Tsunami

A tsunami is a series of powerful ocean waves generated when a large volume of water is suddenly displaced. In the open ocean these waves can travel across entire basins at the speed of a jet aircraft while remaining barely noticeable to ships above. As they approach shallow coastal water, they slow down, pile up and can grow into destructive surges that flood low-lying land far inland.

The word comes from Japanese, meaning "harbour wave", which reflects the fact that tsunamis often strike harbours and coastlines with little apparent warning. They are sometimes mistakenly called "tidal waves", but they are not caused by tides; the moon and sun play no role in generating them.

What causes a tsunami

The most common cause is an undersea earthquake that displaces the seafloor. When a large section of the ocean floor lifts or drops suddenly, it pushes the water above it, setting the whole water column in motion. Not every earthquake produces a tsunami; it generally requires a large quake with vertical movement of the seabed. Other causes include underwater landslides, volcanic eruptions and their collapse, and, rarely, the impact of a large object from space. Sudden rockfalls or calving ice can also trigger local tsunamis.

How tsunamis behave

In deep water a tsunami has a very long wavelength and small wave height, which is why it can pass unnoticed beneath a ship. Because it involves so much water, the energy is spread through the full depth of the ocean. As the wave enters shallow near-shore water, the front slows while the back catches up, causing the water to rise. On shore this may appear as a rapidly rising flood, a wall of water, or, sometimes, an unusual retreat of the sea that exposes the seabed before the waves arrive. A tsunami is not a single wave but a train of waves, and later waves can be larger than the first.

The sign of a coming wave

For people near the coast, the most important natural warning is strong ground shaking from an earthquake, or a sudden withdrawal of the sea. If the ground shakes hard for a long time, or the ocean pulls back unusually far, people should move immediately to high ground or inland without waiting for an official alert. The first wave may arrive within minutes of a nearby earthquake, leaving little time for authorities to react.

Warning systems

Because tsunamis cross oceans slowly, distant shorelines can often be warned well in advance. International networks of seismic sensors detect large earthquakes, and ocean buoys measure changes in water pressure and sea level to confirm that a tsunami has formed. Warning centres then issue alerts to threatened coastlines. For nearby "local" tsunamis, generated close to shore, there may be almost no time, so communities rely on education, sirens, evacuation routes and the natural signs of shaking or receding water.

Impact on coastlines

Tsunamis can cause devastating damage. Fast-moving water and debris smash buildings, roads and bridges, and the force of the flood can strip away soil and structures. Salt water contaminates freshwater supplies, farmland and groundwater. Casualties are often highest among people who remain near the shore, and the risk is greatest in low-lying areas and narrow bays that can funnel and amplify the waves. Economic losses can be immense, particularly in coastal communities that depend on fishing and tourism.

Reducing the risk

Preparedness saves lives. Communities in at-risk regions build vertical evacuation shelters, plant coastal forests that can absorb some energy, and mark safe routes to higher ground. Land-use planning keeps critical facilities away from the most exposed areas, and building codes help structures withstand flooding. Public education ensures that residents and visitors know the warning signs and how to respond. After major events, rebuilding often includes stronger defences, though there are limits to how much any coast can be fully protected.

Common misconceptions

A widespread myth is that a tsunami is a single giant wave that looks like a surfing wave; in reality it is a series of surges, and the highest crest may not be first. Another is that tsunamis are always tall in the open ocean; they are usually small there and only grow near shore. People sometimes believe they are safe if they can see the wave far away, but tsunamis can move faster than a person can run. Finally, not all earthquakes cause tsunamis, and not all tsunamis are caused by earthquakes.

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