Caught off guard by the enormous strength of the Great East Japan Earthquake, the Japan Meteorological Agency will revise the way it issues tsunami warnings for quakes with magnitudes that could exceed 8.
Caught off guard by the enormous strength of the Great East Japan Earthquake, the Japan Meteorological Agency will revise the way it issues tsunami warnings for quakes with magnitudes that could exceed 8.
The warnings will be based on the maximum magnitudes expected for corresponding focal regions beneath the sea. In other words, the agency will assume a worst-case scenario.
The tsunami warnings will be scaled down to appropriate levels after more accurate information on the size of the earthquake is available, the agency said.
Three minutes after the Great East Japan Earthquake struck on March 11, the Meteorological Agency estimated the magnitude at 7.9 and issued "major tsunami warnings."
Initial projections for tsunami heights were 6 meters along the coast of Miyagi Prefecture and 3 meters along the coast of Iwate and Fukushima prefectures.
However, 44 minutes later, after observation data became available, the projections were raised to more than 10 meters for all three prefectures.
The early projections were blamed for delays in evacuations.
A Cabinet Office survey found that 63 percent of evacuees in Iwate Prefecture and 74 percent of those in Miyagi Prefecture did not know that the tsunami height projections were revised upward.
Earthquake magnitudes, released about three minutes after a temblor, are calculated on the basis of maximum amplitudes of seismic records at different locations.
But the promptness of releasing these magnitudes runs the risk of underestimating quakes with magnitudes exceeding 8.
In fact, the Meteorological Agency's initial report estimated the magnitude of the Great East Japan Earthquake at 7.9.
That figure represented only about one-45th of the actual energy released in the earthquake. The correct magnitude, 9.0, was finalized two days later.
Under the new system, for example, an earthquake beneath waters south of Wakayama Prefecture with an initial estimated magnitude of 8.0 would prompt tsunami warnings assuming a magnitude-8.7 quake had occurred. That's the maximum magnitude envisaged by the government's Earthquake Research Committee for three expected simultaneous earthquakes in the Tokai, Tonankai and Nankai focal regions.
Under this scenario, the agency would issue "major tsunami warnings" along the Pacific coast from the Kanto region to the islands of Okinawa and Amami, the agency said.
The magnitude is also expected to exceed 8 for simultaneous earthquakes off Nemuro and off Tokachi, both in Hokkaido.
A study meeting of outside experts on July 27 at the agency failed to agree on whether tsunami height projections should be released as part of the initial warnings. Some of the experts said it would be more effective just to send a message that people should run to higher ground.