Laboratory Experiments on Rock Friction Focused on Understanding Earthquake Mechanics

岩石在实验室摩擦滑动实验中的行为相当好地由所谓的速率和状态变量摩擦定律描述，并构成其基础。尽管这些定律在地震模型中重现了各种各样与地震有关的现象，但摩擦滑动过程中岩石表面微观接触处发生的导致观测到的速度和状态效应的物理机制仍然知之甚少。由于缺乏知识，无法将现有的实验室摩擦数据可靠地外推到地球上。缺乏速度和状态摩擦定律的物理基础，部分原因是难以在实验室中隔离和研究岩石表面之间微米级接触的行为。在这项研究中，纳米压头被用来利用精确控制的接触载荷和几何形状来有效地隔离单个粗糙接触的变形行为。在与抛光的地质样品接触的压头上施加恒定载荷。然后在恒定载荷下(在静态压痕试验中)将压头保持指定的时间，或者在恒定载荷下(在滑动压痕试验中)在试件上平移压头。可以同时进行静态和滑动测试，这使得可以设计类似于两种最常用的岩石摩擦测试的压痕测试，即速度步进测试和滑动-保持-滑动测试。压头/试件的接触面积在整个压痕试验过程中不断确定，从而可以量化接触面积的变化对摩擦变化的影响程度。在机械测试之后，使用扫描和透射电子显微镜对压痕样品进行了广泛的微观结构分析。力学测试和微观结构分析提供了确定由速率和状态摩擦定律描述的微观物理机制所需的基本数据，从而形成了理解地震起源的基础。

The behavior of rocks during frictional sliding experiments in the laboratory is reasonably well described by, and forms the basis for, the so-called rate and state-variable friction laws. In spite of the fact that these laws reproduce a rich variety of earthquake-related phenomena when employed in models of earthquakes, the physical mechanisms that occur at the microscopic contacts between rock surfaces during frictional sliding, which cause the observed rate and state effects, remain poorly understood. This lack of knowledge precludes making reliable extrapolation of existing laboratory friction data to the Earth. The lack of a physical basis for the rate and state friction laws stems in part from the difficulty of isolating and studying the behavior of micron-sized contacts between rock surfaces in the laboratory. In this study a Nanoindenter is used to effectively isolate the deformation behavior of a single asperity contact using a precisely controlled contact load and geometry. A constant load is applied to an indenter tip in contact with a polished geologic specimen. The indenter is then held for a specified time under constant load (in static indentation tests) or translated across the specimen under constant load (in sliding indentation tests). The ability to conduct both static and sliding tests allows indentation tests to be designed which are analogous to the two most commonly used rock friction tests, the velocity-stepping test and slide-hold-slide test. The area of the indenter/specimen contact is determined continuously throughout the indentation tests, allowing the degree to which changes in contact area effect changes in friction to be quantified. Mechanical tests are followed by extensive microstructural analyses of indented specimens using scanning and transmission electron microscopy. The mechanical tests and microstructural analyses provide fundamental data required to identify the microphysical mechanisms described by the rate and state friction laws and thus form a basis for understanding the origin of earthquakes.