The San Andreas Fault Observatory at Depth (SAFOD) drill rig near Parkfield, Calif. in 2004. Credit: Ben van der Pluijm

The San Andreas Fault Observatory at Depth (SAFOD) drill rig near Parkfield, Calif. in 2004. Credit: Ben van der Pluijm

Geologists say they have discovered that an ultra-thin layer of smectitic clay on rocks along deep, older fault lines in the San Andreas fault region provide important lubrication that permits gradual movement rather than earthquake-producing jumps.

Geologist from the University of Michigan and the Ernst-Moritz-Arndt Universität Institut für Geographie und Geologie (Germany) say the smoother “creep” ability occurs over time as the fault creates its own lubricants. Newer faults, which are always being added, lack this lubricating layer and are, therefore, more prone to generating significant seismic events.

Scientists have speculated for some time that a lubricant in the form of fluids or talc (from serpentine) might be easing rock movements. However, when this research group analyzed two-mile-deep rock obtained from the San Andreas Fault Observatory at Depth project, they found that fractured rock surfaces were coated with smectitic clay less than 100 nanometers thick.

“For a long time, people thought you needed a lot of lubricant for creep to occur,” says Ben van der Pluijm, UM’s Bruce R. Clark Collegiate Professor of Geology and Professor of the Environment. “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.”

Van der Plujim explains that newer features of a fault line lack the lubrication system. He says 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 that results in an earthquake.

The findings are reported in the July issue of Geology.

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