Clay-rich fault gouges become frictionally less stable at elevated temperatures

Abstract Large earthquakes nucleate on crustal faults that have accumulated significant slip displacement and field observations show that these faults are ubiquitously clay-rich. Earthquake nucleation requires a reduction in shear resistance for instability to develop. Previous laboratory friction measurements indicate that only stable fault creep should occur in clay-rich faults; a result at odds with observations of widespread earthquake behaviour on mature clay-rich faults in nature. Here we show that the frictional stability of synthetic clay-bearing fault gouges decreases systematically with elevated temperatures commensurate with those found at typical earthquake depths. In materials containing ≤50% clay, the stability of slip decreases with increasing temperature so that gouges display unstable slip at temperatures between 100 and 180°C. At room temperature the same materials host only stable slip. This reduction in stability with increasing temperature coincides with a greater degree of localization observed in the gouge microstructure and with progressive loss of water adsorbed on clay surfaces. Our results indicate that a broad compositional range of clay-bearing fault rocks, and therefore mature faults, can nucleate unstable slip at conditions common to the clay-bearing brittle crust; a result that resolves the apparent paradox that mature clay-bearing faults in nature can nucleate and propagate earthquakes.