Harbour waves
Tsunami is a Japanese word meaning "harbour wave." Tsunamis are not ordinary waves. They’re a series of enormous waves caused by the sudden movement of a huge amount of water. They’re sometimes wrongly called "tidal waves," but they have nothing to do with tides.
What causes tsunamis?
A tsunami starts when something suddenly pushes a large volume of water up or down. This displacement sends waves racing outward in all directions.
| Cause | Share of tsunamis | Example |
|---|---|---|
| Undersea earthquakes | About 80% | Indian Ocean, 2004 |
| Landslides (on land or underwater) | Some | Lituya Bay, Alaska, 1958 |
| Volcanic eruptions | A few | Krakatoa, 1883; Hunga Tonga, 2022 |
| Meteorite impacts | Very rare | The asteroid that ended the age of dinosaurs |
Most tsunamis come from earthquakes at subduction zones, where one tectonic plate sinks beneath another. When stuck plates suddenly slip, the sea floor can jump up several metres, lifting the whole column of water above it.
How tsunamis travel
| Location | Speed | Wave height |
|---|---|---|
| Deep ocean | Up to about 800 km/h, as fast as a jet | Often less than 1 m; ships may not notice |
| Shallow water near shore | Slows to 30–50 km/h | Can grow to 10–30 m or more |
In the deep ocean, a tsunami has an extremely long wavelength, up to hundreds of kilometres. As it reaches shallow water, it slows down, the waves bunch up, and they grow much taller.
A tsunami doesn’t usually look like a giant curling surfing wave. It often looks like a fast-rising tide or a wall of churning water that keeps coming for many minutes.
Warning signs
Nature often gives warnings:
- Strong shaking: if you feel an earthquake near the coast that makes it hard to stand, or lasts a long time.
- Drawback: the sea suddenly pulls back, exposing the sea floor, fish and boats. This happens when the trough of the wave arrives first.
- A loud roar from the ocean, like a train or jet.
If you notice any of these, move to high ground or inland immediately. Don’t wait for an official warning.
Tsunamis come in a series
A tsunami is a series of waves that can arrive 5 to 60 minutes apart, for hours. The first wave is often not the biggest. Never go back to the shore until officials say it’s safe.
Famous tsunamis
| Event | Year | Cause | Impact |
|---|---|---|---|
| Cascadia | 1700 | Magnitude 9 earthquake off BC and the U.S. Pacific Northwest | Flooded coasts; recorded in Japan and in Indigenous oral histories |
| Grand Banks, Newfoundland | 1929 | Magnitude 7.2 earthquake caused an undersea landslide | Struck the Burin Peninsula; 28 deaths; Canada’s deadliest tsunami |
| Lituya Bay, Alaska | 1958 | Landslide into a narrow bay | Water surged 524 m up a hillside, the highest run-up ever recorded |
| Alaska | 1964 | Magnitude 9.2 earthquake | Damaged Port Alberni, BC |
| Indian Ocean | 2004 | Magnitude 9.1 earthquake off Sumatra | Over 225 000 deaths in 14 countries; the deadliest in history |
| Japan (Tōhoku) | 2011 | Magnitude 9.0 earthquake | About 18 000 deaths; Fukushima nuclear accident |
| Hunga Tonga | 2022 | Volcanic eruption | Waves crossed the Pacific |
Warning systems
After 2004, countries expanded tsunami warning systems.
- Seismometers detect large earthquakes within minutes.
- DART buoys on the ocean floor detect changes in water pressure as a tsunami passes.
- Warning centres calculate when waves will arrive and issue alerts.
- Alerts reach the public through sirens, phones, radio and TV. In Canada, alerts go through the Alert Ready system.
Staying safe
Before:
- Know if your home, school or vacation spot is in a tsunami zone.
- Learn the evacuation routes, often marked with signs.
- Have a family plan and an emergency kit.
During:
- If you’re on the coast and feel a strong earthquake, Drop, Cover and Hold On, then move to high ground or inland as soon as the shaking stops.
- Go on foot if possible; roads may be blocked.
- If you can’t get out, go to the upper floors of a strong concrete building.
After:
- Stay away from the coast until officials say it’s safe.
- Avoid floodwater, which may contain debris and chemicals.
Common mistakes
- Calling tsunamis "tidal waves."
- Thinking the first wave is the largest.
- Going to the beach to look when the water pulls back. That’s the time to run.
Try it yourself
- In a long tray of water with a shallow "beach" at one end, quickly lift a board at the other end. Watch how the wave changes as it reaches the shallow end.
- Find your nearest tsunami evacuation route if you live near a coast, or research one in BC.
- Explain why a tsunami might pass under a ship in the deep ocean without anyone noticing.
Quick recap
- Tsunamis are a series of huge waves caused by sudden water displacement, usually from undersea earthquakes.
- They race across oceans and grow tall in shallow water.
- Strong shaking, drawback and a roaring sound are natural warnings.
- The first wave is often not the largest.
- Move to high ground or inland immediately; follow official warnings.