Dynamic Weakening and Melt Generation Along Mature Faults

该项目的重点是大地震期间断层强度的演变。从理论，现场和实验室研究积累的限制突出了热过程的重要性，动态削弱。在成熟的断层带，容纳了许多大地震，其特点是断层泥层的厚度大大超过毫米级的“主滑动面”的剪切局部化，热加压的孔隙流体是特别重要的，以减少故障的强度和限制的程度剪切加热。然而，对于大地震的典型范围内的滑动距离和水力输运特性和其他控制变量的合理估计，预测的温度升高通常会达到熔融的开始，特别是在中低发震深度（例如10 km）。从这些模型计算的断裂能是一致的地震估计。然而，从成熟断裂带的熔融产物的现场证据表明，宏观熔融实际上是相当罕见的。回收的那些熔融产物显示出一定范围的层厚度和晶体含量，这表明显著的剪切加热在熔融开始后持续很长时间。为了更好地理解大地震的动力学和强度演化，以及断层泥固体的熔融如何影响系统行为，需要对有限剪切带中熔融的过渡进行检查。有两个主要的概念挑战：1。摩擦加热的能量输入通常被假定为与有效应力成比例，当产生宏观熔体层时，有效应力消失，并且热力学考虑要求熔体压力平衡法向应力; 2.在熔融开始时，有限剪切区的典型初始晶体含量几乎肯定超过临界固体分数（~50%），该临界固体分数允许在有限有效粘度下的浆料流动，并提供熔融分数随后增加所需的粘性热源。前一种考虑促使更仔细地检查有效摩擦行为的变化，因为熔化首先在高应力（微米级）粗糙接触处开始，并且接触的真实的面积发生变化。后一种考虑表明，作为极端局部化的机制，熔体开始的可能滚动，需要在有限区域内的滑移实际上被容纳在相邻熔融凿颗粒之间的一系列短暂有效剪切表面上。该项目包括多尺度建模工作，结合：1。重点研究，旨在解决关键的微观尺度的相互作用，如确定剪切区宽度的因素，以及由于闪熔而导致的有效摩擦系数的降低; 2.描述断层强度和温度演变的连续模型-包括潜在的额外能量汇和水源的影响;以及3.有针对性的努力，探索断裂特性的熔体转变的动态影响。该项目直接谈到了关于成熟的板块边界断层平均强度的更广泛的科学辩论。此外，断层强度的演变是确定滑动分布是否最好地近似于自愈脉冲或裂纹样行为的关键因素。复发间隔和长期断层动力学也强烈地受到地震期间应力演化的影响，地震破坏的模式也是如此。该项目即将取得的成果将为这些问题和其他更广泛的科学问题提供有价值的见解。

This project focuses on the evolution of fault strength during large earthquakes. Accumulating constraints from theoretical, field and laboratory investigations highlight the importance of thermal processes to dynamic weakening. In mature fault zones that have accommodated many large earthquakes and are characterized by gouge layers that greatly exceed the thickness of the mm-scale "principal slip surfaces" in which shear is localized, the thermal pressurization of pore fluids is particularly important for reducing the fault strength and limiting the extent of shear heating. Nevertheless, for slip distances within the typical range for large earthquakes and reasonable estimates of hydraulic transport properties and other controlling variables, predicted temperature increases do often reach the onset of melting, especially at mid to lower seismogenic depths (e.g.10km). Calculated fracture energies from these models are consistent with seismic estimates. However, field evidence of melt products from exhumed mature fault zones suggests that macroscopic melting is actually quite rare. Those melt products that are recovered display a range of layer thicknesses and crystal contents, which indicate that significant shear heating continued long after melt onset. An examination of the transition to melting in a finite shear zone is required to better understand the dynamics and strength evolution of large earthquakes, and how the fusion of gouge solids affects the system behavior. There are two main conceptual challenges: 1. the energy input for frictional heating is generally assumed to be proportional to the effective stress, which vanishes when macroscopic melt layers are produced and thermodynamic considerations require that the melt pressure balance the normal stress; 2. the typical initial crystal content of a finite shear zone at melt onset almost certainly exceeds the critical solids fraction (~50%) that allows for slurry mobilization at a finite effective viscosity and provides the viscous heat source necessary for the melt fraction to increase subsequently. The former consideration motivates a closer examination of changes to the effective frictional behavior as melting first begins at highly stressed (um-scale) asperity contacts and changes in the real area of contact occur. The latter consideration suggests the likely roll of melt onset as a mechanism for extreme localization, requiring slip in a finite zone to be actually accommodated on a series of short-lived effective shear surfaces between adjacent melting gouge particles. This project encompasses a multi-scale modeling effort that combines: 1. focused studies aimed at resolving key micro-scale interactions such as the factors that determine the shear zone width, and reductions to the effective friction coefficient due to flash-melting; 2. continuum models that describe the evolution of fault strength and temperature - including the influence of potential additional energy sinks and water sources; and 3. targeted efforts that explore the dynamic implications of the melt transition on rupture characteristics. The project speaks directly to the wider scientific debate over the average strength of mature, plate-bounding faults. Moreover, the evolution of fault strength is a key factor in determining whether slip distributions are best approximated by self-healing pulses or crack-like behavior. Recurrence intervals and long-term fault dynamics are also strongly influenced by the evolution of stress during earthquakes themselves, as are the patterns of seismic damage. The results forthcoming from this project will provide valuable insight into these and other broader scientific questions.