Weakening Mechanisms of Rocks at Fast Slip Rates: Example from the Cases without Frictional Melting

Previous laboratory studies on rock friction at slow slip rate ( < 1 mm/s) contribute to form the basis of rate- and state-dependent frictional constitutive laws. However, the frictional parameters determined in conventional frictional experiments at the slow slip rates are probably no longer applicable when fault slip approaches to the average slip rate of order of 1 m/s. Recent progress in experimental studies on rock friction reveals that laboratory simulated faults weaken dramatically at fast slip rate conditions ( > 0.1 m/s) with a large slip-weakening distance ( > 1 m). We focus here on the recently reported weakening processes of simulated faults during high-velocity friction from the cases without frictional melting. Several mechanisms has been proposed so far for the cause of the weakening processes of rock friction; thixotropy of a silica gel formed on quartz-rich rocks, formation of a weak material due to thermal decomposition, and drain-off process of adsorbed moisture from the fault due to frictional heating. High-velocity friction of natural fault gouge samples also showed slip weakening with a large weakening distance. Importantly, the laboratory derived weakening distance of the gouge friction decreased with an increase of normal stress. The exact mechanism that leads to the slip-weakening of the gouge samples is unknown, but it may involve a process in which the cause of the weakening is related to the increase of temperature of the gouge material during the high-velocity slip.