Tsunami

A tsunami is a sequence of waves generated when a large, sudden disturbance displaces water. The most common cause is an undersea earthquake that moves the seafloor vertically, pushing or pulling the water above it. Not every earthquake produces a tsunami: the location, depth, size and pattern of seafloor movement all affect whether enough water is displaced.

Underwater or coastal landslides and volcanic activity can also generate tsunamis. Some fast changes in atmospheric pressure produce meteotsunamis, which resemble earthquake-generated waves but have a meteorological origin. The Japanese word “tsunami” means “harbour wave.” The older expression “tidal wave” is misleading: tides do not cause tsunamis.

Tsunami waves travel outward from their source, sometimes across an entire ocean basin. In deep water, a tsunami can move rapidly while its height at the surface remains small, so a vessel offshore may notice little disturbance. The energy of the wave, however, extends through much of the water column. Wave speed depends on water depth.

As a tsunami enters shallower water, it slows, its wavelength shortens and its height may increase—a process often described as shoaling. The shape of the seafloor and coastline affects how the waves travel and where water rises or inundates land, so impacts can vary sharply between nearby locations. A tsunami does not always arrive as a single towering breaker; it may resemble a fast-rising flood and create strong currents. It is a series of waves, and the first is not necessarily the largest. Successive waves and currents can remain dangerous after the initial arrival.

Tsunamis are sometimes classified by how far they travel and how long they take to reach a particular coast. A local tsunami begins close enough that waves may arrive within minutes, leaving little time for official alerts. A distant, or ocean-basin-wide, tsunami travels much farther and may give warning centers more time to assess the threat, although it can still affect multiple coastlines. These descriptions are relative to the coast at risk rather than different physical kinds of wave.

Tsunami impacts include coastal flooding, erosion and unusually strong currents in bays, harbours and other waterways. The extent of damage depends on the wave, local topography and the exposure of people and infrastructure. Because several waves may arrive over an extended period, the first recession or surge does not mark the end of an event.

Warning systems combine information from seismic networks with measurements of sea level. In the United States, for example, NOAA uses earthquake data to assess a possible source, offshore Deep-ocean Assessment and Reporting of Tsunami (DART) instruments to measure waves in the open ocean, coastal water-level stations to confirm arrival, and forecast models to estimate impacts. International warning cooperation helps share threat information across ocean basins. Methods and alert categories vary among national and regional systems.

Warnings cannot always precede a local tsunami, which may reach shore within minutes. UNESCO's Intergovernmental Oceanographic Commission identifies strong or unusually long ground shaking and an unusual retreat of coastal water as natural warning signs. Monitoring, clear communication, preparedness and evacuation planning reduce risk, but forecasting a tsunami's precise local effects remains challenging because coastal conditions differ from place to place.