Even the gravity has altered (Image: Kyodo/Reuters)

An ultra-thin fault zone packed with slippery clay was behind the massive seismic slip during Japan’s devastating Tohoku earthquake of 2011. The quake was so great that it permanently changed the region’s gravitational field, and was “heard” from space.

To find out how such a large slip – in excess of 50 metres in places – happened, teams of seismologists on board Japan’s deep-sea research vessel Chikyu drilled into the seabed around the plate boundary that ruptured during the Tohoku earthquake. They took measurements in boreholes reaching 844 metres beneath the seabed.

The results, published in a collection of studies this week, revealed a significant presence of smectite, a slippery clay largely responsible for many major landslides in Europe (Science, DOI: 10.1126/science.1243719). Temperature measurements confirmed the fault had a coefficient of friction of only 0.1, making it very likely to shift. Most rocks slip at about 0.5 or 0.6, says James Mori of the Disaster Prevention Research Institute at Kyoto University (Science, DOI: 10.1126/science.1243641).


What’s more, the fault zone was found to be less than 5 metres thick, tens of times thinner than at other subduction zones.

“One of the main goals was to try and explain the massive slip, which we had never seen before in an earthquake,” says Mori. It seems that subduction zones with particularly thin fault zones and a lot of smectite can produce slips of more than 50 metres, he adds, and potentially give rise to large tsunamis.

Infrasound

The Tohoku slip was so big that the infrasound waves generated by the quake propagated more than 200 kilometres through the atmosphere. That disturbed the orbit of the European Space Agency’s GOCE satellite (Geophysical Research Letters, DOI: 10.1002/grl.50205).

Video: Japan earthquake heard from space

GOCE measures small variations in Earth’s gravity field, which changes with the shape of the landscape and density of the crust. Yesterday, researchers at the German Geodetic Research Institute in Munich and Delft University of Technology in the Netherlands released more GOCE data showing that the quake has caused subtle changes in the local gravity field.

“This will continue to help us explain the mechanism of future earthquakes,” says Robert Geller, a seismologist at the University of Tokyo, “but I don’t think it is going to help us to say when or where the next one will occur.”