Earthquakes strike, volcanoes sink

Earthquakes cause volcanoes to sink into the ground, according to a pair of independent studies surrounding recent eruptions in Japan and Chile.

Volcanoes in those countries sank by up to six inches during earthquakes which occurred in 2010 and 2011. This lowering of surface features, called subsidence, created oval-shaped areas, measuring as large as nine by 18 miles. These oval areas lay between 125 and 140 miles from the fault where the quakes were centered.

Each of the two papers, published in the journal Nature Geoscience on June 30, offer different takes on the mechanism behind the behavior.

Youichiro Takada, a geophysicist at Kyoto University in Japan, is lead author of one study that proposes the sinking is caused by magma chambers under volcanoes sinking more than the ground surrounding it during the earthquake. This would largely be due to the softer, hotter rock in such chambers deforming more under pressure than does cooler material.

Matthew Pritchard, a geophysicist at Cornell University, hypothesizes a different process. In his paper, Pritchard's team believes that during the quakes, magma escaped from the chambers, which then settled in, filling the empty space.

After the 2010 earthquake in Chile and the 2011 event in Japan, a pair of research teams, working independently of each other, began to study the areas. Pritchard's team was watching for signs of an impending eruption. What they measured from the satellite data surprised the researchers - instead of a bulging surface which could portend an upcoming event, they found the volcanoes were sinking.

"The observations are so similar in both places. It's just a spectacular observation," Pritchard said.

This is the first time that a string of volcanoes has been observed sinking following a quake in the crust of the Earth. The 2010 earthquake in Maule, Chile, was measured at magnitude 8.8, while the Great East Japan Earthquake of 2011 hit 9.0 on the Richter scale. The process by which these two volcanoes sank into the ground appeared nearly identical to one another, despite the difference in their locations.

It has been known for a while that eruptions often follow large earthquakes, although the causes remain unclear. The papers might also help explain why volcanic eruptions occur during some earthquakes and not others.

"Basically, the volcanic system has to be primed and ready to go for the earthquake to tip it over the edge. If, by chance, no volcanoes are close to that point, no volcanic eruptions are triggered." Pritchard said.

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