Thursday, May 8, 2014
Mechanical and Civil Engineering Seminar
Aseismic Slip and Seismicity Induced by Hydraulic Stimulation of Geological Faults
Frederic Cappa, Associate Professor, Geoazur Earth and Planetary Sciences Laboratory, University of Nice Sophia-Antipolis
The role of fluids in faulting is a central question in geophysics because fluid pressures are suspected to influence the initiation of aseismic slip and seismic rupture. Yet to date few data are available to constrain the fluid-mechanical coupled effects on faulting, at appropriate scales. Here we present unique measurements of induced slip on a natural fault under monitored mechanical and hydraulic conditions. We report continuous measurements of strain, seismicity and fluid pressure (few MPa) during injection-induced rupture of a 10-m long segment of a normal fault. Fault activation displays highly dilatant aseismic slip preceding a seismic period where slip is a-factor-of-4 faster and dilatancy much reduced. We calculate that the slip is initiated by the fault material frictional weakening, the pore pressure increase just being the trigger. Then, there is a competition between high-pressure fluid diffusion in the fault zone and multiple slow ruptures that generate 80% of the seismic energy until the permeability/porosity increase progressively become the predominant control on slip of large fault segments. Detection of variations in these pressure transients and their correlation to seismicity allow capturing irreversible evolutions of fault friction that lead to the instability process. Such experimental results are important in defining mechanisms of natural and induced earthquakes, their preparatory phases and risk assessment.