The quiet, stealthy ones – What’s quiet but sneaks up on you? California’s San Andreas fault is notorious for repeatedly generating major earthquakes and for being on the brink of producing the next big one in a heavily populated area. But the famously violent fault also has quieter sections, where rocks easily slide against each other without giving rise to damaging quakes. These are the quiet 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. The question of why some fault zones creep slowly and steadily while others lock for a time and then shift suddenly and violently, spawning earthquakes, has long puzzled scientists.

When geologist Ben van der Pluijm analyzed samples of rock from an actively creeping segment that was brought up from a depth of two miles below the surface as part of the San Andreas Fault, he found that fractured rock surfaces were coated with a thin layer of clay, less than 100 nanometers thick, which acts something like grease on ball bearings. The nanocoatings occur on the interfaces of broken-up bits of rock in exactly the places where they affect the fault’s “weakness”—how easily it moves.

Van der Pluijm says, “For a long time, people thought you needed a lot of lubricant for creep to occur. What we can show is that you don’t really need a lot; it just needs to be in the right place. It’s a bit like real estate: location, location, location. 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.”

If the fault is greasing itself, then why do earthquakes still occur? Van der Pluijm says, “The problem is that the fault doesn’t always move at strands where the coating sits.” The San Andreas fault is actually a network of faults, with new strands being added all the time. Because it takes some time for the slick nanocoatings to develop in a new strand, the unlubricated, new strand “gets stuck” for a time and then shifts in a violent spasm.

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