Tsunami alert basics
Tsunamis are series of waves usually caused by large undersea earthquakes. Learn the official U.S. message types, the natural warning signs, and why most earthquakes do not cause tsunamis.
Key concepts
- What causes a tsunami
- Warning, Advisory, Watch and Information Statement
- International threat messages
- Natural warning signs
What a tsunami is
A tsunami is a series of ocean waves caused by a sudden, large displacement of water. Most tsunamis are caused by large earthquakes beneath or near the ocean that move the sea floor, but submarine landslides and volcanic eruptions can also cause them.
A tsunami is not a single wave. Waves can keep arriving for hours, the first wave is often not the largest, and dangerous currents can last for days. The time between wave crests can range from about five minutes to two hours.
Most earthquakes do not cause tsunamis
The great majority of earthquakes do not cause tsunamis. Earthquakes on land, small earthquakes, and many deep earthquakes cannot move enough sea floor to disturb the ocean significantly. That is why tsunami warning centres evaluate each significant earthquake and issue messages that range from a warning to a statement that there is no threat.
It also means you should not guess from a headline. A magnitude or a location near the coast does not by itself tell you whether a tsunami was generated. Official tsunami messages are the authority, and EQRadar never infers a tsunami alert from earthquake data.
U.S. tsunami message types
In the United States, the National Tsunami Warning Center and the Pacific Tsunami Warning Center issue four main types of message for U.S. and Canadian coasts. A single bulletin can assign different levels to different stretches of coast, so always check the level for your own area.
For other countries, the Pacific Tsunami Warning Center issues tsunami threat messages and information statements. A threat message describes potential threats to coasts and advises following instructions from national and local authorities; it is not the same as a U.S. tsunami warning. Cancellation messages end a previous alert for the areas they list.
| Message | Threat | What to do |
|---|---|---|
| Warning | Dangerous coastal flooding and powerful currents | Move to high ground or inland; follow local officials |
| Advisory | Strong currents and waves dangerous to those in or very near water | Stay out of the water, away from beaches and waterways |
| Watch | Not yet known | Stay tuned for more information; be prepared to act |
| Information Statement | No threat, or a very distant event whose hazard is not yet determined | No action suggested at this time |
Natural warning signs
Near the coast, nature may warn you before any official message does. Natural warning signs include a strong or long earthquake, a loud roar from the ocean, and unusual ocean behaviour such as a fast-rising flood of water or water suddenly receding to expose the sea floor.
If you notice any of these signs, even just one, a tsunami could be coming. Protect yourself during the shaking first, then move inland or to higher ground as soon as you can do so safely. Do not wait for an official warning, and never go to the shore to watch.
Tsunami messages in EQRadar
EQRadar's Tsunami Alerts page shows the latest bulletin from each product published by the U.S. Tsunami Warning Centers, keeping the categories exactly as issued. A message with no expiry time is shown as unknown rather than cleared, and a failed check is never shown as an all-clear.
EQRadar is not an official warning channel and can lag the source. Official messages reach the public through NOAA Weather Radio, emergency alerts, local officials and tsunami.gov.
Three takeaways
- Tsunamis are series of waves, usually from large undersea earthquakes; most earthquakes do not cause one.
- U.S. messages are Warning, Advisory, Watch and Information Statement, and one bulletin can assign different levels to different coasts.
- Strong or long shaking, an ocean roar or unusual water movement are natural warnings: move to higher ground without waiting.
Sources
- Tsunami Message Definitions — NOAA/NWS U.S. Tsunami Warning Centers
- Tsunami Frequently Asked Questions — NOAA/NWS U.S. Tsunami Warning Centers
- During a Tsunami — NOAA National Weather Service