Mechanisms underlying lubrication of faults and implications for Earthquake nucleation
Yijue Diao and Rosa M. Espinosa-Marzal
University of Illinois at Urbana-Champaign
The friction between two adjacent tectonic plates under shear loading may dictate seismic activities. To advance the understanding of the mechanisms underlying the frictional strength of fault rocks, we investigate the frictional characteristics of calcite in aqueous environment. By conducting nanoscale friction-force measurements as a function of load, sliding velocity, and electrolyte concentration, three pathways of energy dissipation have been distinguished: viscous shear at low normal stress, in which the confined film behaves as a non-Newtonian fluid; a shear-promoted thermally activated sliding at intermediate stresses, to some extent similar to dry friction, but revealing the influence of hydrated ions localized on the calcite surface; and pressure-solution facilitated slip at sufficiently high loads. The latter is a direct result of the significant reactivity of calcite in an aqueous medium, enhanced under high stresses, which we can prove by visualizing the contact and quantifying it by thermodynamic means. This nanoscale evidence can be extended to fault weakening: when a fault undergoes creep slow enough for pressure solution to come into play, a slip can be triggered, which alters fault dynamics at the level of single-asperities. We thus hypothesize that earthquakes ruptures might be triggered and perhaps propagated by fault weakening induced by pressure solution of calcite asperity contacts.