Japan's 2011 megaquake reactivated dormant faults

The megaquake that shook the east coast of Japan in March 2011 reactivated dormant faults near Fukushima's beleaguered nuclear reactors, geologists warn.

"A strong quake may occur in the Futaba fault, only 5 to 6 kilometres away," says Dapeng Zhao of Tohoku University. The fault runs parallel to the coast, right past both of Fukushima's nuclear plants.

Zhao's team used data from more than 6000 quakes between June 2002 and October 2011 to make a 3D map of the crust in the area that suffered the largest aftershock recorded on land after the March megaquake: a quake on 11 April in Iwiki. The images show fluid rising through the Pacific plate as it sinks under Japan. They say the March quake transferred stress to surrounding faults, causing them to adjust in aftershocks. The fluid in the Iwiki fault lubricated it, causing it to slip almost immediately in a large, magnitude 7 aftershock (Solid Earth, DOI: 10.5194/se-3-43-2012).

Seismic readings from the Fukushima area reveal stresses and rising fluids similar to those found at Iwiki, so "the possibility of quake generation becomes much larger than before the 11 March quake", says Zhao.

A magnitude 7 quake would release far less energy than the magnitude 9 megaquake, but because the Futaba fault is so close to Fukushima, it would shake both power stations more strongly.

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