Why do Some Earthquakes Cause Tsunamis When Others Don’t?

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Damage caused by the 2011 Japanese tsunami. Photo by Tamaki Seto

Damage caused by the 2011 Japanese tsunami. Photo by Tamaki Seto

In the past decade, the devastating impact of major tsunamis has made the news on a number of occasions, most notably the 2011 Japanese tsunami and the notorious Boxing Day tsunami of 2004. But the earth experiences very many earthquakes each day, and each year sees on average around 15 major earthquakes of at least magnitude 7 (M7.0) and one of ≥M8.0 – and in this context, damaging tsunamis are few. As a reader asked recently, why aren’t we seeing more tsunamis, even when there are pretty big earthquakes?

What Causes a Tsunami?

Tsunamis are not just large waves: they are generated by vertical displacement of sea water by some kind of submarine activity. A tsunami is in fact a series of waves with a very long wavelength (the distance between one wave and the next) and long wave period (time between waves). Although they are usually the result of earthquakes, they can also be caused by other mechanical movements displacing water – such as submarine landslides or volcanic eruptions.

The devastating impact of a major tsunami comes from the fact that, unlike the waves we normally see, they travel through the whole depth of the ocean. While the water is deep, the waves are barely visible but when they make landfall (they can travel across oceans at very high speed) the speed of the wave reduces, the faster-moving water behind piles up and very large breaking waves result on land (the United States Geological Survey reports estimates that the wave which broke on the island of Sumatra on Boxing Day 2004 may have reached 15m in places).

Which Earthquakes Cause Tsunamis?

How a tsunami is generated. Image credit: Anthony Liekens

How a tsunami is generated. Image credit: Anthony Liekens

For an earthquake to cause a tsunami, certain conditions must be satisfied; size, depth of water, and type of movement must all be right to generate the huge waves.

Although there is no absolute size rule, an underwater earthquake of around M7.0 might be considered capable of producing the required displacement. And an M7.0 earthquake is very large – ten times the size of an M6.0 and 100 times the size of an M5.0 (the earthquake scale is logarithmic).

Large earthquakes, however, do not always generate tsunamis – for example, an M8.6 off Sumatra in 2005 caused a 3m tsunami which killed over 1300 people, while another in the same region and of the same magnitude in 2012 generated no tsunami. Although both occurred in deep water (another necessary condition), they were characterised by different types of earth movement. The first involved sudden vertical movement over a long area of a fault: the second involved lateral (strike-slip) faulting within a tectonic plate.

Earthquakes: Movement Doesn’t Always Cause a Tsunami

In summary, then, it’s clear that not all earthquakes generate tsunamis. The International Tsunami Warning Center identifies the main causes of tsunamis as “large, shallow earthquakes with an epicentre or fault line near or on the ocean floor.” So a small earthquake, even if it fulfills all the other conditions, will not generate a major tsunami – and, in reality, such major disasters are mercifully few and far between.

Resources

International Tsunami Warning Center. About Tsunamis. (2013). Accessed 14 January 2013.

Open University. Waves, Tides and Shallow-Water Processes. (2005). Butterworth Heinemann.

USGS. Indian Ocean tsunami triggers deadly earthquake. (2005). Accessed 14 January 2013.

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© Copyright 2013 Jennifer Young, All rights Reserved. Written For: Decoded Science
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