Washington's Coast
Washington State Department of Ecology

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The Cascadia Subduction Zone

Earthquakes and Washington's coast
The surface of the earth is made of plates. These plates are always on the move, shifting over or under each other. When plates move suddenly, an earthquake occurs. Part of the earth's crust, the Juan de Fuca Plate, is spreading away from the Pacific plate, several hundred miles offshore. The Juan de Fuca plate is being pushed under the North American plate -- a process called subduction.
Locking plates, build-up of stress

The Juan de Fuca Plate is pushing deep under the North American Plate. The colliding edges of these plates are locked, one plate pressed into the other. As the plates press and move, stress builds up -- until the lock breaks.
sliding plates: earthquake

Slip - and an earthquake occurs
The plates slip suddenly, causing a subduction zone earthquake.

The next big one
Although the last giant earthquake hit Washington's coast 300 years ago, scientists say that the Cascadia Subduction Zone may be storing up energy to be released in the next catastrophic earthquake. The stresses detected off Washington's coast could generate a huge earthquake (magnitude 9 or more). That would rival the largest earthquake ever recorded. (A magnitude 8.7 earthquake would release 1,000 times the energy of the magnitude 6.6 earthquake that struck Los Angeles in 1994.)
When it hits
Violent shaking will last at least two minutes. The ocean floor will drop, most likely creating a tsunami that will strike long stretches of the coast. The central Olympic peninsula will suddenly rise, while some coastal areas will sink below sea level.
If an earthquake occurs at the coast
  • Drop, cover and hold. Get under a sturdy object and hold on. Watch for falling objects.
  • As soon as the shaking is over, move to high ground or inland. Do not wait for an official warning.
  • Stay away from the coast. Waves may continue to arrive for hours.
  • Listen to your local radio station for an official "All Clear" notice before returning to the coastal area.
  • Be alert for aftershocks.
Protect yourself and your family
  • Develop a family disaster plan. Everyone needs to know what to do on their own to protect themselves from an earthquake.
  • Be familiar with local Emergency Management earthquake and tsunami plans. Know where to go to survive a tsunami.
  • Be prepared to survive on your own for a minimum of three days.
  • Prepare a disaster supply kit for your home, automobile and work.
  • Take a first aid course and learn survival skills. Knowledge is your greatest defense against potential disaster.
Source: Earthquake images redrawn from "Northwest Exposures: The Geologic Story of the Northwest," Alt and Hyndman.

Earthquakes, Federal Emergency Management Agency (FEMA). What to do during an earthquake, how to prepare, how to protect property, what to tell children, and other frequently asked questions.

Earthquake Hazards Program - Pacific Northwest, USGS. Earthquakes threaten the Pacific Northwest, which includes Washington, Oregon, and northern California. USGS earthquake studies of this region are intended to help reduce the losses the earthquakes may cause.

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