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 researchers said they discovered that the smooth movement, called creep, occurs because the fault creates its own lubricants -- slippery clays that form ultra-thin coatings on rock fragments.
The question of what causes creep has long puzzled scientists. Some have speculated fluids facilitate slippage, while others have focused on serpentine -- a greenish material that can become a kind of slippery talc.
But when van der Pluijm and colleagues analyzed samples of rock from an actively creeping fault two miles deep they found very little talc. Instead, they found fractured rock surfaces coated with a thin layer of smectitic clay, less than 100 nanometers thick, that acts as a lubricant.
"For a long time, people thought you needed a lot of lubricant for creep to occur," van der Pluijm said. "What we can show is that you don't really need a lot. It just needs to be in the right place."
The study that included assistant research scientist Anja Schleicher appears in the July issue of the journal Geology.