Cascadia Subduction Zone

Earthquakes

The Oregon coast is vulnerable to great earthquakes that can occur on the offshore Cascadia Subduction Zone (CSZ), which runs from British Columbia to northern California. CSZ earthquakes are believed to occur every 200 to 600 years. The Cascadia subduction zone is the convergent boundary between the large North America plate and the small Juan de Fuca and Gorda plates to the west. These smaller plates plunge beneath the North America plate at a rate of 1 to 1¹/₂ inches per year. As the Juan de Fuca and Gorda plates push into North America, stress accumulates. Earthquakes are caused by the abrupt release of this slowly accumulated stress. The last great earthquake on the Cascadia fault zone occurred in January 1700.

Tsunamis

These quakes can cause tsunamis, large water waves generated by seismic deformation of the sea floor. Most tsunamis (pronounced tsoo-nah-mee) are caused by shallow submarine earthquakes. Fault motion causes changes to the ocean bottom that generate large waves. Subduction earthquakes such as those that occur off the Oregon coast are particularly effective in generating tsunamis. As a tsunami nears the shore, it both slows in speed and grows in height so that a tsunami unnoticed at sea, may grow to be several meters or more in height as it reaches land.