Cascadia Tsunami Hazards | British Columbia | Washington | Oregon | Northern California

Tsunami Hazards in Coastal areas of Cascadia

Coastal areas in the Cascadia Region are at risk for tsunamis – like the tsunami that devastated communities around the Indian Ocean after a great earthquake on Dec. 26, 2004. Tsunamis on Cascadia are not frequent, but could arise either from a large earthquake on the Cascadia Subduction Zone, or from a distant earthquake strong enough to send a tsunami across the ocean. Tsunami Escape Route signs have been installed on the coasts of Alaska, Hawaii, Washington, Oregon, Alaska and California.

Although tsunamis do not occur frequently, they can cause a tremendous amount of damage in coastal areas when they do happen. To learn about the physics of tsunamis and why they happen, visit The Wave the Shook the World – NOVA’s reconstruction of the 2004 tsunami, or the University of Washington Tsunami site.

Tsunami!, a 1999 article from “Scientific American” about tsunamis, inundation modeling and warning systems, provides useful background information. The National Tsunami Mitigation Program provides a coordinated national effort to assess tsunami threat, prepare community response, issue timely and effective warnings, and mitigate damage. Tsunami Warnings for the Cascadia coast are generated by the Alaska Tsunami Warning Center.

Every state in the Cascadia Region has issued a brochure warning people of the danger of tsunamis and what they can do to avoid being killed by one. These brochures are linked in the mitigation section of this site. Some useful information about tsunamis is available from the Oregon State brochure about tsunamis and is reproduced below. This information applies to all coastal areas in the Cascadia Region. Surviving a Tsunami�Lessons from Chile, Hawaii, and Japan is a USGS pamphlet about well-documented tsunami events in several countries.

What is a Tsuami (soo-nah-mee)?

A tsunami is a series of sea waves usually caused by displacement of the ocean floor by an undersea earthquake. The waves travel at speeds up to 600 miles per hour, sometimes crossing the entire Pacific Ocean. As tsunamis enter shallow water near land, they increase in height and can cause great loss of life and property damage where they come ashore. Remember, a tsunami is not one wave but a series of waves. People have died when they assumed they were safe because they had survived one large wave, only to be caught by a later arriving, larger wave.

When does a Tsunami occur?

It can occur any time of day or night after an undersea earthquake. Experts believe that a tsunami caused by an undersea earthquake near the coast could strike the coast within 5-30 minutes after the earthquake, before official warning is possible. Undersea earthquakes thousands of miles away can cause smaller tsunamis on the Oregon coast but will take several hours to arrive, generally allowing time for official warning.

Tsunami Facts

Two kinds of tsunamis could affect the coast:

1. Tsunamis generated by undersea earthquakes just off the coast can strike the coast within five to thirty minutes, possibly disrupting power lines and communications and leaving little time for an official warning. The actual ground shaking of the earthquake may be the only warning you receive.
2. Tsunamis generated by earthquakes occurring thousands of miles away will take several hours to reach the coast. There would be time for official warning, but if you are in an isolated area, you may not hear the official warning, you will feel no earthquake, and the only warning you may have is a sudden unexpected change in sea level. If the sea level suddenly drops or rises without any obvious reason, nature may be warning you of impending danger, and you should move inland or to high ground immediately.

Recent research shows that at any time the Pacific Northwest can experience large earthquakes and accompanying tsunamis, and that tsunamis have affected the coast on a regular basis over time. Scientists have not yet had time to do local studies that will be able to tell you how high a tsunami may be in any one area and how far you must run inland or how high you must climb to escape. The best general advice available today is to (1) go up to 2 miles inland if you are in a low-lying area or at least 100 feet above sea level if you can; and (2) go on foot if at all possible because of traffic jams and earthquake damage.

A tsunami wave increases in height as it approaches shore. Typical wave heights from tsunamis occurring in the Pacific over the last 80 years have been between 21 to 45 feet at the shoreline. A few waves, however, have been much higher- as much as 100 feet or more because of local conditions. Also, tsunamis may affect local areas differently, causing great damage and loss of life in one area but little in another.

Past Tsuanmi History

On January 26, 1700, a massive earthquake occurred off the coast of Washington. The earthquake resulted in a tsunami which hit Japan’s coast with waves up to 10 feet high. Information about the tsunami is available from many sources, including:

• PNSN – The Pacific Northwest Seismic Network
• PGC – The Canadian Pacific Geoscience Centre
• The Orphan Tsunami – A book on dating the 1700 Cascadia tsunami using Japanese historic archives; by Brian Atwater et al.

In 1964, an earthquake along the coast of Alaska caused a tsunami which affected the coast from Alaska to California and Hawaii. The 1964 Alaskan tsunami killed 110 people and destroyed property in many areas along the coast from Alaska to California. In Washington alone, this tsunami caused $105,000 in damage. It is important to note that a tsunami can remain deadly for the duration of the wave activity, and not just at first impact. The first and second waves from the 1964 Alaska earthquake struck Crescent City, California at low tide and did little damage. The third and fourth waves, however, killed 11 people and caused $7.4 million in damage. A description of the 1964 earthquake and tsunami, including runup information, is available from NOAA. Another description is on the University of Washington Tsunami site.

Tsunami Hazard Information You Should Know

The following information is taken from a publication from Humboldt State University:

What Can I Do Ahead of Time to Protect Myself and My Family From a Tsunami?

• Make disaster plans beforehand. Talk to the people you live with about what may happen during a strong earthquake or other disaster. If you live or work in a low-lying coastal area, know where to go to survive a tsunami. Hold earthquake/tsunami drills at home or at work.
• Assemble a portable disaster supply kit. Have a kit available in your car, at home and at work. Your kit should include a portable radio with fresh batteries, water, first aid supplies, flashlight, and extra clothes or a blanket. Put your kit in a backpack and leave it in an easy-to-reach place.
• Contact local emergency officials. Find out what areas are most vulnerable to tsunami hazards, which areas are safe, and which routes are best for evacuation.
• Take a first aid class. Learn survival skills, talk with your family, friends and neighbors. Knowledge is your greatest defense against any potential disaster.

IF YOU FEEL A STRONG EARTHQUAKE WHICH LASTS A LONG TIME WHEN YOU ARE ON THE COAST:

• Protect yourself during the earthquake. Duck, cover and hold if inside and watch for falling objects if outside until the earthquake is over.
• Move to higher ground immediately. Gather your family members and evacuate quickly. Leave everything else behind. A tsunami may be coming within minutes. Go on foot if at all possible. If there is no high ground, move inland away from the coastline.
  DO NOT WAIT FOR AN OFFICIAL WARNING
• Stay away from the coast. A later wave may be higher than the first! Damaging waves may continue to arrive even hours later.
• Listen to your radio. Wait until an official all clear signal has been given before returning to low-lying areas.
• Never go to the coast to watch for a tsunami if you hear that a warning has been issued. Tsunamis move faster than a person can run. Also, incoming traffic hampers safe and timely evacuation of coastal areas.
• Tsunamis are not surfable! They are not V-shaped or curling waves. Most frequently they come onshore as a rapidly-rising turbulent surge of water choked with debris.
• All tsunamis – like hurricanes – are potentially dangerous, even though they may not damage every coastline they strike.
• An earthquake in your area is a natural tsunami warning. Do not stay in low-lying coastal areas after a strong earthquake has been felt.

Damaging tsunamis are very rare. Our coastlines are vulnerable but tsunamis are infrequent. Understand the hazard and learn how to protect yourself, but don’t let the threat of tsunamis ruin your enjoyment of the beach.

General Links for All Areas

• Western State’s Seismic Policy Council Tsunami Hazard Mitigation Committee
• The National Tsunami Mitigation Program – A coordinated national effort to assess tsunami threat, prepare community response, issue timely and effective warnings, and mitigate damage.
• The Alaska Tsunami Warning Center.
• TsuInfo Alert – A periodical for Emergency Managers on Pacific Tsunami risk.

Information for British Columbia

• “Prepare for a Tsunami in Coastal British Columbia” from the Provincial Emergency Program

Information for California

• Living on Shaky Ground: How to Survive Earthquakes and Tsunamis on the North Coast of California

Information for Oregon

• Tsunami! How to survive the hazard on the Oregon coast
• Earthquake and giant Seawave Potential in Oregon
• “Tsunami warning systems and procedures – guidance for local officials”
• Tsunami legislation and Risk Reduction Efforts in Oregon

Information for Washington

• Tsunami Links from the Washington State Dept. of Natural Resources
• The Washington EMD Tsunami Program