Surprise! Megaquakes Caught Sinking Volcanoes

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Tinguiririca volcano, Chile, one of five volcanoes that subsided following the 8.8-magnitude Maule, Chile earthquake in 2010.
Luis Lara

We already know that megaquakes can level cities and launch tsunamis, but they've now been implicated in the sinking of volcanoes in Chile and Japan as well.

Another eruption of this active volcano isn't the only pending disaster facing Washington's Mt. Rainier.
DCI

Two teams of scientists working independently on volcanoes in Japan and Chile discovered that after mega earthquakes in 2011 and 2010, some nearby volcanoes dropped as much as 15 centimeters (6 inches). The two teams have published their findings in a pair of papers in the June 30 issue of the journal Nature Geoscience.

"The observations are so similar in both places," commented Matthew Pritchard of Cornell University, the lead author of one of the papers. "It's just a spectacular observation."

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In both locations the scientists used satellite data to look for deforming ground around the volcanoes before and after the massive 2011 magnitude 9.0 Tohoku earthquake in Japan and the 2010 magnitude 8.8 Maule earthquake in Chile. In the Maule case, Pritchard's team wasn't even looking for subsidence. They were on an entirely different search -- for any signs of increased volcanic activity -- when they stumbled onto the changes in the sinking volcanoes.

"There's probably nothing special about it," Pritchard told DNews. Similar subsidence is probably happening after the biggest quakes in Alaska, Indonesia and other major subduction zones in which megaquakes are possible. These two events are just the first to be detected because they happened when the right instruments were in orbit to get the data, he explained.

Pritchard also pointed out that the sinking ground is very local, and has nothing to do with the larger, overall mountain building going on in these places, caused by the colliding tectonic plates which are the cause of the megaquakes and volcanoes in the first place.

As for why the volcanoes sank at all, nobody is sure, but they have some ideas. The Japanese researchers Youichiro Takada and Yo Fukushima of Kyoto University suspect that the violent quaking caused subsidence of magma and heat-weakened rocks inside the five volcanoes found to have subsided, which then caused the ground above to fall as well.

For their part, Pritchard's team wonders if the megaquake rattled loose mineral deposits in the hydrothermal system of five Chilean volcanoes – essentially clearing the pipes – so that trapped fluids could escape and deflate the volcanoes somewhat. It's even possible that there were different mechanisms for the sinking in the different locations, all of which could have implications for how the volcanoes behave in the future.

“These remarkable observations highlight that large earthquakes cause significant changes in volcanic regions and may therefore influence volcanic hazard,” commented Sigurjón Jónsson, a geologist at King Abdullah University of Science and Technology.