Mechanochemical Processes dictating Calcite's Frictional Characteristics

地震是反复发生的破坏性事件。然而，要准确预测它们的发生和规模仍然很困难。这是因为地震发震断层破裂的机制还没有完全被理解。水在断层动力学中起着重要作用；然而，断层界面的复杂性阻碍了确定影响断层行为的具体机制。在这里，研究人员调查了方解石--一种在断层泥中发现的矿物--在水环境中的摩擦强度。为了研究富含水的流体的影响，他们使用了一个理想化的颗粒界面，该界面由两个1毫米的单晶相互滑动组成，中间有或没有流体。通过原子力显微镜在微观尺度上表征了界面包含的粗糙接触。这项研究与天然断层有关，因为施加在界面上的应力和滑动速率与地质环境中的应力和滑动速率相当。摩擦特性是用一种新的技术来量化的，这种技术测量位移的精度为1埃(0.1万分之一毫米)。这使得可以研究所涉及的机制，例如接触上的晶体溶解/再结晶。该项目的结果改进了断层摩擦模型，有助于改进在存在反应性流体的情况下对地震危险性的评估。该项目促进了对研究生的支持，对本科生的培训，以及对K-12学生的接触，特别是来自科学方面代表性不足的群体。该团队遵循两条研究主线。在实验水平上，该项目提出了机械力-化学界面反应及其对方解石摩擦特性的影响的认识。它揭示了晶界压解的潜在机制。评估了不同因素的影响：流体化学、表面电位、层状硅酸盐或受限流体膜的存在。定量分析了滑动速度、应力、温度、接触时间和表面形貌对摩擦和粘着的影响。这项工作的关键是对表面力仪器进行改进。新的开发允许测量方解石湿接触处的蠕变变形，精度为0.1 nm，以及界面力(分离压力、粘附力和摩擦力)。这提供了在各种条件下压力溶液蠕变与静、动态摩擦之间的关系的洞察力。实验的简单几何结构允许研究导致宏观摩擦的微观机制，并测试描述它们的理论。在理论水平上，基于剪切辅助的热激活滑移理论，模拟了单粗糙面和多粗糙面接触的摩擦，考虑了压力溶液蠕变引起的接触老化。将新的模型参数与速率-状态摩擦本构方程及其经验参数进行了比较和检验。因此，该项目填补了在理解水环境中断层摩擦的微观和宏观尺度之间的空白。该奖项由极端事件预测和弹性(PREEVENTS)计划共同资助。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Earthquakes are recurring and devastating events. To predict accurately their occurrence and magnitude remains, however, difficult. This is because the mechanisms underlying the rupture of earthquake-generating faults are not fully understood. Water plays an important role in fault dynamics; yet, the complexity of fault interfaces has hindered identifying the specific mechanisms influencing fault behaviors. Here, the researchers investigate the frictional strength of calcite - a mineral found in fault gouges - in aqueous environment. To study the effects of water-rich fluids, they use an idealized grain interface consisting of two 1-mm single crystals sliding against each other, with or without fluids in between them. The interface contains asperity contacts characterized at the microscale by atomic force microscopy. The study is relevant to natural faults because the stresses and slip rates applied to the interface are comparable to those of geological settings. Frictional characteristics are quantified using a new technique which measures displacements with an accuracy of 1 angstrom (0.1 millionth of a mm). This allows investigating the involved mechanisms, such as crystal dissolution/recrystallization on contacts. The project's results improve models of fault friction, which contributes improving the assessment of seismic hazards in the presence of reactive fluids. This project promotes support for a graduate student, training for undergraduate students and outreach to K-12 students, notably from groups underrepresented in Science. The team follows two main lines of research. At the experimental level, the project advances the knowledge of mechano-chemical interfacial reactions and their influence on calcite's frictional characteristics. It informs the mechanisms underlying pressure-solution at grain boundaries. The influence of different factors is evaluated: fluid chemistry, surface electric potential, the presence of phyllosilicates or confined fluid films. The effects of sliding velocity, stress, temperature, contact time and topography, on friction and adhesion are also quantified. Essential to this work is a modification of the Surface Forces Apparatus. The new development allows measuring creep deformation at wet calcite contacts with a precision of 0.1 nm, as well as interfacial forces (disjoining pressure, adhesion and friction). This provides insight on the relationship between pressure-solution creep and static and dynamic friction over a wide range of conditions. The simple geometry of the experiments allows to investigate the microscopic mechanisms responsible for macroscopic friction, and to test the theories that describe them. At the theoretical level, friction at single- and multi-asperity contacts are modeled based on the shear-assisted thermally activated slip theory, accounting for contact aging due to pressure-solution creep. The new model parameters are compared to and tested against the rate-and-state friction constitutive equations and their empirical parameters. The project, thus, fills the gap between the microscopic and the macroscopic scales in the understanding of fault friction in aqueous environment.This award is co-funded by the Prediction of and Resilience against Extreme Events (PREEVENTS) program.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.