A man walks with his dog through an area destroyed by an earthquake and tsunami

Rubble replaced many Japanese towns after the 2011 Tohoku earthquake, which was preceded by subtle changes in geological zones off Japan’s east coast. Credit: Issei Kato/Reuters

Seismology

Fault reversal a harbinger of large quakes

Seafloor slippage may have presaged catastrophic 2011 event in Japan.

Monitoring the direction of seafloor movement in geological zones off the coast of Japan could help seismologists to assess the risk of earthquakes.

Gordon Lister and his colleagues at the Australian National University in Canberra looked at a catalogue that lists thousands of earthquakes around the globe and describes the movement of geological faults during those quakes. The team focused on the floor of the Pacific Ocean east of Japan, where earthquakes seem to cluster in linear zones.

The scientists studied fault movements during the three months before and the nine months after the magnitude-9.1 Tohoku earthquake that occurred in March 2011. Well before that cataclysmic event, the seafloor slipped in one direction during small quakes. But in the days before the big quake, the direction of slippage reversed.

Detailed measurements of the seafloor’s movements could help to assess whether a large quake is about to strike, the authors say.