A sudden push
A large part of the seabed lifts or drops in seconds. The whole column of water above it moves, from the floor to the surface.
Science and safety
Tidal wave: the word and the real thing
“Tidal wave” sounds clear, yet it hides at least four different phenomena. Here is what each one is, what causes it, and why the safest habit is to learn the right names.
~700 km/htsunami speed in deep ocean
100s of kmwavelength of a tsunami
0connection between tsunamis and tides
Why “tidal wave” is the wrong name
Old texts used the phrase for any sudden surge of sea water. The picture in the mind is of a wall of water, so the name stuck in films, news headlines and novels.
The trouble is the word tidal. Tides are the slow daily rise and fall of the sea caused mainly by the Moon’s gravity. A destructive wave from an earthquake has nothing to do with that pull, so scientists replaced the old term with the Japanese word tsunami, meaning “harbour wave”.
A tsunami also does not look like a curling surfing wave. Out at sea it is only a gentle swell, often less than a metre tall, which is why ships may not notice it. Near the shore it behaves more like a fast, rising flood.
Four waves people call a tidal wave
The cause is the key difference. Once you know the cause, you also know the warning signs.
| Phenomenon | Cause | Warning | Where it happens |
|---|---|---|---|
| Tsunami | Undersea earthquake, landslide, volcano | Shaking, sea retreating, official alerts | Any coast, mostly around the Pacific and Indian oceans |
| Tidal bore | Rising tide squeezed into a river mouth | Published tide tables | Qiantang River, Bay of Fundy, Severn, Amazon |
| Storm surge | Wind and low pressure during a storm | Hurricane or cyclone forecasts | Low-lying shorelines |
| Rogue wave | Waves combining in open water | Almost none | Deep ocean |
How a tsunami grows
The physics is neat: speed depends on depth, and that one fact explains almost everything.
Fast and long
In water four kilometres deep, wave speed is about the square root of gravity times depth, roughly 200 metres per second. The waves are very long and very low.
Slow and tall
On the shelf the water gets shallow and the front slows, while the back is still arriving. The energy piles up into height and force.
The first wave is often not the largest. A tsunami can arrive as a series spread over an hour or more, so stay on high ground until officials end the warning.
Two events that changed warning systems
On 26 December 2004 a magnitude 9.1 earthquake off Sumatra sent a tsunami across the Indian Ocean. Hundreds of thousands of people were affected, and there was almost no regional alert network at the time.
On 11 March 2011 a magnitude 9.0 earthquake off Japan produced waves that reached far inland on the Tōhoku coast. Japan’s alerts and sea walls saved many lives, yet the scale showed how hard it is to be fully ready.
Since then, ocean buoys, seabed sensors and faster messaging have improved the lead time in many regions, but people on the shore remain the last and most important link.
What to do near the coast
Know your evacuation route
Look for blue tsunami-zone signs and learn the way to higher ground from where you sleep, work and swim.
Move at the first natural sign
If shaking is strong or long, do not wait for a text message. Go up and inland on foot if roads are clogged.
Stay away from the shore
Do not walk onto the exposed seabed or go to watch the waves. The next one can come in minutes.
Wait for the all-clear
Return only when local authorities announce that the danger has passed.
Frequently asked questions
What is a tidal wave?
In everyday speech it means a huge wave that floods the coast. Scientists avoid the term because such waves are not caused by tides. The correct name is usually tsunami.
Is a tidal wave the same as a tsunami?
People use the words as if they were the same, but a tsunami is made by a sudden disturbance such as an undersea earthquake, landslide or volcanic eruption, while tides come from the pull of the Moon and Sun.
How fast does a tsunami travel?
In deep ocean a tsunami can move at about 700 to 800 kilometres per hour, similar to a jet airliner. It slows down as the water gets shallower, but grows taller.
What is a tidal bore?
A tidal bore is a wave that moves up a river or narrow bay against the current when a strong incoming tide is funnelled into a shrinking channel. It is real, regular and tied to the tide.
What should I do if I feel a strong earthquake near the sea?
Move to high ground or inland straight away. Do not wait for an official alert and do not go to the shore to watch. Stay away until authorities say it is safe.
Learn the names, then learn the signs
Calling it by the right name is the first step to understanding which warning applies.
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