The Role of Fluid Pressure in Earthquake Triggering: Insights from an Experimental Study of Frictional Stability of Carbonates

Abstract:

Fluid overpressure has been often proposed as one of the primary mechanisms that facilitate earthquake slip along faults. However, elastic dislocation theory combined with rate- and state- friction laws suggests that fluid overpressure may inhibit the dynamic instabilities that result in earthquakes, by reducing the critical rheological fault stiffness, kc. This controversy poses a serious problem in our understanding of earthquake physics, with severe implications for seismic hazard and human-induced seismicity. Nevertheless, currently, there are only a few systematic studies on the role of fluid pressure under controlled, laboratory conditions for which the evolution of friction parameters and slip stability can be measured.

We have used a biaxial rock deformation apparatus within a pressure vessel, in order to allow a true triaxial stress field, in a double direct shear configuration. We tested carbonate fault gouge, Carrara marble, sieved to a grain size of 125 microns. Normal stresses and confining pressure were held constant throughout the experiment at values of 5 to 40 MPa, and the pore fluid pressure was varied from hydrostatic up to near lithostatic values. Shear stress was induced by a constant displacement rate and sliding velocities varied from 0.1-100 microns/s, in order to evaluate slip stability via rate- and state- dependent frictional parameters, such as (a-b), Dc. With increasing fluid pressure we observe an evolution of (a-b) from slightly velocity strengthening to velocity neutral and a reduction in Dc from about 100 to 20 microns. Our analysis on carbonate fault gouges indicates that the increase in fluid pressure not only favour fault reactivation but it also makes the fault more prone to generate earthquake instabilities.