Experimental and theoretical investigation of rock friction at seismic slip rates

PI提出了新的实验数据和理论，描述了地震滑动过程中细粒泥的热弱化。他们假设，细粒断层泥中的颗粒由于弹性剪切模量的固有降低而热软化，并且响应于断层泥层的快速加热而塑性增加。高速衰减过程直接关系到地震的传播物理和许多主要板块边界断裂系统的异常低的分解剪应力状态、危险的长寿命滑坡、错位断层和低角度断层的运动的起源。他们假设一个简单的非线性弱化模型和支持的实验证据，解释的主要属性的初始高速弱化阶段的热控制的塑性增加和屈服强度的下降，在故障中的接触微凸体在摩擦热响应。它依赖于在升高的温度和应力下岩石塑性的充分理解的概念，并通过考虑Wachtman-Anderson关系（弹性模量随温度的降低）和双曲正弦蠕变定律（考虑在升高的应力和温度下的粗糙蠕变），解释了在高速摩擦试验中观察到的3至4个弱化因子。他们计划测试这样一个假设，即闪光弱化是一个真正的绝热过程，其中摩擦引起的热量被限制在沿沿着粗糙接触的非常薄的热边界层中，使大部分粗糙和附着的晶粒不参与加热和变形。初始数据实验数据证实，随着速度从0.14 m/s增加到2.5 M/s，摩擦系数从~0.5到低至0.15有系统的演变。然而，它并不支持目前的简单形式的闪光减弱假说。为了限制理论的发展，他们将做以下工作：（1）获得剪切带中平均温度的热记录，以便我们可以适当地限制断层泥流变学的温度依赖性。(2)在不同的速度和法向应力下，用SEM检查剪切带组构，以观察粒度分布的性质以及塑性变形和凿痕颗粒熔化开始的证据。(3)检查成分（粘土与结晶岩）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.