oCollaborative Research: Rock Friction, Nanoindentation, and Atomic Force Microscope Experiments Focused on Understanding Earthquake Mechanics

该项目的意义和重要性。地球上断层上的地震是如何发生的？令人难以置信的是，S地壳是由物理过程控制的，这些物理过程发生在沿着断层接触的粗糙岩石表面之间的微观接触处。尽管经验摩擦方程成功地描述了实验室摩擦实验的结果，并在地震的计算机模型中产生了各种与地震有关的现象，但这些方程缺乏物理基础。也就是说，发生在微观接触并在实验中引起观察到的摩擦效应的物理机制的确切身份和性质仍然未知。这些描述的经验性反映了分离和研究微观断层接触处的物理过程的难度。如果没有良好的物理理解，我们在将方程可靠地应用于自然界中的地震、更好地全面了解地震过程以及最终做出可靠的地震预测方面的能力仍然有限。在这项革命性的研究中，我们将利用最先进的材料科学测试方法，即原子力显微镜和纳米压痕，为更粗尺度的摩擦观测提供物理基础，从而大大改善对地震过程的了解。这项工作可能让我们了解到，我们是否有可能在地震前检测到断层上的加速蠕变，从而在地震发生前几天到几个小时预测地震，这将挽救许多人的生命，减轻对建筑环境的破坏。从力学和材料科学的科学领域来看，这些通过识别和连接许多长度尺度上的摩擦行为而获得的新见解，具有远远超出地球物理的潜在应用，例如，在许多工程系统中，包括硅基微机械设备。技术说明。拟议研究的首要目标是隔离和识别摩擦界面上的粗糙接触处发生的物理机制。我们研究的一个更具体的主要目标是了解摩擦状态的起源？演化？对速率和状态摩擦的影响，其最简单的表现是？静态？摩擦力随时间呈准静态接触。为此，我们将进行协调的、跨学科的合作，利用实验室实验在广泛的长度尺度上调查摩擦现象。一个结果将是开发出本构方程，允许将这些机制外推到与地震相关的高温和更长时间。我们将在布朗大学进行岩石的宏观摩擦实验，在橡树岭国家实验室的纳米压痕蠕变、粘合和摩擦实验中进行微尺度到纳米尺度的压痕实验，在宾夕法尼亚大学的原子力显微镜中进行纳米尺度的粘合和摩擦实验。为了在不同类型的实验之间建立联系，并分离状态演化效应的不同来源，我们将在所有三组实验中改变相同的环境条件。这些测试包括湿度、PH值(液体中)和温度的函数。在宏观岩石摩擦实验中，这三个环境因素都被证明对演化效应有影响。纳米压痕和原子力显微镜测量应该使我们能够确定对宏观行为负责的粗糙尺度上的过程。

Significance and importance of the project. The initiation of earthquakes on faults in the Earth?s crust is controlled, incredibly, by physical processes that occur at microscopic contacts between rough surfaces of rock that touch along the fault. Despite the success of empirical friction equations in describing the results of laboratory friction experiments and producing a variety of earthquake-related phenomena in computer models of earthquakes, these equations lack a physical basis. That is, the precise identity and nature of the physical mechanisms that occur at microscopic contacts, and give rise to the observed friction effects in experiments, remain unknown. The empirical nature of the descriptions reflects the difficulty of isolating and studying the physical processes at microscopic fault contacts. Without a sound physical understanding, we remain limited in our abilities to reliably apply the equations to earthquakes in nature, to obtain a better general understanding of the earthquake process, and to ultimately make reliable predictions of earthquakes. In this transformative study, we will make use of state-of-the-art materials science testing methods, namely atomic force microscopy and Nanoindentation, to provide a physical basis for friction observations at a coarser scale and thereby gain a much improved understanding of the earthquake process. This work may allow us to learn whether we are likely to be able to detect accelerating creep on faults just prior to an earthquake and thereby predict earthquakes days to hours before an earthquake, which would save many lives and mitigate damages to the built environment. From the perspective of the scientific fields of mechanics and materials science, these new insights, gained by identifying and connecting frictional behavior across many length scales, have potential application well beyond geophysics, for example, in many engineered systems, including silicon-based micromechanical devices. Technical description. The overarching goals of the proposed studies are to isolate and identify the physical mechanisms that occur at asperity contacts at frictional interfaces. A more specific major goal of our study is to understand the origin of the friction state ?evolution? effect in rate and state friction, the simplest manifestation of which is an increase in ?static? friction with the time of quasi-stationary contact. To that end, we will conduct a coordinated, interdisciplinary collaboration that will employ laboratory experiments that investigate frictional phenomena over a wide range of length scales. One outcome will be to develop constitutive equations that will allow extrapolation of these mechanisms to the elevated temperatures and longer times relevant for earthquakes. We will perform macro-scale friction experiments on rocks at Brown University, micro-scale to nano-scale indentation creep, adhesion, and friction experiments in the Nanoindenters at Oak Ridge National Laboratory, and nano-scale adhesion and friction experiments in atomic force microscopes at the University of Pennsylvania. To make connections between the different types of experiments and to isolate different origins of the state evolution effect, we will vary the same environmental conditions in all three sets of experiments. These include tests as a function of humidity, pH (in liquid), and temperature. All three environmental factors have been demonstrated to influence the evolution effect in macroscopic rock friction experiments. Nanoindentation and AFM measurements should allow us to determine the processes on an asperity scale responsible for the macroscopic behavior.