水热作用下花岗岩和辉长岩的中-高速摩擦性质研究

The frictional properties of seismic faults are perceived as one of the most important factors that control the nucleation and propagation of earthquakes. In the past 20 years, intermediate- to high-velocity (ca. mm/s–m/s) rock friction studies deepened our understanding on earthquake physics, in particular the dynamic weakening mechanisms of seismic faults. However, by now, technical challenges related to heating technology and pore fluid confinement during rapid shear impeded to perform experiments under conditions typical of the ambient conditions of natural faults (e.g. hydrothermal conditions). This limits our understanding on coseismic faulting within natural fault zones during earthquakes. Recently, a new externally heated pressure vessel (temperature up to 500°C; pore pressure up to 70 MPa) have been developed for the rotary-shear rock friction apparatus at State Key Laboratory of Earthquake Dynamics. Thanks to this technical improvement, in this project, we plan to carry out a comparative study of intermediate- to high-velocity frictional properties of granite and gabbro, which are both typical crystalline rocks at seismogenic depth, under hydrothermal conditions. The main concerns are (1) the effects of pore water and ambient temperature on the tribochemical processes and friction velocity dependences of the two rocks at intermediate slip rates; (2) the frictional melting and melt lubrication effects under hydrothermal conditions; (3) the mechanisms that control the frictional behaviors of the two rocks at intermediate to high velocities.

地震断层的摩擦滑动性质是控制地震成核和破裂传播的最重要因素之一。过去20年里，岩石中-高速（mm/s–m/s量级）摩擦实验深化了对地震物理过程，特别是断层动态弱化机制的认识。然而，限于快速滑动中加温方式和孔隙水密封等技术问题，目前国际上仍未开展过更接近自然断层环境条件下（如水热条件）的中-高速摩擦实验，这在一定程度上限制了对野外断层带同震断层作用的深入了解。最近，地震动力学国家重点实验室为旋转剪切岩石摩擦设备研制了外加温孔隙压力容器（温度最高达500°C，孔隙压最高达70 MPa）。得益于这一技术突破，本项目拟对壳内孕震深度范围内两种代表性的结晶岩——花岗岩和辉长岩开展水热条件下的中-高速摩擦性质实验研究。重点将探索（1）孔隙水和背景温度对两种岩石中速滑动下摩擦化学作用和摩擦速度依赖性的影响；（2）水热条件下的高速摩擦熔融与熔体润滑效应；（3）两种岩石中-高速摩擦行为的主控机制。

获取水热条件下断层的摩擦滑动性质是约束野外断层带滑动行为和地震成核条件的有效手段。本项目使用地震动力学国家重点实验室的旋转剪切岩石摩擦设备及配套的外加温孔隙压力容器（温度最高达500°C，孔隙压最高达70 MPa），对壳内孕震深度范围内代表性的结晶岩——花岗岩和辉长岩开展了水热条件下的低速至高速摩擦性质研究。取得的主要认识如下：（1）断层同震高速滑动起始阶段的磨损和局部熔融会通过产生细颗粒物质带来孔隙流体的圈闭效应，促进瞬态热压作用的发生。考虑这一效应，热压作用在渗透性相对较好的断层带也可能发生，因此可能是野外断层带上非常普遍的弱化机制；断层同震滑动中的等效渗透率也因而是一个快速演化的复杂物理量，难以被常规的渗透率实验估计准确。（2）辉长岩在滑动速率大于1 μm/s、孔隙水为液态和超临界态的条件下经历数百毫米位移的摩擦滑动后，均能呈现出显著的滑动弱化。滑动带内生成的非晶化物质的水岩相互作用是这一现象的可能原因。这些结果表明位移对断层准静态摩擦强度具有重要的影响。（3）背景温度能够促进辉长岩的动态弱化，flash heating可以解释这一现象；断层动态弱化随深部的变化未来值得在地震动态破裂的模拟中得到应用。（4）花岗岩和辉长岩在正应力相对较低的条件下，常温至400°C 均表现为速度弱化，这与前人的研究结果显著不同，反映了正应力对断层摩擦滑动稳定性具有重要影响。实验研究中需要同时满足正应力和温度的原位条件才可能准确地评估断层稳定性和地震危险性。