Laboratory Experiments on Rock Friction Focused on Understanding Earthquake Mechanics

Brown University, Providence RI

Investigators

Abstract

The behavior of rocks during frictional sliding experiments in the laboratory is reasonably well described by, and forms the basis for, the so-called rate and state-variable friction laws. In spite of the fact that these laws reproduce a rich variety of earthquake-related phenomena when employed in models of earthquakes, the physical mechanisms that occur at the microscopic contacts between rock surfaces during frictional sliding, which cause the observed rate and state effects, remain poorly understood. This lack of knowledge precludes making reliable extrapolation of existing laboratory friction data to the Earth. The lack of a physical basis for the rate and state friction laws stems in part from the difficulty of isolating and studying the behavior of micron-sized contacts between rock surfaces in the laboratory. In this study a Nanoindenter is used to effectively isolate the deformation behavior of a single asperity contact using a precisely controlled contact load and geometry. A constant load is applied to an indenter tip in contact with a polished geologic specimen. The indenter is then held for a specified time under constant load (in static indentation tests) or translated across the specimen under constant load (in sliding indentation tests). The ability to conduct both static and sliding tests allows indentation tests to be designed which are analogous to the two most commonly used rock friction tests, the velocity-stepping test and slide-hold-slide test. The area of the indenter/specimen contact is determined continuously throughout the indentation tests, allowing the degree to which changes in contact area effect changes in friction to be quantified. Mechanical tests are followed by extensive microstructural analyses of indented specimens using scanning and transmission electron microscopy. The mechanical tests and microstructural analyses provide fundamental data required to identify the microphysical mechanisms described by the rate and state friction laws and thus form a basis for understanding the origin of earthquakes.