Experimental and theoretical investigation of rock friction at seismic slip rates

PIs提供了新的实验数据和理论来描述地震滑动过程中细粒沟槽的热弱化。他们假设，细粒凿泥中的颗粒热软化是由于凿泥层的快速加热导致弹性剪切模量的内在降低和塑性的增加。高速减弱过程直接关系到地震传播物理特性和许多主要板块边界断裂系统的低解剪应力异常状态、危险的长期滑坡、断层错位运动和低角度分离的起源。他们假设了一个简单的非线性弱化模型和支持的实验证据，以热控制的方式解释了初始高速弱化阶段的主要特性，即在摩擦加热的响应下，断层中接触颗粒的塑性增加和屈服强度降低。它依赖于众所周知的高温和高温下岩石塑性的概念，并考虑到在高速摩擦试验中观察到的3到4弱的因素，考虑到弹性模量随温度降低的Wachtman-Anderson关系和在高温和高温下解释粗糙蠕变的双曲正弦蠕变定律。他们计划测试一种假设，即闪变是一个真正的绝热过程，在这种过程中，摩擦产生的热量被限制在沿粗糙体接触的非常薄的热边界层中，而粗糙体的大部分和附着的晶粒不参与加热和变形。初始数据实验数据证实，当速度从0.14 m/s增加到2.5 m/s时，摩擦系数从~0.5系统地演变到低至0.15。然而，它并不支持闪电减弱假说的简化电流简单形式。为了约束理论发展，他们将做以下工作：(1)获得剪切带平均温度的热记录，以便我们可以适当地约束泥流变的温度依赖性。(2)在不同的速度和正应力下，用扫描电镜检查剪切带结构，以观察颗粒尺寸分布的性质以及泥质颗粒塑性变形和熔化的证据。(3)检查成分（粘土与结晶岩石）和水的存在对摩擦性能的影响。(4)修改旋转剪切装置，使其运行速度慢2至4倍，以便在更宽的速度范围内扩展测量。

The PIs present new experimental data and theory that describe the thermal weakening of fine-grained gouges during earthquake slip. They postulate that the particles in fine-grained gouges thermally soften due to an intrinsic decrease in the elastic shear modulus and increases in plasticity in response to rapid heating of the gouge layer. High-speed weaken processes directly bear on earthquake propagation physics and the origin of the anomalously low state of resolved shear stress on many major plate boundary faults systems, hazardous long run out landslides, and the motion on misaligned faults and low angle detachments. They suppose a simple non-linear weakening model and supporting experimental evidence that explains the principal properties of initial high velocity weakening phase in terms of a thermally controlled increase in the plasticity and decrease in yield strength of the contacting asperities in fault in response to frictional heating. It relies on the well understood concepts of rock plasticity at elevated temperatures and stresses and accounts for the factor observed of 3 to 4 weakening seen in high speed friction tests by considering both the Wachtman-Anderson relationship for the temperature dependent decrease of the elastic modulus and the hyperbolic sine creep law to account for asperity creep at elevated stresses and temperatures. They plan to test the assumption that flash weakening is a truly adiabatic process where the heat caused by the friction is confined to a very thin thermal boundary layer along the asperity contact leaving the bulk of the asperity and attached grains uninvolved in the heating and deformation. The initial data experimental data confirms there is a systematic evolution of the friction coefficient from ~0.5 to as low as 0.15 as velocities increase from 0.14 m/s to 2.5 M/s. However, it does not support the simplified current simple form of the flash-weakening hypothesis. To constrain the theoretical development They will do the following: (1) Obtain thermal record of the average temperatures in the shear zone so we can properly constrain the temperature dependence of the gouge rheology. (2) Examine the shear zone fabrics with a SEM at different velocity and normal stresses to look at the nature of the grain size distribution and evidence for the onset of plastic deformation and melting of gouge particles. (3) Check for the effects of composition (clay vs. crystalline rock) and presence of water on the friction properties. (4) Modify the rotary shear apparatus so that it will run 2 to 4 times slower to extend measurements over a wider velocity range.