汶川地震断层岩在孔隙水压条件下的中-高速摩擦性质研究

Frictional properties of faults in dynamic sliding are of great importance in fault mechanics and earthquake dynamics, as fault friction has significant influence on stress drop, energy budget and coseismic displacement during earthquakes. In recent 20 years, intermediate to high-velocity rock friction experiments have been launched extensively in fault mechanics studies, aiming to simulate dynamic frictional sliding of faults during coseismic stage. In order to get a better understanding on mechanisms of high-angle reverse faulting and complex surface rupture and slip distribution during the Wenchuan earthquake, we try to conduct intermediate to high-velocity friction experiments on fault gouges and cataclasites from the Wenchuan earthquake fault zone in the presence of pressurized pore water, using a rotary-shear low to high-velocity frictional testing machine equipped with a newly-tested pressure vessel. The main concerned issues include (1) the effects of pressurized pore water on intermediate to high-velocity frictional properties of the Wenchuan earthquake fault zone and their homogeneity. (2) The physico-chemical changes of the studied fault rocks during the experiments. (3) The velocity dependence of frictional sliding in the intermediate velocity regime. (4) The mechanisms controlling the frictional behaviors of fault rocks. Based on these experimental explorations, we will compare the experimental results in this study with those obtained in the air-dry condition and discuss the implications of these results for earthquake dynamics.

断层动态滑动的摩擦性质是断层力学和地震动力学中的重要问题，因为地震过程中断层的摩擦显著影响着应力降、能量分配及同震位移。近20年来，断层力学领域兴起了岩石中-高速摩擦实验研究，其目的正是在于在模拟同震阶段断层的动态摩擦滑动。为了更好地认识汶川地震高角度逆冲断层作用的动力学机制以及复杂的地表破裂和位移分布，本项目拟使用旋转剪切低速-高速摩擦实验机和新近调试完成的压力容器，对汶川地震断层带断层泥和碎裂岩样品开展孔隙水压条件下的中-高速摩擦实验研究。重点关注的科学问题包括：(1) 孔隙压力水对汶川地震断层带中-高速摩擦性质及其均一性的影响；(2) 断层岩在实验中所经历的各种物理化学变化；(3) 中速域的摩擦滑动速度依赖性；(4) 决定断层岩摩擦性状的物理机制。在此基础上，我们将本研究中的实验结果和干燥条件下的进行对比，探讨这些结果的地震动力学意义。

断层的摩擦滑动性质对地震成核与破裂传播具有一阶控制作用。现今断层力学的观点认为，地震成核受控于断层在低速下与速率和状态相关的摩擦滑动性质，而微小的预滑如何演变为大地震以及地震破裂传播过程中断层的强度如何演化则受控于断层中速至高速（约mm/s至m/s量级）摩擦滑动性质。为了更好地认识2008年Mw7.9级汶川地震的机制，我们对龙门山断裂带的断层泥、角砾岩和碎裂岩样品开展了低速至高速（10 μm/s – 2.1 m/s）摩擦实验研究。实验使用了装配有压力容器的旋转剪切低速至高速摩擦实验机，实验条件包括干燥、含水和孔隙水压条件。我们对实验后的样品进行了仔细的微结构观察、X射线衍射分析和BET比表面积测量。为更好地估计实验中温度和孔隙压力的演化，进一步约束断层弱化机制，我们还开展了热压作用的数值模拟。这些研究工作获得了如下认识：（1）断层泥在滑动速率小于10–100 mm/s的范围内存在着速度强化，暗示着在地震过程中存在着一个阻碍破裂加速传播的力学障碍。（2）温度升高在断层泥的动态弱化中起到了至关重要的作用，仅仅是纳米颗粒物质不足以造成断层的弱化。（3）Flash heating是干燥条件下最可能的断层主导弱化机制；同时，在含水条件下，flash heating可能触发局部滑动带内孔隙压的迅速升高，造成断层在远低于常规所认为的特征弱化速率的情况下呈现快速动态弱化，从而显著地促进动态破裂的传播。（4）孔隙水的汽化能够促进断层同震滑动中的流体增压效应，对断层带的温度升高也起到明显的缓冲作用。（5）无论是干燥还是含水条件，地震断层滑动中的颗粒磨碎效应都仅仅吸收了很小一部分能量。