The Pacific Northwest sits next to a powerful tectonic boundary called the Cascadia Subduction Zone. A major earthquake along the Cascadia Subduction Zone offshore the Pacific Northwest could cause coastal land to drop by at least about 6 feet (roughly 2 meters) in mere minutes. The flood waters would then move farther inland – not because the sea level suddenly rises, but because the ground suddenly drops. This significantly enlarging flood-prone areas and permanently altering ecosystems, infrastructure, and settlement zones from northern California to British Columbia.

Source: Earth.Com