Experimental and theoretical investigation of rock friction at seismic slip rates

PI提供了新的实验数据和理论，描述了细粒泥浆在地震滑动过程中的热减弱。他们假设，细粒泥层中的颗粒由于弹性剪切模数的内在降低和对泥层的快速加热而增加的塑性而热软化。高速减弱过程直接与地震传播物理、许多主要板块边界断裂系统的异常低剪应力状态、危险的长时间滑坡以及错位断裂和低角度滑脱的运动有关。他们假设了一个简单的非线性弱化模型和支持实验证据，解释了初始高速弱化相的主要性质，即断层中接触的凹凸体对摩擦加热的响应导致塑性的热控增加和屈服强度的降低。它依赖于人们熟知的高温和应力下岩石塑性的概念，并通过考虑Wachtman-Anderson关系和双曲正弦蠕变定律来解释在高温应力和温度下的粗糙度蠕变，从而解释了在高速摩擦试验中观察到的3到4个削弱的因素。他们计划测试闪光减弱是一个真正的绝热过程的假设，在这个过程中，摩擦产生的热量被限制在沿着粗糙面接触的非常薄的热边界层中，而大部分粗糙面和附着的颗粒不参与加热和变形。初始数据实验数据证实，当速度从0.14m/S增加到2.5M/S时，摩擦系数从~0.5到0.15有系统的演化，但并不支持闪变假设的简化电流简单形式。为了限制理论的发展，他们将做以下工作：(1)获得剪切带平均温度的热记录，以便适当地约束断层泥流变性的温度依赖性。(2)在不同的速度和法向应力下，用扫描电子显微镜观察剪切带组构，观察其粒度分布的性质以及泥粒开始塑性变形和熔化的证据。(3)检查成分(粘土与结晶岩)和水对摩擦性能的影响。(4)对旋转剪切仪进行改造，使其运行速度慢2-4倍，以便在更宽的速度范围内进行测量。

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.