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.