Physically-Based Fault Zone Constitutive Responses and Consequences for Earthquake Dynamics

断层本构响应的物理描述正被用来解决地震动力学中的关键问题。 主要有两个主题：（1）在目前设计地壳地震序列模型的尝试中，利用速率和状态本构参数的实验激发的温度变化（因此深度变化），已经注意到，随着状态演化滑动距离L向实验室范围内的值减小，出现了重要的新特征。 这些是一个人口的小事件的出现，聚集向基地的孕震区，以及由此产生的异质残余应力模式，从这些事件上的最早阶段的大事件的地震辐射的影响。 这个过程似乎很有希望解释许多大型事件中最初犹豫不决的辐射，称为“地震成核阶段”。 为了在现有计算机上进行这样的计算，必须使L比实验室值大得多，实验室值的数量级为10微米，因为所需的数值网格尺寸是L的一个大因子（10^5数量级）。 然而，随着L向这些值减小，有趣的行为正在出现。 该项目通过结合新的数值研究，与简化模型的渐近分析相协调，随着L的大小减小，为解释和外推提供基础，从而解决小L范围问题。(2)热弱化效应被认为发生在大地震的快速滑动过程中，导致有效摩擦系数从实验室般的值（当传播的破裂前缘首次到达断层上的一个点时）减小到快速和大滑动发生时的低得多的值。 这个问题正在解决的基础上，分别侧重于最早阶段的滑动，熔化发生之前，并在活跃的pseudotachylyte发展的成熟阶段的初步研究。 当滑移率高（1 m/s）但总滑移仍然很小时，在可以量化的意义上，粗糙接触处的闪速加热预计是主要的热弱化过程。 初步分析捕捉了一些可用的实验功能。 目前正在研究如何解决部分熔融时滑移范围大得多的问题。 其中包括一个观点，支持pseudotachylyte的意见，即一个粒状断层泥成为液化通过发展少量的部分熔融，并自我调节的过程中的速度减弱的字符发展，因为这个高压相渗透到相邻的断层壁。 这些概念正在开发的目的是将它们与数值模拟和理论的破裂模式如何取决于本构响应，研究破裂动力学的后果。这将有助于理解主要断层系统如何在实际较低的总驱动应力下运行，即使当启动滑动所需的局部应力大得多时，并量化最小平均应力水平，对于该最小平均应力水平，破裂一旦在局部高剪切应力或低有效正应力的位置启动，就可以传播很长的距离。

Physically based descriptions of fault constitutive response are being used to address key problems in the dynamics of earthquakes. There are two main topics: (1) In current attempts to devise models of crustal earthquake sequences, using experimentally motivated temperature variation (hence depth variation) of rate and state constitutive parameters, it has been noticed that important new features emerge as the state-evolution slip distance L is decreased towards values in the laboratory range. These are the emergence of a population of small events that is clustered towards the base of the seismogenic zone, and the effect of the resulting heterogeneous residual stress patterns from those events on the earliest phases of seismic radiation in large events. This process seems promising to explain the initially hesitant radiation in many large events, known as the "seismic nucleation phase". To fit such calculations on present computers, L must be made much larger than laboratory values, which are of order of magnitude 10 microns, since the required numerical grid size scales with a large factor (of order 10^5) times L. Yet interesting behavior is emerging as L is reduced towards those values. This project addresses the small L range by a combination of new numerical studies, coordinated with asymptotic analysis of simplified models, as L is decreased in size, to provide a basis for interpretation and extrapolation.(2) Thermal weakening effects are thought to occur during rapid slip in major earthquakes, causing the effective friction coefficient to diminish from lab-like values, present when a propagating rupture front first reaches a point on a fault, to much lower values when rapid and large slip occurs. This problem is being addressed by building on initial studies that focus separately on the earliest phases of sliding, before melting occurs, and on the mature stage of active pseudotachylyte development. Flash heating at asperity contacts is expected to be the primary thermal weakening process when slip rates are high ( 1 m/s) but total slip is still small, in a sense that can be quantified. A preliminary analysis captures some features of available experiments. Some ideas are being developed on how to address the much larger slip range when partial melting occurs. These include a view, supported by pseudotachylyte observations, that a granular fault gouge becomes liquefied through development of small amounts of partial melt, and that a self-regulated process of velocity-weakening character develops as this highly pressurized phase permeates into the adjoining fault walls. These concepts are being developed for purposes of integrating them with numerical simulations and theory on how the mode of rupture depends on constitutive response, to examine consequences for rupture dynamics. That will contribute to understanding how major fault systems can operate at realistically low overall driving stresses, even when the stress needed locally to initiate slip is much larger, and to quantifying the minimum average stress level for which a rupture, once initiated at a location of locally high shear stress or low effective normal stress, can propagate over large distances.