Slip Sliding Away

Abstract

GEOPHYSICS During earthquakes, very high stresses within the crust press the two sides of the fault together so hard that they should be effectively locked together by friction. In the laboratory, rocks are similarly difficult to rip apart. Yet in the landscape setting, faults rupture suddenly and easily. Various explanations for this conundrum have been put forward, including fault lubrication by fluids or weakening by seismic vibrations. Recent experiments suggested that the rocks themselves may become slippery during rupture if they are heated or interact with fluids; silica gel may lubricate quartz rocks and fine powder may ease sliding in carbonate rocks. Hirose and Bystricky have found support for another hypothesis: fault weakening through dehydration of embedded phyllosilicate clays. They carried out high-velocity friction experiments on natural serpentinite (a phyllosilicate) under conditions mimicking an earthquake and measured the heat generated by friction and the resulting rock strength. An observed increase in humidity implied that water was lost from the serpentinite during sliding. Dehydration requires temperatures of about 500°C, which the authors argue might be attained where bumpy asperities rub together. — JB Geophys. Res. Lett. 34 , L14311 (2007).