A wave unlike the ones at the beach
The word tsunami comes from Japanese and is usually translated as “harbor wave.” It describes a series of long ocean waves caused by a sudden displacement of a large volume of water. Ordinary waves are pushed along by wind and affect only the surface layer. A tsunami involves the whole water column, from the seafloor to the top, which is why it can carry so much energy.
Tsunamis are sometimes called “tidal waves,” but that is a misnomer. Tides are driven by the gravity of the Moon and Sun, and tsunamis have nothing to do with them.
What triggers a tsunami
The most common cause is a large undersea earthquake. Most of the biggest examples occur at subduction zones, where one tectonic plate slides beneath another. The plates can lock together for decades or centuries while strain builds. When the stuck section finally slips, the seafloor can jerk upward or drop by a few meters over a wide area, and the water above is lifted or lowered with it. That sudden change is the starting point, so the earthquake causes at subduction zones are central to tsunami risk.
Not every earthquake produces a tsunami. Quakes that are deep, small, or that mostly slide sideways rather than moving the seafloor up and down are less likely to. Magnitude is only one clue.
Other triggers exist:
- Underwater or coastal landslides, which push water aside as they fall.
- Volcanic activity, including collapses of volcanic flanks and some explosive volcanic eruptions near or under the sea.
- Rarely, meteorite impacts in the ocean, which are possible but very uncommon in human history.
Landslide and volcanic tsunamis are often more local, but they can still be very dangerous because they strike with little time for warning.
How they travel
In the open ocean, a tsunami has a very long wavelength, which is the distance from one crest to the next. It can be many tens or even hundreds of kilometers. Its height there is often only a small fraction of a meter to perhaps a meter or so, so a ship at sea may hardly notice it.
What makes it distinctive is speed. For waves this long, the speed depends mainly on the depth of the water. Roughly, speed is the square root of gravity multiplied by depth. In water about 4,000 meters deep, that works out to about 200 meters per second, which is around 700 kilometers per hour, comparable to a jet airliner. This is a rough illustration, not an exact forecast for any particular event, but it shows why a tsunami can cross an ocean basin in many hours.
Check the arithmetic: gravity is about 9.8 meters per second squared. Multiply by 4,000 to get 39,200. The square root of 39,200 is a little under 198, so roughly 200 meters per second. Multiply by 3,600 seconds per hour to get about 713,000 meters per hour, or about 710 kilometers per hour.
Tsunamis also lose relatively little energy on the way. Energy spreads out and some is reflected, but the wave can remain destructive at great distances. Tsunamis can bend around islands and reflect off coastlines, so the pattern of arrivals can be complicated.
Why coasts are affected
As the wave approaches shallower water near the coast, it slows down. The front of the wave slows first and the back catches up, so the wave becomes shorter and taller. This effect is known as shoaling. The energy that was spread over a deep column of water is squeezed into a smaller volume, and the wave grows in height.
On land, a tsunami often does not look like a breaking surf wave. It may arrive as a rapidly rising surge, like a fast-flooding tide, and it can push far inland, especially across low, flat ground, river mouths and bays. The maximum height reached on land is called run-up. Local factors strongly affect the outcome, such as the shape of the seafloor, the shape of the bay, and whether reefs, wetlands or buildings absorb or channel the flow.
Tsunamis usually come as a series of waves, sometimes tens of minutes or more apart. The first wave is not always the largest, so danger can continue for hours. Sometimes the water first recedes dramatically from the shore, which can expose the seabed. This does not always happen, and relying on it is unwise.
Warning systems and staying safe
Monitoring networks use seismic instruments and pressure sensors on the ocean floor, along with coastal tide gauges, to detect a tsunami and estimate its size. Some of that information relies on satellite communication to relay data. In regions with such systems, authorities can issue alerts, and the time available varies widely: a distant source may allow hours, while a nearby one may leave only minutes.
General safety advice is consistent across many countries, but the details depend on place, so you should follow local guidance and official instructions where you live or visit.
- If you are near the coast and feel a strong or long-lasting earthquake, do not wait for an official message. Move to high ground or farther inland.
- If an official tsunami warning is issued, follow evacuation instructions and routes.
- Stay away from the shore until authorities say it is safe, because later waves can be larger.
- If you travel to coastal areas, learn the local signs and evacuation routes. Travelers can check these when they arrive, as part of general travel safety.
The wider picture
Tsunamis are a reminder that Earth’s hazards are connected. Plate motion builds up stress, an earthquake releases it, and the ocean carries the effects far from the source. The same waves that are barely noticeable far from shore become destructive when they meet the coast. Understanding the physics behind them, and paying attention to warnings, is the best protection scientists and communities have.