Megaquake Heightened the Risk to Tokyo
by Richard Kerr, from Science Magazine
Every rupturing fault, large or small, passes its accumulated stress on to neighboring faults, so March's magnitude-9 Tohoku earthquake bequeathed a huge amount of stress to faults offshore and across central Japan. Researchers reported at the meeting that the redistributed stress activated distant, long-quiescent faults, the first time that has been recorded. Most disturbingly, the stress instantly raised the risk of a major quake on some faults, including one beneath greater Tokyo and its 30 million people.
Geophysicist Shinji Toda of Kyoto University in Japan and colleagues took advantage of Japan's dense network of seismometers to compare post-Tohoku seismic activity with calculations of how the megaquake changed the stress on various faults. They found an increase in small to moderate quakes inland across the island of Honshu starting at the time of the Tohoku quake. Many faults that normally rupture when squeezed by added stress went silent. At the same time, long-quiet faults began failing when the sudden added stress pulled them apart. And areas of heightened seismicity remain unusually active, they reported. Added stress from the megaquake seems to have triggered at least five of the seven large, unusually distant aftershocks.
Looking at particularly threatening faults, Toda and his colleagues calculated that the magnitude 9 quake drove several offshore and inland faults closer to the breaking point. And small quakes are still striking all those faults at elevated rates after Tohoku. There was already a chunk of tectonic plate clogging the works beneath Tokyo that sets off quakes like the magnitude-7.3 shock of 1855 (Science, 5 January 2007, p. 37). If the group's stress-transfer calculations hold for the activated Tokyo area, “the seismic hazard for Tokyo has now likely doubled,” Toda said. That would raise the chances of another quake in the series from 35% in the next 30 years to at least 50%. The group's best advice: further strengthen Tokyo's already quake-resistant structures and buy more earthquake insurance.