Every 15 months or so, an unfelt earthquake occurs in western Washington and travels northward to Canada’s Vancouver Island. The episode typically releases as much energy as a magnitude 6.5 earthquake, but it does so gradually over a month.

New technology is letting University of Washington researchers get a much better picture of how these episodic tremor events relate to potentially catastrophic earthquakes, perhaps as powerful as magnitude 9, that occur every 300 to 500 years in the Cascadia subduction zone in western Washington, Oregon and British Columbia.

“Depending on where the tremor is, a different part of the fault is being loaded,” said Abhijit Ghosh, a UW doctoral student in Earth and space sciences, who is presenting the most recent findings Monday (Dec. 13) at the annual meeting of the American Geophysical Union in San Francisco.