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A tsunami is a sea wave that results from large-scale seafloor displacement caused by a large earthquake, major submarine slide, or exploding volcanic island.

A seiche (pronounced "saysh") is a series of standing waves in a fully- or partially-enclosed body of water caused by earthquakes or landslides. Seiche action can affect harbors, bays, lakes, rivers, and canals.

While tsunami and seiche events are infrequent in Puget Sound, it is important to be aware of their dangerous potential. Early warning signs of a tsunami include a sudden or unexpected recession of water. The first wave will be followed by additional waves a few minutes or even a few hours later. Wave size typically increases over time, and coastal flooding may often precede the largest wave.

Recent studies regarding the potential for a large Cascadia Subduction Zone earthquake off the coasts of Washington, Oregon, and Northern California indicate the local tsunami waves may reach nearby coastal communities within minutes of a major quake.

Local studies of an earthquake on the Seattle Fault also indicate a potential for tsunamis and seiches.

tsunami

While tsunami and seiche events occur infrequently in Puget Sound, it is important to be aware of their very real and dangerous potential, an awful reality illustrated by the December 2004 earthquake and tsunami in Southeast Asia. Early warning signs of the arrival of tsunami waves is usually typified by a sudden and unexpected recession of water; the first wave will be followed by additional waves a few minutes or even a few hours later. Wave size typically increases over time, and coastal flooding may often precede the largest waves.

Not all earthquakes produce tsunamis. To generate a tsunami, an earthquake must occur underneath or near the ocean, be very large (approximately magnitude 7 or greater), and create vertical movement of the sea floor. However, recent studies regarding the potential for a major Cascadia Subduction Zone earthquake off the Washington, Oregon, and Northern California coastlines indicate the local tsunami waves may reach nearby coastal communities within minutes of the earthquake thereby giving little or no time to issue warnings.

Local studies of the Seattle fault zone also indicate a potential for tsunamis. Scientists interpret the evidence of irregular sand sheets in the northern Puget Sound area as a result of a tsunami generated by an earthquake on the Seattle fault about 1,000 years ago. Similar evidence in Lake Washington sediments suggests a recurrence interval of 300 to 400 years.

The West Coast/Alaska Tsunami Warning Center is responsible for tsunami warnings for California, Oregon, Washington, British Columbia, and Alaska. The Pacific Tsunami Warning Center provides warnings to international authorities, Hawaii, and U.S. territories within the Pacific Basin.

These agencies may issue the following types of bulletins:

  • Information: A message with information about an earthquake that is not expected to generate a tsunami.
  • Advisory: An earthquake has occurred in the Pacific Basin, which might generate a tsunami.
  • Watch: A tsunami was or may have been generated, but is at least two hours travel time to the area in Watch status.
  • Warning: A tsunami was or may have been generated which could cause damage. People in the warned area are strongly advised to evacuate.

If you're near a coastal beach, here are ways to know a tsunami may be imminent and you need to seek higher ground:

  • A warning siren may sound.
  • Seawater may recede quickly.
  • The ground may shake, indicating an earthquake has occurred.
  • Your NOAA Weather Radio issues a warning that a tsunami may be headed to your area.
Learn about tsunami risk in your community
  • Find out what areas are most at risk from tsunamis in your community. Learn the established tsunami evacuation routes and pick the safest route from your home, school, workplace, or any other place you'll be where tsunamis present a risk. Try to go to pick area at least 100 feet above sea level or up and two miles inland, away from the coastline. Determine routes you would take by car, and also by foot.
  • Make arrangements for housing in the event you need to evacuate your home, and always have your emergency kit ready to go.
  • Follow flood preparedness precautions. Tsunamis are large amounts of water that crash onto the coastline, creating floods.
  • Establish meeting places and phone numbers in case family members are separated.
When you're away from home
  • Learn if the area you are visiting is at risk from tsunamis.
  • Check with the hotel, motel, campground operators, or local officials for tsunami evacuation information and how you would be warned. It is important to know designated escape routes before a warning is issued.
  • Find out if your NOAA Weather Radio will work where you are visiting. If so, set it so it can receive information about that area.

When a tsunami watch is issued, listen to a NOAA Weather Radio, Coast Guard emergency frequency station, or local media for emergency information.

  • Stay away from the beach. Watching a tsunami from the beach or cliffs could put you in grave danger. If you can see the wave, you are too close to escape it.

  • Follow instructions issued by local authorities. Recommended evacuation routes may be different form the one you use, or you may be advised to go to even higher ground.

  • Take your disaster supplies with you.

  • Get to higher ground and as far inland as possible. Officials cannot reliably predict either the height or local effects of tsunamis.

  • Tsunami waves may continue for hours. Do not assume that after one wave the danger is over. The next wave may be larger than the first one.

  • Continue listening to your NOAA Weather Radio or monitoring local media for emergency news. The tsunami may have damaged roads, bridges, or other places that may be unsafe.

  • Helped injured or trapped persons. Give first aid where appropriate. Do not move seriously injured persons unless they are in immediate danger of further injury.

  • Use the telephone only for emergency calls.

  • Stay away from flooded and damaged areas until officials say it is safe to return.

  • Never drive through flooded areas - cars can be carried away by just two feet of water.

  • Stay away from debris in the water; it may pose a safety hazard to boats and people.

  • Return home only after authorities advise it is safe to do so.

  • Stay out of the building if waters remain around it. Tsunami waters, like flood waters, can undermine foundations, causing buildings to sink, floors to crack, or walls to collapse.

When re-entering buildings or homes following a tsunami, use extreme caution:

  • Wear sturdy shoes.

  • Use battery-powered lanterns or flashlights instead of candles, lighters, matches, or other ignition devices that could create a hazardous fire situation.

  • Examine walls, floors, doors, staircases, and windows to make sure the building is not in danger of collapsing.

  • Inspect foundations for cracks or other damage.

  • Look for fire hazards. There may be broken or leaking gas lines, flooded electrical circuits, submerged electrical equipment. Fire is the most frequent hazard following floods.

  • Check for gas leaks. If you smell gas or hear a blowing or hissing noise, open a window and quickly leave the building. Turn off the gas if you can and contact the utility company.

  • Look for electrical system damage. If you see sparks or broken or frayed wires, or smell burning insulation, turn off the electricity at the main electrical panel. Note: stepping into water or water damage to the electrical panel may present an electrical hazard as you try to turn it off - if this is the case, contact an electrician first for advice.

  • Do not use appliances or lights until an electrician has checked the electrical system.

  • Check for sewage and water line damage. If you suspect damage, avoid using the toilets and contact a plumber. Contact the utility company for any damage to water pipes.

  • Use a stick to poke through debris in flooded areas.

  • Watch for loose plaster, drywall, and ceilings that could fall.

  • Open doors and windows to help dry the building.

  • Shovel mud while it is still moist to give walls and floors an opportunity to dry.

  • Only use tap water if local health officials advise it is safe.

  • Check food supplies. Any food that has come in contact with flood waters may be contaminated and should be thrown out.

  • Take photographs to document damage for insurance purposes.

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