ruptures, we study the mechanical responses due to crack propagation in fluid-saturated porous media. The main contents of this project are as follows. We aim at developing analytic models for mode-II subsonic and intersonic crack moving uniformly in porous media by accounting for the effects of filtration, energy loss and inertia. We will develop analytic solutions for the problems of a seimi-infinite crack subjected to suddenly applied forces on the crack faces, propagating steadily, and suddenly stopping. Based on the models developed, we will analyze the effects of pore fluid on crack-tip stress singularity, stress fields near the slip plane, effective stress change, energy release rate, and cracking stability. We will also analyze the effects of surficial permeability of the slip plane on crack propatation. This project has academic innovations in the study of crack propagation, the understanding of wave propagation, and the analysis of rupture stability. It is expected that this study could obtain something new about fault slip and provide theoretical foundation for further understanding earthquakes.

天然地震中的断层破裂速度可以达到Rayleigh波速度甚至是超过地层横波速度，理解这种快速破裂现象是地震力学面临的关键问题。已有研究发现地下水流动对震前滑移面附近的应力场分布具有重要影响。为进一步认识流体运移是如何影响同震断层破裂，本项目基于孔隙介质理论开展有关裂纹扩展的理论研究。主要内容包括：1.综合考虑流体运输、能量耗散和惯性效应，理论研究裂纹在快速扩展（包括亚音速扩展和跨音速扩展）中的力学响应，建立流体饱和孔隙介质中裂纹瞬间开裂、稳态传播和突然止裂问题的数学解析模型。2.分析断层动态破裂过程中孔隙流体对尖端奇异性、应力场分布、有效应力变化、能量释放率和断层破裂稳定性的影响。3.研究滑移面渗透性对裂纹扩展的影响。本研究在地壳岩石中的裂纹扩展、弹性波动特性研究以及断层稳定性分析等方面具有学术创新意义，期望够获得有关地震断层破裂和波动的新认识，为进一步认识地震灾害提供理论依据。

越来越多的观测表明地下水的流动对地壳岩石中的裂纹扩展有着重要影响。注水诱导的微地震、水力压裂以及震后断层稳定性变化等物理现象都与地下流体以及流-固耦合孔隙弹性效应密切相关。然而，流体的运输、能量耗散以及地层弹性和渗透率如何综合影响孔隙岩石的破裂以及断层滑动尚不十分明确。基于完整的孔隙介质波动力学模型研究流体对裂纹的扩展影响对地震成核以及油气资源开采具有重要意义。本项目基于Biot理论，研究了流体饱和孔隙介质中裂纹在动态载荷作用下的力学响应，分析了裂纹尖端附近的应力场分布、应力强度因子以及断层泥水力-机械性质的影响，取得了如下成果：（1）相比于传统的干燥裂纹，含流体裂纹能够显著降低裂纹面上的有效正应力，在相同断裂韧度条件下，含流体的模式I裂纹较干燥裂纹不易扩展。（2）地层渗透率、流体粘度以及地层的弹性性质显著影响了模式I应力强度因子的松弛频率。当裂纹长度与慢纵波波长处于同等两级时，模式I应力强度因子随频率增加而下降最快。（3）孔隙流体的存在也会降低模式II应力强度因子。相比于干燥裂纹，流体饱和裂纹应力强度因子峰值会降低10%左右。（4）应力强度因子的松弛频率主要受低渗区域渗透率的影响，例如低渗断层泥能够降低松弛频率从而使得正应力在长时间上对地震成核过程中发挥作用。本项目还研究了含裂纹孔隙介质的动力学有效性质（频率相关的有效模量）以及相应的波动特征，发现波动诱导的慢纵波能够在地震频段（1-1000Hz）主导弹性波的频散和衰减，并与地层渗透率密切相关。