"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.