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.

摩擦力在许多社会关注的领域中起着关键作用，包括运输和制造业。在地球科学中，了解摩擦力对于更好地了解与地震和山体滑坡相关的危害至关重要。材料的摩擦特性已经研究了几个世纪，但其时间依赖性背后的物理和化学仍然不清楚。然而，在地震和山体滑坡期间摩擦特性的微小波动可能对这些事件的规模和速度产生巨大影响。滑动界面（如构造断层）上的摩擦通常用所谓的“速率和状态相关摩擦”定律来描述。这些经验定律解释了滑动速度（“速率”）和界面的演化特性（称为“状态”）;后者是滑动历史的函数，很难直接观察到。速率和状态框架被广泛用于摩擦滑动模型。 但是，相应的定律不能准确地描述与地震有关的一系列条件下的实验室观测结果。在这里，该团队的目标是更好地了解岩石摩擦特性的物理基础。研究人员使用计算机模拟来模拟沿着构造断层存在的被称为断层泥的细磨岩石颗粒层的行为。我们的目标是测试是否岩石摩擦和它的时间依赖性是在晶粒尺度上由晶粒间的相互作用。模拟输出受到实验观测的约束：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它们。 因此，该团队逐渐揭示了地震发生断层行为背后的物理学。除了其强大的社会相关性，该项目还为早期职业科学家提供支持，并为本科生提供培训。为了模拟凿痕的行为，研究人员采用了离散元法模拟。他们使用模型的几何形状和加载条件，旨在模仿标准的岩石摩擦实验，如“速度步”和“滑动保持reslide”协议。他们测试了这样一个假设，即在实验室中观察到的岩石摩擦力是由颗粒-颗粒接触尺度上的时间无关性质决定的。这种创新的方法不同于更传统的假设，时间相关的塑性或化学键合在微观接触的速率和状态的依赖性摩擦的来源。颗粒模拟与最成功的速率和状态相关的摩擦方程滑动协议，这些方程准确地描述实验（“速度步骤”和“幻灯片保持”协议）是一致的。它们更好地匹配实验室数据，用于那些方程失败的滑动协议（例如，遵循“滑动-保持”协议的重新滑动）。此外，颗粒模拟的输出允许研究模型的速率和状态相关摩擦样行为的来源。 研究小组发现，如果泥颗粒的动能被围压适当地归一化，它会产生一个与模拟一致的速度依赖性估计，并且在实验室数据的范围内。研究人员继续探索颗粒流模型，将其与现有方程无法很好解释的更广泛的滑动协议进行比较（例如，“滑动-保持-再滑动”和“正常应力-步骤”实验）。 他们还将模拟中断层泥层的压实/膨胀与实验观察进行比较;目的是评估孔隙度对断层泥滑动行为的作用。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。