To learn more, Saffer and three colleagues investigated slow movements along a fault off the Japanese coast. They drilled 1 kilometer beneath the seafloor to extract a dozen dark-gray, clay-rich sample cores, each about the size of a soda can, from the fault. Back at the lab, they shaved 1-millimeter wafers from each core and sheared the slices between two steel blocks, at varied velocities, to simulate a fault slipping in a long, controlled motion. The rock’s response to these pressures surprised Saffer.
Initially the clay resisted the sliding motion, and then it slowly began to weaken. In an actual fault, the rock would be stationary under growing tectonic pressure, then would slowly give way. Although it’s not completely clear why the rocks held and released energy this way, Saffer believes the flatness of clay particles and the way clay absorbs water are part of the answer. Knowing that clay-rich rocks can release tectonic energy in a controlled way could explain how a fault might creep along, he says, and might help scientists predict a fault’s next move.