Dynamic Stress Stimulates Flow in Fractures : 2 Laboratory Observations of Permeability Enhancement

12 We report on laboratory experiments designed to investigate the effect of 13 dynamic stressing on permeability of fractured rock. Berea sandstone samples 14 were fractured in-situ under triaxial stresses of 10's of MPa, and deionized 15 water was forced through the incipient fracture under conditions of steady and 16 oscillating pore pressure. We find that dynamic stresses produced by pore 17 pressure oscillations induce transient increases in effective permeability. The 18 magnitude of permeability increases scale with the amplitude of pore pressure 19 oscillations, and permeability changes persist well after the stress perturbation. 20 Maximum values of permeability enhancement are 5x10 m over a 21 background permeability of 1x10 m. Permeability recovery following 22 dynamic stressing occurs as the inverse square root of time. The recovery 23 indicates that a reversible mechanism, such as clogging/unclogging of 24 fractures, as opposed to an irreversible one, like micro fracturing, causes the 25 transient permeability increase. Our data demonstrate the feasibility of 26 dynamically controlling permeability of fractured systems. The result has clear 27 consequences for earthquake dynamic triggering mediated by permeability 28 enhancement in fault zones due to shaking from near and distant earthquakes. 29 30