Assessing the roles of wear and roughness on dynamic fault friction

孕育地震的断层自然是粗糙的。断层表面上的粗糙斑块可能会发生碰撞和锁定，从而阻止断层上的任何进一步移动。当这些粗糙的斑块破裂时，地震就开始了，地震特征由断层表面的摩擦过程控制，这些摩擦过程随着地震发生而演变。这个项目将通过探索在地震过程中可能调节断层摩擦的两种机制来促进我们对地震物理学的理解。第一种是加热导致流体压力增加，这可能会通过润滑促进地震中的持续滑动；第二种是由于为润滑液产生更多空间而变硬，这可能会阻止地震。为了更好地了解这些过程，PI将使用布朗大学独一无二的形变仪器在类似地震的条件下进行实验。PI将测试天然断层粗糙度和磨损过程对两个相互竞争的过程的作用。这项工作的结果将促进我们对地震物理学的理解，并将为新的和现有的地震破裂模型的开发和修改提供信息。这些模型通过提高对地震过程的理解，在减轻世界范围内的地震灾害和风险方面发挥了至关重要的作用。该项目还将使PI能够继续参与Deep Cores，这是一个为普罗维登斯当地公立学校开发和实施STEM课程的计划。Deep Cores旨在扩大未被充分代表的群体对STEM领域的参与，并提高公众的科学素养。描述地震滑动过程中地质材料摩擦行为的基于物理的本构方程的实验验证是推进基于物理的地震动态破裂模型的关键步骤。这项工作将利用布朗大学新改装的Tullis旋转剪切仪进行几套岩石动态摩擦实验，研究地震期间可能调节断层摩擦行为的两种机制：热孔隙流体加压弱化(TPW)和扩容硬化(DH)。当摩擦加热的孔隙流体的热膨胀速度快于断层孔隙时，就会出现TPW。在地震滑动期间排水不良的情况下，这会导致孔隙压力增加，从而降低作用在断层上的剪应力，从而削弱断层。相反，剪切作用会导致新的微裂缝的形成，增加总的孔隙体积，从而降低孔隙流体压力，加强断层。在地震中，只有当水汽含量最小时，TPW才是显著的。为了阐明磨损和断层粗糙度对动摩擦力的作用，并探索TPW和DH值之间的平衡，PI在滑动速率高达1m/S、高围压(45-60 Mpa)和高孔压(25-40 Mpa)的情况下对渗透率和滑动表面粗糙度都不同的试件进行实验，模拟自然界中断层粗糙度的范围。力学数据将与微观结构分析和微观机械建模相结合，以指导对结果的分析和解释。这些实验将是第一个在滑动速率为1米/S的情况下独立控制和提高孔压、围压和法向应力的实验。它将建立条件，在天然断层粗糙度的增强下，允许TPW在不同渗透率的样品中发展，还将建立DH在高位移的粗糙表面上抵消TPW的程度。该项目由地球科学、地球物理计划和建立的刺激竞争研究计划(EPSCoR)联合资助。该奖项反映了美国国家科学基金会的法定使命，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。

Faults that host earthquakes are naturally rough. Rough patches on fault surfaces can collide and lock, preventing any further movement on a fault. Earthquakes begin when these rough patches break, and earthquake characteristics are controlled by frictional processes at the fault surface that evolve as the earthquake occurs. This project will advance our understanding of the physics of earthquakes by exploring two mechanisms that may regulate the friction of faults during earthquakes. The first occurs when heating leads to increased pressure in fluids, which can promote continued slip in earthquakes through lubrication, and the second is hardening due to producing more space for the lubricating fluids, which can impede earthquakes. To better understand these processes, the PI will conduct experiments at earthquake like conditions using a one-of-a-kind deformation apparatus at Brown University. The PI will test the roles of natural fault roughness and wear processes on the two competing processes. The results from this work will advance our understanding of earthquake physics and will inform the development and modification of new and existing earthquake rupture models. These models play a vital role in mitigating earthquake hazard and risk worldwide by improving the understanding of earthquake processes. This project will also enable the PI’s continued participation in DEEPS CORES, a program that develops and implements STEM curriculum for local Providence public schools. DEEPS CORES aims to expand participation in STEM fields from under-represented groups and to improve science literacy of the general public.Experimental validation of physics-based constitutive equations that describe the frictional behavior of geologic materials during seismic slip is a critical step in advancing physics-based dynamic rupture models for earthquakes. This work will use the newly modified Tullis Rotary Shear Apparatus at Brown University to conduct several suites of dynamic rock friction experiments investigating two mechanisms that may regulate the frictional behavior of faults during earthquakes: thermal pore-fluid pressurization weakening (TPW) and dilatancy hardening (DH). TPW occurs as frictionally heated pore fluids thermally expand faster than the fault pores. In poorly drained conditions during seismic slip, this leads to increases in the pore pressure that decrease the shear stress acting on the fault thereby weakening the fault. DH has the opposite effect in which shearing causes the formation of new microcracks increases total pore volume, thereby reducing pore fluid pressure and strengthening faults. TPW will only be significant during earthquakes if DH is minimal. To elucidate to roles of wear and fault roughness on dynamic friction and explore the balance between TPW and DH, the PI is conducting experiments at slip rates up to 1 m/s, elevated confining pressures (45-60 MPa), and elevated pore pressures (25-40 MPa) on samples with both variable permeability and sliding surface roughness mimicking the range of fault roughness measured on faults in nature. Mechanical data will be combined with microstructural analysis and micromechanical modelling to guide the analysis and interpretation of results. These experiments will be the first with independently controlled and elevated pore pressure, confining pressure, and normal stress at slip rates of 1 m/s. It will establish conditions under which wear processes, enhanced by natural fault roughness, allow TPW to develop in samples with varying permeabilities and will also establish the extent to which DH counteracts TPW on rough surfaces at high displacements.This project is jointly funded by the Division of Earth Sciences, Geophysics Program and the Established Program to Stimulate Competitive Research (EPSCoR).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.