Giant earthquakes in subduction zones do not just create tsunamis — they can also cause nearby volcanic regions to sink, possibly altering the risk of eruptions.

In subduction zones, one plate of Earth's crust plunges beneath another. Quakes cause the overriding plate to expand and subside. Volcanoes on these plates subside even further, according to satellite radar data from two regions.

Youichiro Takada and Yo Fukushima at Kyoto University, Japan, measured drops in volcanic regions of up to 15 centimetres near the fault that broke in the 2011 magnitude-9.0 Tohoku earthquake. Separately, Matt Pritchard at Cornell University in Ithaca, New York, and his colleagues measured subsidence of up to 15 centimetres within weeks of the 2010 magnitude-8.8 Maule earthquake off the coast of Chile.

The authors of the Japanese study suggest that the subsidence occurred because reservoirs of magma below the volcanoes sank. By contrast, the authors of the Chilean study say that hydrothermal reservoirs may have drained, causing the ground above to collapse.

Nature Geosci. http://dx.doi.org/10.1038/ngeo1857; http://dx.doi.org/10.1038/ngeo1855 (2013)