Precursive pore pressure changes associated with premonitory slip during stick-slip sliding

Abstract

Undrained, triaxial-compression, pore pressure experiments have been conducted to monitor pore pressure changes during stick-slip sliding of 35° sandstone precuts. Pore pressure changes reflect changes in pore volume (i.e., dilatancy). During the stick portion of the stick-slip cycle, there are linear increases in the displacement, force, and pore pressure that indicate elastic compaction. Just prior to the major seismic-slip event, premonitory slip always occurs. Coinciding with the onset of premonitory slip, the linear rises in pore pressure and force become nonlinear and level off or decrease immediately prior to the major seismic-slip event. With increasing total displacement along the precut there is an increase in the amount of premonitory slip that precedes individual stick-slip events and a corresponding increase in the precursive drop in pore pressure. During the major seismic-slip event, the stress on the specimen is relieved resulting in a corresponding coseismic drop in pore pressure. The precursive drop in pore pressure is related to observed microfracturing in grains immediately adjacent to the sliding surface. It is suggested that if dilatancy in the form of increased fracture porosity along the sliding surface is to occur before stick-slip events, premonitory slip is necessary.