Experimental investigation of flash weakening in limestone

Flash heating and weakening could operate during earthquake nucleation and propagation. We performed 27 friction experiments in a compression-torsion apparatus on ring-shaped limestone samples at sub-seismic to seismic slip rates 340 mm/s, centimetric displacements and normal stresses of 8 MPa. Friction decreases dramatically at slip rates of 50e150 mm/s. Flash weakening was contemporaneous with a peripheral temperature rise of w90 C measured with an infrared camera. The peripheral temperature yields a lower limit to the slipping zone temperature. The decrease in friction may result from weakening of the asperity contacts due to decarbonation of calcite induced by (1) flash heating or (2) mechanically-activated reactions. However, X-Ray powder diffraction and Raman Spectroscopy analyses do not reveal the presence of decarbonation products in the slipping zone. Instead, White Light Interferometry and Field Emission Scanning Electron Microscope observations reveal the presence of a smooth sliding surface made of nanometric-particles. The mechanical data can be fit by the rate- and state-dependent friction model or by a quadratic model (the latter proposed for powder lubrication). We conclude that flash heating and weakening and powder lubrication may operate together to decrease dynamic friction in limestone in experiments and, for the conditions investigated here, in nature.