Cohesive strengthening of fault zones during the interseismic period: An experimental study

There is widespread evidence indicating that faults regain a portion of their strength during the interseismic period. Here, we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid-rock reactions in fault zones. The triaxial experiments consisted of fracturing cores of Fontainebleau sandstone under dry conditions, forming a localized shear failure zone (stage 1). The specimens were then reacted hydrothermally under isostatic conditions, allowing the fault damage zone to compact, consolidate and strengthen (stage 2). Following reaction, the specimens were then reloaded to failure under nominally dry conditions, so that the increase in cohesive strength of the fault could be measured (stage 3). Experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction. After 6 hours of reaction at the highest temperatures (927°C) and pore pressures (200 MPa), cohesion increases by as much as 35 MPa. Microstructural. examination of the specimens showed that the gouge particles within the fault compacted and cemented together, exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed. A theoretical treatment of the data was conducted using these experiments in combination with results on time-dependent changes in fault cohesion presented by Tenthorey et al. (2003). We find that the rate-controlling process in our experiments has an activation energy (Q) of approximately 70 kJ mol^-1. We use this information to develop a model for time-dependent cohesive strengthening in fault zones within the continental seismogenic regime. We conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluid.