https://phys.org/news/2018-07-tremors-shove-washington-state-west.html

Scientists believe an episode of tremors could someday trigger a so-called megaquake on the offshore fault called the Cascadia Subduction Zone. The 700-mile-long fault runs from Vancouver Island to northern California, and could unleash earthquakes of up to magnitude 9.0. It's one of the biggest of faults in the U.S.

While older models suggest that the locked zone is mostly shallow and offshore, the location of these tremors indicates that in a big earthquake, layers of rock jerking past each other may take place closer to Seattle than previously thought.