Large tsunamis are rare relative to other kinds of natural disasters, but when they do occur they can devastate coastal populations.

INTRODUCTION
A tsunami is a series of waves – usually in an ocean – that is created when a large volume of water is displaced by a powerful event such as earthquake, volcanic eruption 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 kilometres per hour.

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 metres higher than the normal sea level and so can devastate coastal regions, knocking over trees and buildings and carrying debris far inland.

This video footage of the tsunami that struck Japan in 2011 shows the immense power that these waves have.

PROTECTION AGAINST TSUNAMIS
Three things are critical to protect vulnerable coastlines and the people and infrastructure there from tsunamis.

First: a way of detecting tsunamis as soon as they form. Second: a way of alerting people to the danger. And third: coastal defences, evacuation plans and other steps to minimise the impact of any tsunami that does strike.

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 centres 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 centres send text messages to millions of mobile phones soon after getting the alert themselves.

People can also join free international warning systems – such as CWarn – to receive text messages if a tsunami threatens their location. Radio, television and loud-speakers or sirens are also important ways to warn people.

Coastal defences 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 defences 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.

REPORTING TIPS
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, Pacific Tsunami Warning Center and C-Warn.org, 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 — in particular the fate of survivors — who may be injured, homeless, missing or at risk from water-borne diseases.

This kind of natural disaster raises many ethical challenges which are explored in this article in the Pacific Journalism Review [PDF], about the experiences of journalists from New Zealand who reported on the Indian Ocean tsunami of 2004.

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. For story ideas and to keep track of news and views about tsunamis in developing nations, see SciDev.Net’s tsunami collection.

CASE STUDY: Indian Ocean tsunami
On 26 December 2004 a large (magnitude 9.0) earthquake struck off the coast of Sumatra, Indonesia and it triggered the most devastating tsunami in recorded history — killing an estimated 230,000 and destroying entire towns.

When the earthquake struck, the seabed directly above it rose by several metres and this forced a massive volume of water upwards. The waves were up to 30 metres high and in some places they travelled two kilometres inland.

Indonesia bore the worst of the effects and had the greatest number of casualties but there was also considerable damage in Sri Lanka, Thailand and India. The tsunami also hit Bangladesh, Somalia, South Africa, Myanmar and other countries. For a full list and the number of victims in each country see this table of data on Wikipedia.

The tsunami created a widespread humanitarian crisis, with hundreds of thousands of people left without shelter, food and water supplies. Many people are still trying to rebuild their lives years later.

At the time, there was no tsunami warning system in the Indian Ocean – unlike in the Pacific where earthquakes and tsunamis are much more frequent. Since then, a system has been set up.

LINKS TO OTHER TOOLKIT PAGES
Communicating Risk
Reporting on Disasters

FURTHER READING
International Tsunami Information Centre
Pacific Tsunami Warning Center
C-Warn.org
Tsunami Society International
NOAA tsunami pages