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Earth

Tohoku megaquake shows big tremors make volcanoes sink

By Michael Marshall

30 June 2013

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Height loss is a curious side-effect of huge quakes

(Image: Clover/SuperStock)

Five Japanese volcanoes are a bit stouter than they were just a few years ago. Within a day of the 2011 megaquake, they shrank by up to 15 centimetres. The same thing happened to a string of Chilean volcanoes after a magnitude 8.8 quake ripped through the centre of the country in 2010.

This sinking effect could be common to most big earthquakes. “There’s every reason to suspect this is a widespread feature,” says Matt Pritchard of Cornell University in New York, who studied the Chilean volcanoes. Nobody had noticed the subsidence before, bar a few hints in the 1990s, because satellite imaging was not sensitive enough to detect it.

Pritchard and colleagues used a satellite called ALOS to monitor five volcanic areas in the southern Andes, before and after the magnitude-8.8 earthquake struck on 27 February 2010. They found that the volcanoes subsided up to 15 centimetres within weeks of the quake.

Youichiro Takada and Yo Fukushima of Kyoto University in Japan used the same satellite to monitor volcanoes on Japan’s main island of Honshu. Within a day of the Tohoku megaquake that devastated parts of northern Japan in 2011, five volcanoes subsided – and by the same amount that Pritchard saw in Chile.

On the slide

What causes the subsidence is not clear. Pritchard suspects that the shaking opens cracks in the rock, allowing water trapped underground to escape to the surface in hot springs, and triggering subsidence.

Takada and Fukushima have a different theory. They suspect that volcanoes’ magma chambers can be deformed by quakes, allowing the rock above to settle.

Regardless of the mechanism, does the subsidence make volcanoes in the affected region more likely to erupt? Possibly. Tamsin Mather of the University of Oxford, who was not involved in the subsidence studies, has shown that big earthquakes slightly increase the frequency of such eruptions over the following 12 months.

“If you’ve got a volcano that’s en route to an eruption, the earthquake can accelerate it,” she says. None of the volcanoes affected by the Chile and Japan quakes has since erupted, probably because they were too stable for the quakes to trigger a blast.

“The reaction of the volcanoes looks like it’s influenced by the state each volcano is in,” says Mather.

Journal reference: Nature Geoscience, DOI: 10.1038/NGEO1857 (Japan) and 10.1038/NGEO1855 (Chile)

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