The role of water lubrication in critical state fault slip

Abstract

The conventional driving mechanisms including elevating the pore pressure or far-reaching poroelastic effects, may not necessarily explain the injection-triggered seismicity that can be up to 35–90 km far away from injection wells. In this experimental study, we examined the mechanism of water lubrication for the triggered seismicity where the fluid pressure was negligible compared to the in-situ stresses. The results show that fault slip can occur upon saturation when the ratio of the applied shear stress to the fault peak shear strength exceeds a critical threshold, which is defined to quantitatively assess the critical stress state for faults. The microscale slip test using atomic force microscopy reveals the mechanism responsible for the significant reduction of friction between granite surfaces from dry to wet state. A nanometer-thick water layer was attached on the granite surfaces, causing the shear plane shifting from the direct granite/granite interface to the water layer, and meanwhile the adhesive force between granite surfaces was significantly reduced by water intercalation, both of which played the dominate role in triggering fault slip. In practice, slip may be associated with in-situ fluid flow into far-field faults, which causes the increase of water and lubricates the fault zone.