Japan's Earthquake Off the (Seismic Risk) Map
TOKYO—The most surprising thing about the magnitude-8.9 earthquake that hit Japan today is that it was a surprise. Despite what may be the world's most intensive effort to map faults and assess risks by a notoriously earthquake-prone and earthquake-conscious nation, such a strong quake was not anticipated for the region, says University of Tokyo geophysicist Robert Geller.
The earthquake occurred 130 kilometers east of Sendai and 373 kilometers northeast of Tokyo, along or very near the boundary between two tectonic plates, where the Pacific plate is being drawn under the Japanese islands. Movement along plate boundaries is known to be capable of producing major earthquakes. And Japan's latest national seismic risk map gave a 99% chance of a magnitude-7.5 or greater quake occurring in that area in the next 30 years, Geller says.
Although today's quake technically satisfies that prediction, the logarithmic scale used for measuring the power of earthquakes means that a magnitude-8.9 earthquake releases well over 100 times more energy than does a magnitude-7.5 quake. "I don't think those hazard assessments are meaningful," Geller says.
Geller believes the quake is the strongest to hit Japan since the start of reliable observations over a century ago. It is also more than 1000 times the force of the magnitude-6.3 quake that struck Christchurch, New Zealand, on 22 February.