Acoustic emission and microslip precursors to stick-slip failure in sheared granular material

We investigate the physics of laboratory earthquake precursors in a biaxial shear configuration. We conduct laboratory experiments at room temperature and humidity in which we shear layers of glass beads under applied normal loads of 2-8 MPa and with shearing rates of 5-10 μm/s. We show that above ~ 3 MPa load, acoustic emission (AE), and shear microfailure (microslip) precursors exhibit an exponential increase in rate of occurrence, culminating in stick-slip failure. Precursors take place where the material is in a critical state - still modestly dilating, yet while the macroscopic frictional strength is no longer increasing. Key Points Sheared granular media exhibits precursor events to stick-slip failure Rate of precursor activity increases exponentially prior to stick-slip failure Acoustic emission and microslip precursors are correlated in time