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.
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.
CSZ earthquakes are believed to occur every 200 to 600 years, and the last major event in the CSZ is estimated to have occurred 300 years ago.
In the event of an earthquake, move to high ground until told to return to lower lying areas that are susceptible to tsunamis.
You may need to survive on your own after an emergency. This means having your own food, water, and other supplies in sufficient quantity to last for at least three days. Local officials and relief workers will be on the scene after a disaster, but they cannot reach everyone immediately. You could get help in hours, or it might take days. In addition, basic services such as electricity, gas, water, sewage treatment and telephones may be cut off for days, or even a week or longer.
Earthquakes can cause tsunamis – large water waves generated by seismic deformation of the sea floor.
Subduction earthquakes, such as those that occur off the Oregon coast, tend to generate 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.
If you hear the Tsunami alarm – a siren sound reminiscent of the old air raid sirens that cycle up and down every 5 seconds for 3 minutes – it is important that you find a way to get to high ground immediately. You might only have a few minutes to do so. This alarm system is tested at 11am on the first Wednesday of each month. The sirens blast for one minute during the test.
Tsunami Evacuation Maps for Oregon Coastal Cities: