Tsunami quake was second largest on record

Seismogram of the 26 December 2004 earthquake Copyright: Pennsylvania Department of Conservation and Natural Resources

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The earthquake that triggered last year’s tsunami in the Indian Ocean was 2.5 times more powerful than previously thought, making it the second largest ever recorded.

Research published yesterday (31 March) in Nature puts the quake’s magnitude at 9.3 on the Richter scale, second only to the one measuring 9.5 that struck Chile in 1960.

The research by Seth Stein and Emile Okal at Northwestern University, Evanston, United States — which was submitted to the journal before this week’s earthquake in the region — highlighted the continued threat of large earthquakes in the region.

Last year’s earthquake occurred on the boundary of two plates of the Earth’s crust: the India and Burma plates.

When the earthquake struck, pressure created by the India plate pushing beneath the Burma plate was released along a zone that ‘slipped’ by an average of 13 metres, leading to the tsunami.

Researchers initially feared only 400 kilometres of this ‘rupture zone’ had slipped, leaving large unstable sections that could trigger another tsunami.

But a separate paper, also in yesterday’s edition of Nature, by Sidao Ni at the University of Science and Technology of China and colleagues, shows that the entire 1,200 kilometre long zone had ruptured.

Stein and Okal say that this suggests it is unlikely that another tsunami would be generated along the plate boundary affected by last year’s earthquake. But their paper warns that that large earthquakes close to two fault lines further south could trigger tsunamis.

One of these is the Sunda fault — the site of Monday’s (25 March) earthquake, which had been predicted by researchers earlier this month (see Tsunami quake ‘increased risk of further disasters’).

Scientist believe that the latest earthquake did not cause a tsunami because it was deeper, less powerful, and occurred below a shallower body of water than last’s years one (see Scientists explain why latest quake caused no tsunami).

Kerry Sieh of the Tectonic Observatory at the California Institute of Technology, United States, says in an article accompanying the Nature research papers, that ongoing analysis of satellite imagery and on-the-ground observations of changes to the elevation of the region’s coastlines will reveal much more about last year’s earthquake and tsunami.

Link to full paper by Stein and Okal in Nature 

Link to accompanying article by Sieh in Nature  

Read more about tsunamis in SciDev.Net’s Tsunami update.