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

项目的意义和重要性。 地震是由地球上的断层引发的？令人难以置信的是，地球的地壳受到物理过程的控制，这些物理过程发生在沿着断层接触的岩石粗糙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，发生在微观接触的物理机制的确切身份和性质，并引起实验中观察到的摩擦效应，仍然是未知的。 描述的经验性质反映了分离和研究微观断层接触的物理过程的困难。 没有一个健全的物理理解，我们仍然在我们的能力有限，可靠地应用方程的地震在自然界中，以获得更好的一般理解的地震过程，并最终作出可靠的预测地震。 在这项变革性的研究中，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，从而更好地了解地震过程。 这项工作可以让我们了解我们是否有可能在地震发生前检测到断层上的加速蠕变，从而在地震发生前几天到几小时预测地震，这将挽救许多生命并减轻对建筑环境的破坏。 从力学和材料科学的科学领域的角度来看，这些新的见解，通过识别和连接在许多长度尺度上的摩擦行为，有潜在的应用远远超出了电子物理学，例如，在许多工程系统，包括硅基微机械设备。技术说明。建议的研究的总体目标是隔离和识别的物理机制，发生在粗糙接触摩擦界面。 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？进化？影响率和国家摩擦，其中最简单的表现是增加？静电干扰？摩擦力随时间的准静态接触。 为此，我们将进行协调的跨学科合作，将采用实验室实验，研究各种长度尺度上的摩擦现象。结果之一将是开发本构方程，允许将这些机制外推到与地震相关的高温和更长时间。我们将在布朗大学的岩石上进行宏观尺度的摩擦实验，在橡树岭国家实验室的纳米压痕仪中进行微米尺度到纳米尺度的压痕蠕变、粘附和摩擦实验，在宾夕法尼亚大学的原子力显微镜中进行纳米尺度的粘附和摩擦实验。为了在不同类型的实验之间建立联系，并隔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。这三种环境因素都对宏观岩石摩擦实验中的演化效应产生了影响。纳米压痕和原子力显微镜测量应该使我们能够确定粗糙尺度上的宏观行为负责的过程。

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.