Dynamic Weakening and Melt Generation Along Mature Faults

本项目重点研究大地震时断层强度的演变。从理论、现场和实验室研究中积累的约束突出了热过程对动态弱化的重要性。在成熟的断裂带中，发生过多次大地震，断层泥层厚度大大超过剪切作用的mm尺度“主滑动面”厚度，孔隙流体的热加压对于降低断层强度和限制剪切加热的程度尤为重要。然而，对于大地震典型范围内的滑动距离，以及对水力输送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融化的开始，特别是在中低发震深度（例如10km）。这些模型计算出的裂缝能与地震估计一致。然而，从挖掘出的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。这些被回收的熔体产品显示出一系列的层厚度和晶体含量，这表明在熔体开始后很长一段时间内，显著的剪切加热仍在持续。为了更好地理解大地震的动力学和强度演变，以及泥质固体的融合如何影响系统行为，需要对有限剪切带中向熔融过渡的研究。有两个主要的概念挑战：1。通常假设摩擦加热的能量输入与有效应力成正比，当宏观熔体层产生时，有效应力消失，热力学考虑要求熔体压力平衡法向应力；2. 熔体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%），这允许在有限有效粘度下的浆料动员，并为随后的熔体分数增加提供必要的粘性热源。前一种考虑促使我们更仔细地研究有效摩擦行为的变化，因为熔化首先开始于高应力（微米尺度）粗糙接触，并且实际接触面积发生变化。后一种考虑表明，可能的熔体开始滚动是一种极端局部化的机制，需要在有限区域内的滑动实际上被容纳在相邻熔融泥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，包括：1。重点研究解决关键的微观尺度相互作用，如决定剪切带宽度的因素，以及由于闪融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统模型——包括潜在的额外能量汇和水源的影响；和3。有针对性的努力，探索熔体转变对破裂特征的动态影响。该项目直接说明了关于成熟的板块边界断层的平均强度的更广泛的科学争论。此外，断层强度的演变是决定滑移分布是否最好地近似于自愈脉冲或裂纹行为的关键因素。地震本身的应力演变也强烈地影响着地震的重现间隔和长期断层动力学，地震破坏的模式也是如此。这个项目即将产生的结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.