Earthquake fault dynamics: Insights from laboratory experiments

Abstract

The determination of rock friction under the conditions of seismic slip in the Earth crust (slip rates of the order of 1 m/s or more and normal stress of hundreds of MPa) is of paramount importance in earthquake mechanics. Fault friction controls the stress drop, the mechanical work and the frictional heat generated during the slip. However the essential engine of earthquakes is buried at several kilometers depth and only remote, indirect measurements, which are not sufficient to fully characterize fault dynamics, are available. Elucidating constraints are derived from experimental studies performed in powerful apparatuses applying rotary shear motion to rock samples. The experiments indicate that when slip velocities and normal stresses approach those of actual earthquakes, a significant decrease in friction kicks-in (of up to one order of magnitude), which we term fault lubrication, both for cohesive rocks (silicate-built, quartz-built and carbonate-built) and non-cohesive rocks (clay-rich, anhydrite, gypsum and dolomite gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger a number of physicochemical processes (gelification, decarbonation and dehydration reactions, melting and so on) whose products are responsible for fault lubrication.