The magnitude 9 earthquake that shook Japan in 2011 more than doubled the risk that a big quake will rattle Tokyo in the next five years, geologists report May 2 in Geophysical Research Letters.
The 2011 quake hit 373 kilometers northeast of Tokyo. Afterwards, seismic activity around Japan’s capital spiked, and small earthquakes are now three times as frequent as they were before the 2011 event. The ground movement comes from a piece of Earth’s crust that’s wedged between the Eurasian plate, on which Tokyo sits, and the Pacific plate, which slides into the mantle beneath the Eurasian plate.
Shinji Toda of Tohoku University in Japan and Ross Stein of the U.S. Geological Survey in Menlo Park, Calif., suggest that the 2011 quake transferred stress to the wedged fragment, triggering the increase in seismic activity. The pair calculates that the added stress raised the probability of a magnitude 7 or larger earthquake striking Tokyo in the next five years, from 6.6 percent to 17.2 percent.