Tsunamis – Causes and background

TSUNAMIS – DANGER FROM THE BLUE

What causes the giant waves? And how can one protect themselves?

(19/11/2014) Almost ten years ago, the coasts of South Asia and East Africa in the Indian Ocean were surprised by the deadliest tsunami on record. At least 230,000 people lost their lives on the morning of Boxing Day, around 1.7 million became homeless. What do we know today about the origin of tsunamis? And how can we protect ourselves against them?

Infographic: the 2004 tsunami.
Infographic: the 2004 tsunami. Click on the image to enlarge.

Tsunamis: initially inconspicuous, then fatal

Tsunamis originate from marine earthquakes, landslides, volcanic eruptions or meteorite impacts – shocks that set kilometre-high towers of water into vibration. Like an acoustic signal, the tsunami spreads out in all directions.

In open sea, the waves are up to 50 centimetres high; it is only in flatter waters that they become compressed, break and reach land at a height of up to one hundred metres. 

The tsunami 2004 saw waves of more than 30 metres in height. When hitting the coast the waves and the pressing water masses caused devastation. Not only the waves, but also the enormous suction was fatal. The drawback pulled thousands of people into the deep sea.

As a result of returning to port after a perfectly normal day at sea and discovering a completely destroyed shore, Japanese fisherman named the phenomenon tsunami - ‘harbour wave’. 

Dangers and occurance

Their inconspicuousness makes tsunamis the most fatal natural disaster of all. Until now, though, they are very rare: only one percent of sea quakes trigger measurable tsunamis. 

The majority of tsunamis occur in the Pacific Rim of Fire, now and then they also originate on the European coasts – such as in 1908 off Italy in the Mediterranean, where almost 100,000 people were killed in Messina. The tsunami off Japan on 11th March 2011 did not immediately claim so many lives, but it triggered the nuclear disaster at Fukushima.

Warning systems: High tech and ancient knowledge

But a tsunami does not necessarily cause a disaster like in South Asia in 2004, when it hit people unexpectedly and without warning. Tsunami alerts can be given in advance, since the waves can already be located in the sea.

In the Pacific there has been a tsunami early warning system since 1968. In response to the disaster an early warning system developed by the GFZ German Research Centre for Geosciences was put into operation in the Indian Ocean in 2008. And this is how it works: Sensors fixed to the ocean floor register every seismic shock. The measured data is sent to GPS buoys and then forwarded to satellites. Buoys and satellites deliver additional data about the ocean movement to scientific control centres. Tsunami alerts are sent out from there: to media outlets, but also via SMS and mobile apps.

However, local knowledge, handed down through the generations, also plays an important role. Only a few kilometres from the epicentre of the earthquake almost an entire island survived: Simeulue in Indonesia. In 1907 a tsunami swept many people to their deaths there. Because the survivors had passed on the memory verbally, their descendents were spared the same destiny. Through a Welthungerhilfe project, the residents of Simieulue have since rebuilt their island.

"The Great Wave off Kanagawa", a famous colour woodcut by the Japanese artist Katsushika Hokusai (1760-1849).
"The Great Wave off Kanagawa", a famous colour woodcut by the Japanese artist Katsushika Hokusai (1760-1849).
A mother with her children by the destroyed Aceh plains. © Maedje
A mother with her children by the destroyed Aceh plains. © Maedje
Clean-up operations and reconstruction take a long time, as this photo from 2007 shows. © Schlueter
Clean-up operations and reconstruction take a long time, as this photo from 2007 shows. © Schlueter
Fishermen on the way to their fishing grounds off the island of Simeulue (in 2006), only a few kilometres away from the epicentre of the quake. © Lohnes
Fishermen on the way to their fishing grounds off the island of Simeulue (in 2006), only a few kilometres away from the epicentre of the quake. © Lohnes
Facts about Tsunamis

Tsunamis result from sudden and extensive vertical movement of parts of the ocean floor during an earthquake. The movement of the ocean floor leads to oscillation of the water masses.

Only one percent of sea quakes trigger measurable tsunamis:

  • if it has a strength of at least 7 on the Richter scale;

  • if it happens close to the surface of the earth

  • and if it causes the ocean floor to make sudden vertical movements.
2004 tsunami
Map: coastal regions affected by the 2004 tsunami.
Coastal regions affected by the 2004 tsunami. Click on the image to enlarge it.

The tsunami caused disasters in eight countries - more than 1.7 million people became homeless.

We help in emergencies

In many countries Welthungerhilfe provides fast and effective emergency aid during natural disasters or armed conflict. Learn more:

Our work during disasters and conflicts

In times of climate change

Some geologists believe that due to global warming earthquakes, volcanic eruptions and tsunamis will occur more often in the future. The Earth is a system in which the effects of climate change, such as the rise of sea levels or the thawing of ice and gas hydrates, have far-ranging consequences for the topology and stability of the Earth's strata.