The central-California section of the San Andreas Fault creeps rather than generating large earthquakes in sudden slip events because minerals in the fault zone exert only weak friction, suggests a study published online this week in Nature Geoscience. The samples from the fault zone also revealed that, unlike in the surrounding rock, bonds in the material that fills the fault do not heal after rupture, adding to the fault’s weakness.

Brett Carpenter and colleagues analysed samples obtained at 2.7 km depth from an actively slipping part of the San Andreas Fault. The researchers found that both the frictional strength of the fault and the rate of healing varied systematically across the fault zone, and was lowest where the fault creeps at present.