The Cascadia Subduction Zone is a massive geologic fault that last ruptured in January 1700. But while this fault has stayed quiet for centuries, it regularly generates small tremors that accompany gradual, nondisruptive movement along the fault.

The tiny tremor events and slow slippage are known collectively as “episodic tremor and slip.” Seismic waves associated with these tremor events are recorded and tracked by the UW’s Pacific Northwest Seismic Network. Other groups track the associated slow motion of the plates using GPS measurements. These paired types of events occur regularly and seem to fluctuate with tidal cycles, but they originate deep underground and their cause has been mysterious.

A pair of papers published Jan. 29 provides new confirmation of speculations about a cause of these events. Taken together, the papers show that fluids deep underground create fractures in the rock, and that this creates rumblings that match what we observe at the surface.

Marine Denolle, an assistant professor of Earth and space sciences at the UW, and Joan Gomberg, an affiliate associate professor at the UW who is based with the U.S. Geological Survey, are co-authors on the AGU Advances paper about the experimental findings. They talk with UW News about slow slip earthquakes in this Q&A.