Tiny clays curb giant earthquakes

Scientists have found why some areas of the San Andreas fault experience repeated earthquakes and other areas do not.

Professor Ben van der Pluijm at the University of Michigan and Professor Laurence Warr of Ernst-Moritz-Arndt University in Greifswald, Germany, said the famously violent fault also has quieter sections where rocks easily slide against each other without giving rise to damaging quakes.

The relatively smooth movement, called creep, happens because the fault creates its own lubricants - slippery clays that form ultra-thin coatings on rock fragments.

When van der Pluijm and colleagues analysed samples of rock from an actively creeping segment they found a thin layer of smectitic clay, less than 100 nanometers thick, that acts something like grease on ball bearings.

"The clays are growing in the fault zone, and the fault is coating its own pieces of fragmented rock. At some point there's enough coating that it begins to drive the behavior of the fault, and creeping kicks in," van der Pluijm said.

But earthquakes still occur because the fault doesn't always move at strands where the coating sits. Because it takes some time for the slick nano-coatings to develop in a new strand, the non-lubricated, new strand "gets stuck" for a time and then shifts in a violent spasm.

The study is published in the July issue of Geology.