The granular physics contribution to rate- and state-dependent fault friction

颗粒物理对速率和状态相关的断层摩擦的贡献

• 批准号: 1946434
• 负责人: Allan Rubin
• 金额: $ 32.78万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-02-01 至 2024-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1946434&HistoricalAwards=false
• 关键词: granular; physics; contribution; rate; state

Friction plays a critical role in many areas of societal interest, including transportation and manufacturing. In Earth Sciences, understanding friction is critical for a better understanding of the hazards associated with earthquakes and landslides. The friction properties of materials have been studied for centuries, but the physics and chemistry underlying their time dependence remain obscure. Yet, small fluctuations in friction properties during earthquakes and landslides can have tremendous effects on the size and speed of these events. Friction on sliding interfaces such as tectonics faults are usually described by the so called "rate- and state-dependent friction" laws. These empirical laws account for the sliding speed ("rate") and for the evolving properties of the interface termed "state"; this latter, a function of the slip history, is difficult to observe directly. The rate-and-state framework is widely used to model frictional sliding. But the corresponding laws fail to accurately describe laboratory observations for a range of conditions relevant to earthquakes. Here, the team aims to better understand the physics underlying the frictional properties of rocks. The researchers use computer simulations to model the behavior of granular layers of finely-ground rock, called gouge, that are present along tectonic faults. The goal is to test whether rock friction and its time dependence is governed at the grain scale by grain-to-grain interactions. The simulation outputs are constrained by experimental observations: in many cases they describe them better than the most successful rate-and-state friction laws. The team, thus, gradually unveils the physics underlying the behavior of earthquake-generating faults. In addition to its strong societal relevance, this project provides support for an early career scientist as well as training for undergraduate students. To model the behavior of the gouge, the researchers employ Discrete Element Method simulations. They use model geometries and loading conditions designed to mimic standard rock-friction experiments, such as "velocity-step" and "slide-hold-reslide" protocols. They test the hypothesis that rock friction as observed in the laboratory is governed by time-independent properties at the grain-grain contact scale. This innovative approach differs from more traditional ones which assume that time-dependent plasticity or chemical bonding at microscopic contacts are the source of the rate-and-state dependence of friction. The granular simulations are consistent with the most successful rate-and-state-dependent friction equations for sliding protocols where those equations accurately describe experiments ("velocity-step" and “slide-hold” protocols). They better match laboratory data for sliding protocols where those equations fail (e.g., the reslides following "slide-hold" protocols). Furthermore, output of the granular simulations allows investigating the source of the rate-and-state-dependent friction-like behavior of the model. The team finds that if the kinetic energy of the gouge particles is suitably normalized by the confining pressure, it produces an estimate of the velocity dependence that is consistent with the simulations and within the ballpark of laboratory data. The researchers continue exploring the granular flow model by comparing it to a wider range of sliding protocols that are not well explained by existing equations (e.g., "slide-hold-reslide" and "normal-stress-step" experiments). They also compare the compaction/dilation of the gouge layers in the simulations to experimental observations; the goal is to evaluate the role of porosity on the gouge sliding behavior.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

摩擦在许多社会关注的领域（包括运输和制造）中起着至关重要的作用。在地球科学中，理解摩擦对于更好地理解与地震和滑坡相关的危害至关重要。已经研究了材料的摩擦特性数百年，但是它们的时间依赖性的物理和化学仍然晦涩难懂。然而，地震和滑坡期间摩擦特性的微小波动可能会对这些事件的大小和速度产生巨大影响。在滑动界面（例如构造断层）上的摩擦通常由所谓的“速率和状态依赖性摩擦”定律描述。这些经验定律解释了滑动速度（“速率”）以及所谓的“状态”界面的不断发展的特性；后来，很难直接观察到滑动历史记录的功能。速率和状态框架被广泛用于建模摩擦滑动。但是相应的法律无法准确描述与地震相关的一系列条件的实验室观察。在这里，团队旨在更好地了解岩石摩擦特性的基础物理学。研究人员使用计算机模拟对沿构造断层呈现的细岩石颗粒层的颗粒层的行为进行建模。目的是测试岩石摩擦及其时间依赖性是否通过晶粒对粒度相互作用来控制晶粒尺度。模拟输出受到实验观察的限制：在许多情况下，它们比最成功的速率和状态摩擦定律更好地描述了它们。因此，该团队逐渐揭示了地震生成断层行为的基础物理学。除了具有强大的社会相关性外，该项目还为早期职业科学家以及对本科生的培训提供了支持。为了建模凿子的行为，研究人员员工离散元素方法模拟。他们使用模型的几何形状和旨在模拟标准岩石摩擦实验的加载条件，例如“速度步骤”和“幻灯片呈偏斜”协议。他们检验了一个假设，即实验室中观察到的岩石摩擦受晶粒晶粒接触量表的时间独立特性的控制。这种创新的方法与更传统的方法有所不同，这些方法假设在微观接触处的时间依赖性的可塑性或化学键是摩擦的速率和状态依赖性的来源。颗粒模拟与滑动协议的最成功的速率和状态依赖性摩擦方程是一致的，在这些方程中，这些方程准确地描述了实验（“速度步骤”和“滑动”协议）。它们更好地匹配了这些方程失败的滑动协议的实验室数据（例如，以下“幻灯片”协议重复）。此外，颗粒模拟的输出允许研究模型的速率和状态依赖性摩擦行为的来源。该小组发现，如果岩体的动能通过限制压力适当地归一化，则会产生与模拟并在实验室数据的球场内相符的速度依赖性的估计。研究人员通过将其与现有方程未很好地解释的广泛的滑动协议进行比较（例如，“幻灯片呈较高的”和“正常的压力步骤”实验），继续探索颗粒流模型。他们还比较了模拟中岩石层的压实/扩张与实验观察结果；目的是评估孔隙率在岩石滑动行为上的作用。该奖项反映了NSF的法定任务，并使用基金会的知识分子优点和更广泛的影响评估标准，被视为通过评估来获得珍贵的支持。