What is a Tsunami? 

A tsunami is a series of long waves caused by a large and sudden displacement of the ocean. The speed of a tsunami depends on the depth of the water it’s traveling through. The deeper the water, the faster the tsunami. In the deep ocean, tsunami move as fast as a jet plane. As they enter shallow water near land, they slow to 20 or 30 mph. As they slow down, tsunami grow in height. When they arrive on shore, most are less than 10 feet high. In some cases, they can be 100 feet in height.

News Release - California Geological Survey Issues New Tsunami Hazard Maps for SLO County 

Tsunami Alert Levels 

Tsunami Warning

A Tsunami Warning is issued when a tsunami with the potential to generate widespread inundation is imminent, expected, or occurring. Warnings let you know that dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after initial arrival.

Warnings may be updated, adjusted geographically, downgraded, or canceled based on updated information and analysis.

Tsunami Advisory 

A Tsunami Advisory is issued when a tsunami with the potential to generate strong currents or waves dangerous to those in or very near the water is imminent, expected, or occurring. The threat may continue for several hours after initial arrival, but significant inundation is not expected for areas under an Advisory.

Advisories may be updated, adjusted geographically, upgraded to a warning, or canceled based on updated information and analysis.

Tsunami Watch
A Tsunami Watch is issued when a tsunami may later impact the Watch area. The Watch may be upgraded to a Warning or Advisory or canceled based on updated information and analysis.

Tsunami Information Statement
A Tsunami Information Statement is issued when an earthquake or tsunami has occurred of interest to the message recipients. In most cases, Information Statements are issued to indicate there is no threat of a destructive basin-wide tsunami and to prevent unnecessary evacuations. Information Statements for distant events requiring evaluation may be upgraded to a Warning, Advisory, or Watch based on updated information and analysis.

Tsunami Information

Where do tsunami happen?

Large tsunami can flood low-lying coastal areas more than a mile inland. Not all tsunami are the same, and a tsunami may act differently due to local and coastal geographical features. A small tsunami in one place may be very large and violent tsunami a few miles away.

Tsunami can occur in all of the world’s oceans, inland seas, and in any large body of water. Certain areas are at high risk for tsunami. This is due to their location near local tsunami sources and exposure to distant tsunami sources. It is also due to the shape of the nearby seafloor and coastal geographic features. Low-lying areas such as beaches, bays, lagoons, harbors, and river mouths leading to the ocean are most at risk

U.S. West Coast’s tsunami hazard level
The West Coast states of Washington, Oregon, and California have experienced tsunami from as far away as Alaska, South America, Japan, and Russia. The most damaging on record is the tsunami caused by the 1964 Great Alaska earthquake. More recently, harbors in the region were damaged by events in Japan (2011) and Chile (2010). Locally, the greatest threat is from the Cascadia subduction zone.

Tsunami safety
Although tsunami cannot be prevented, there are things that can be done before, during, and after a tsunami that could save lives.

What is my risk?
A tsunami can strike any coastline, but the threat is not the same everywhere. To better understand your risk, find out if your home, school, workplace, or other places you visit are in a tsunami hazard zone and if your community has had tsunami in the past.

How will I know if a tsunami is coming?
There are two ways you might learn that a tsunami is coming: A Tsunami Warnings from local officials or a natural tsunami warning. Both are equally important. Be prepared to take action to whichever warning you receive first.

Official tsunami warning
An official Tsunami Warning from local officials will likely be broadcast through local radio and television, NOAA Weather Radios, wireless emergency alerts, or marine radio. There may not always be time for an official Tsunami Warning to be issued, so it’s important to understand natural warning signs.

Natural tsunami warnings
Natural tsunami warnings include a strong earthquake, a loud roar from the ocean, and strange ocean behavior. The ocean could look like a fast-rising flood, wall of water, or it could drain away suddenly. If you experience any of these natural warnings, a tsunami could be coming. A natural tsunami warning may be your first or only warning that a tsunami is coming.

Preparing for a tsunami

It’s easy to prepare for a tsunami. Many of the things you should do to prepare for a tsunami are the same as those for other hazards in your community.

If your home, school, workplace, are in a tsunami hazard zone, you should do the following:

  • Ensure you can receive official Tsunami Warnings. Get a battery-operated NOAA Weather Radio, sign up for Reverse 9-1-1, and make sure your mobile devices are set to receive wireless emergency alerts.
  • Make a family emergency plan. Meet with your family to discuss the plan. Practice your plan and keep it up to date.
  • Map out routes to safe places on high ground. Plan to evacuate on foot if you can; roads may be impassable due to damage, closures, or traffic jams.
  • Practice walking your routes, even in darkness and bad weather. This will make evacuation quicker and easier during an emergency.
  • Put together a portable disaster supply kit with items your family and pets may need. Since you do not know where you’ll be during a tsunami, prepare kits for work and cars too. Consider storing essential documents or supplies with family or friends outside of the tsunami hazard zone. 

After an official Tsunami Warning

  • Stay out of the water and away from beaches and waterways.
  • If local officials direct you to evacuate, use your emergency plan and move quickly to your safe place. If you do not have a safe place or cannot reach it, go as high or far inland as possible.

After a natural tsunami warning

If you feel a strong or long earthquake or the ocean sounds or acts strange, a tsunami could arrive within minutes:

  • In case of an earthquake, protect yourself. Drop, cover, and hold on. Be prepared for aftershocks.
  • Act. Do not wait for an official warning. Move to high ground immediately.
  • If there is earthquake damage, avoid fallen power lines and stay away from weakened structures.
  • When you’re in a safe place, get more information from local radio or television. Continue to stay tuned throughout the event.

If you’re on the beach or near water and feel a large earthquake, move quickly to high ground or inland away from water. Get more information from radio, television, or your mobile device.

What should I do after a tsunami?

After a tsunami, listen to local officials. They will assess the damage and tell you when it’s safe to return. Make sure that you:

  • Stay out of the tsunami hazard zone until local officials say it’s safe. Remember, the first wave may not be the last or the largest and the danger may last for hours or days.
  • Keep listening to local radio or television to get updates about when it’s safe to return.
  • Limit non-emergency phone calls to keep the lines open for emergency responders.
  • Stay away from areas with damage.
    • Stay out of any building that has earthquake or tsunami damage, or that has water around it until an official tells you it’s safe to enter.
    • Avoid fallen power lines and broken utility lines.

If you are on the water:

  • Make sure you have a way to receive Tsunami Warnings. The U.S. Coast Guard will issue marine broadcasts on your marine VHF radio’s channel 16.
  • Your harbormaster, the U.S. Coast Guard, and local emergency management offices are good sources for tsunami safety information for boaters in your area.
  • Plan and put together a disaster supply kit to keep on board your boat. Remember that the harbor may be damaged during a tsunami and you may not be able to return to it. Be prepared to remain at sea for a day or more.
  • If you’re in a harbor, you should leave your boat and move quickly to a safe place on land
  • If you’re at sea, you should move to a deeper depth and stay away from until officials tell you the danger has passed.

What causes a tsunami

The most common cause of a tsunami is a large earthquake below or near the ocean floor. Landslides, volcanic activity, weather, and asteroids can also generate tsunami.

Not all earthquakes generate tsunami. Whether an earthquake generates a tsunami depends on its location, size, and depth. In general, earthquakes that generate tsunami:

  • Occur under or very near the ocean
  • Have magnitudes over 7.0
  • Are less than 62 miles below Earth’s surface.

An earthquake may cause the ocean floor to suddenly rise or fall. This sudden vertical movement of the ocean floor is what typically sets a tsunami in motion.

Tsunami can also occur when a landslide displaces ocean water. A landslide-generated tsunami may be larger than a tsunami generated by an earthquake and can affect nearby coasts within minutes. These tsunami usually lose energy quickly and rarely travel great distances.

  Downloadable Tsunami Preparedness Fact Sheet