Control of Transient Slip Weakening During Gypsum Dehydration

Abstract:

The understanding of fault mechanics is of first order importance to unravel earthquake triggering. Among the parameters influencing fault reactivation and earthquake triggering, the influence of pore-fluid pressure and friction on stability of fault zones have been a focus of recent work based on geological, geophysical and experimental analyses. Here, the effects of dehydration reactions on hydraulic and mechanical properties of rock are analysed to better understand the conditions required to trigger earthquakes. Triaxial experiments are conducted using gypsum and a direct shear sample assembly that allows a constant normal stress to be applied and permeability to be measured during sliding. The evolutions of shear stress, pore-fluid pressure and permeability are continuously measured throughout the experiment until dehydration reaction reached completion. Tests are conducted with a temperature ramp from 70 to 150 °C and with different effective confining pressures (50, 100 and 150 MPa) and velocities (0.1 and 0.4 µm.s^-1). Results show that gypsum dehydration induces transient stable slip weakening that is controlled by pore-fluid pressure and permeability evolutions followed by unstable slip on fully dehydrated product.

The evolution of microstructures and mineralogy during the experiment are inferred from SEM and XRD analyses of deformed samples collected at different key stages during repeated tests. The microstructural analysis shows clear evidence of dehydration reactions related to the development of S-C-C' structures where dehydration product is preferentially localized along shear and schistosity planes. A conceptual model is then proposed to explain transient slip weakening during dehydration reactions incorporating the key role played by permeability, and to provide a framework to define the conditions required to trigger unstable events during dehydration reactions.