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A tsunami is a series of waves, almost always in an ocean, hat is created when a large volume of water is displaced by a powerful event such as earthquake, volcanic eruption, landslide or explosion. About 80 percent of tsunamis occur in the Pacific Ocean.

Tsunamis that originate in the deep ocean initially have a very small wave height that is barely detectable but the waves move very fast — at up to 500 to 1,000 kilometers per hour.

Once the water is displaced, the tsunami comes in the form of a small wave that moves very fast, from 500 to 1000 kilometers per hour. As the tsunami approaches shallower water near land, waves are forced to slow down but gain a significant rise in height.

As the waves reach shallower water around coastlines, friction causes them to slow down and this makes them rise in height.

In some cases, the arrival of a tsunami on a shore will be preceded by a sudden retreat of the sea, which can serve as a warning to anyone nearby to evacuate the area.

By the time the waves reach land, they can be as much as ten meters higher than the normal sea level and so can devastate coastal regions, knocking over trees and buildings and carrying debris far inland.


Because of the size and speed of tsunamis, there is no way to stop a tsunami once it starts. The construction of coastal defenses yield mixed results as they are often not high or strong enough to protect against a tsunami. Therefore, the best protection is to be aware of when a tsunami is coming and plans for evacuation. The earthquakes that cause most tsunamis are hard to predict but when an earthquake happens, it will be noted immediately by seismologists around the world who can issue warning alerts.

Shoreline tide gauges or floating buoys in the deep ocean can measure any change in sea level to confirm whether a quake has triggered a tsunami. The buoys can relay this information via satellite to land-based centers where staff can issue a full tsunami alert, often with predictions of wave heights, speed and direction of travel.

The information can be used to warn people to take action to protect themselves — such as by moving quickly to high ground. In some countries warning centers send text messages to millions of mobile phones soon after getting the alert themselves. Radio, television and loud-speakers or sirens are also important ways to warn people.

Coastal defenses such as sea walls and mangrove forests can help to protect people, but recent research — and the Japanese tsunami of 2011 — suggest that in a very powerful tsunami even these defenses can be inadequate. And if an earthquake occurs very close to a coastline, there may simply not be enough time to get a warning to people who live nearby.

Not every tsunami is big enough to threaten people’s lives. Nor do all oceanic earthquakes cause tsunamis. So when an earthquake strikes, it is important for journalists to assess the risk so they can report responsibly (see also Communicating Risk).

Reliable sources of information include the International Tsunami Information Centre and Pacific Tsunami Warning Center which all monitor and raise alerts about tsunamis.

As with other kinds of natural disasters, citizens are increasingly important to journalists as sources of instant, on-the-ground information that they can share via mobile phones and the internet.

For a discussion of how this kind ‘citizen journalism’ played a role in media coverage of the 2004 Indian Ocean tsunami, see this Poynter article “Taking Tsunami Coverage into Their Own Hands.”

Although the immediate impact of a tsunami is the most devastating, there are many ongoing stories to report in its aftermath such as the recovery effort and those who may be injured, homeless, missing or at risk of water-borne diseases.

Other lasting effects include the social and economic impacts of damage to infrastructure, such as the damage that salty sea water can do to agricultural land or supplies of drinking water, and damage to roads, schools, hospitals and power supplies.

A big part of the story of a tsunami will be about how well prepared countries and communities were (and this is also an angle that journalists can use to report on tsunamis before they happen) and whether any human factors contributed to people’s vulnerability.

For example, some scientists and environmentalists — see Mangroves: Nature’s defence against Tsunamis[PDF] — say dense coastal mangrove forests can reduce the impact of tsunamis by absorbing some of their energy. They warn that excessive deforestation and coastal development could lead to greater damage and more deaths if a tsunami strikes.

But, as reported here by SciDev.Net, other research concludes that mangroves can offer little protection against a powerful tsunami. The reality will vary from place to place and will depend of course on the size and strength of the tsunami waves.

The Earth Journalism Toolkit page on Reporting on Disasters has more advice that will be relevant to journalists who report on tsunamis.


An Example New York Times Article on the 2011 Japanese Earthquake and Tsunami
International Tsunami Information Centre
Pacific Tsunami Warning Center
Tsunami Society International
NOAA tsunami pages