Microscopic wear grooves on slickensides: Indicators of paleoseismicity

Abstract

The frictional wear of Westerly granite surfaces polished with 0.3-μm Linde A polishing compound was studied on triaxial compression specimens for slip up to 3 mm at confining pressures to 2.9 kbar. A stable sliding to stick-slip transition occurs at 0.3-kbar confining pressure. Frictional wear occurs on the polished sliding surface only when the specimens undergo stick-slip. The initial wear on the surface consists of microscopic carrot-shaped grooves whose length rarely exceeds the maximum slip accompanying one stress drop during stick-slip sliding. The preferred orientation of the carrot-shaped grooves is such that the tip points in the direction of motion of the surface containing the grooves. A groove length equivalent to individual slip distance results in a small groove length to total slip ratio (<1:5) that may be used as a criterion for stick-slip sliding. A small groove length to total slip ratio also occurs on two natural slickenside surfaces, and on the basis of the laboratory experiments these natural surfaces are thought to be samples from fault zones that were once seismically active. The experimental observation that maximum groove length closely approximates the slip during the seismic event suggests that the seismic moment of prior earthquakes on a fault may be calculated without prior knowledge of seismicity.