New evidence suggests that tectonic plate slippage and nonvolcanic tremor near the Cascadia subduction zone both are signs of processes taking place 25 miles deep at the interface of the Juan de Fuca and North American plates. “We are quite confident that each episodic tremor and slip event will increase the stress on the megathrust fault,” one of the researchers said. “If a megathrust earthquake were to begin off the Washington coast, one might expect it to occur during one of these slow slip events.”