Science, Maths & Technology

Samoan tsunami

Updated Wednesday 30th September 2009

Dave Rothery looks at the cause of the tsunami that struck Samoa in September 2009

The tsunami that struck Samoa yesterday has the potential to be the biggest tsunami disaster since the Boxing Day (26 December) 2004 tsunami that devastated coasts around the Indian ocean and took nearly 300,000 lives. The death toll this time will be much less than that, but it seems likely to rise as more reports are gathered and it may exceed the 550 killed on 17 July 2006 when a tsunami hit southern Java and will certainly be worse than one that hit Sumatra in September 2007.

Like those, this tsunami was caused by an undersea earthquake, at a subduction zone where one tectonic plate is being pushed down below another. In this example, the floor of the Pacific ocean is being pushed westwards below the Tonga island arc. Plates do not slide past each other smoothly. Instead, strain builds up until the deformation is relieved in a major jerk. In this case, the 'jerk' began at a relatively shallow depth of about 18 km, and the seafloor above it was probably jolted upwards by several metres. This sudden displacement caused a series of waves on the sea surface, which became higher and steeper when they ran ashore. Local reports speak of waves reaching more than 5 metres above sealevel and rushing 100 metres inland.

Unlike the situation in the Indian ocean back in 2004, the Pacific ocean has a pretty good tsunami warning system, and evacuation of Samoa's capital, Apia, did occur although I am not sure whether this was achieved before any tsunami waves were likely to hit it, because the earthquake was so close by that the waves would arrive in less than an hour. Also the earthquake happened so early in the day, just before 7am local time, that many people may not have been aware of the situation.

Students of the Open University short course Volcanoes, earthquakes and tsunamis , which is supported by the book Teach Yourself Volcanoes, Earthquakes and Tsunamis will doubtless soon be discussing the issues and implications raised by this event. There are many current news reports, and already a rather good entry on Wikipedia.

Samoan region of the Pacific ocean [data courtesy of US Geological Survey] Copyrighted image Icon Copyright: data courtesy of US Geological Survey
Samoan region of the Pacific ocean

Earthquakes in the Samoa region in the 24 hours before the time stated at the top (there were none in the previous week). The largest blue square locates the epicentre of the magnitude 8.0 quake that caused the tsunami. The others are smaller aftershocks.  Samoa is the group of islands north of the earthquake swarm, Tonga lies to the south, and the outlying islands of the Fiji group are visible near the western edge of the map. The map covers a 10 by 10 degree block, approximately 1000 km across. Data courtesy of USGS.

Take it further

Volcanoes, earthquakes and tsunamis

Teach Yourself Volcanoes, Earthquakes and Tsunamis by David Rothery
published by Hodder Education


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