Researchers: 30% chance of magnitude-9 quake in 30 years

January 02, 2012

By SHIGEKO SEGAWA / Staff Writer

Chances are 30 percent that a magnitude-9 earthquake will strike along the Japan Trench between the Tohoku region and Chiba Prefecture within the next 30 years, a government research council said.

The Headquarters for Earthquake Research Promotion on Nov. 24 released revised probabilities for earthquakes occurring off eastern Japan between the Sanriku coast and the Boso Peninsula, taking magnitude 9-class earthquakes into account for the first time.

Following the failure to prepare for the March 11 Great East Japan Earthquake, which had a magnitude of 9.0, the headquarters started reviewing its methods to evaluate magnitudes and probabilities of earthquakes, and present those forecasts to the public.

The headquarters said an earthquake of the same type as the Great East Japan Earthquake, which ruptured entire stretches between the Sanriku coast and Ibaraki Prefecture, had a zero probability of recurring within 30 years because the latest disaster released so much energy.

But the research body said it was difficult to determine if all the built-up energy was released in all parts of that area near the Japan Trench, meaning that a quake of a different type could still occur there.

The region off the northern Sanriku coast was not included in the focal area of the March 11 disaster, while the areas off Fukushima Prefecture and further south were not among the core part of the catastrophic seismic rupture, the headquarters said.

Based on devastating tsunami following historical earthquakes--including the 1611 Sanriku-Oki, the 1677 Boso-Oki and the 1896 Meiji Sanriku-Oki events--the headquarters concluded that a magnitude-8.6 to 9.0 earthquake could hit that area.

Statistical analysis of the records of large earthquakes in the area led the research body to estimate that such an earthquake had a 30-percent chance of striking within 30 years.

The headquarters also said a magnitude-7.1 to 7.6 earthquake had a 90-percent chance of striking off the northern Sanriku coast within 30 years; a magnitude-7.0 to 7.3 event could occur off Miyagi Prefecture with a 60-percent probability within 30 years; a magnitude-7.4 earthquake could hit off Fukushima Prefecture at a 10-percent probability; and a magnitude-6.7 to 7.2 temblor has a 90-percent probability of occurring off Ibaraki Prefecture.

Both large crustal deformation and aftershocks have followed the Great East Japan Earthquake near its focal region. But the latest revision did not take aftershocks into account. When aftershocks are included, the probabilities of magnitude-7-class earthquakes may exceed the latest forecasts in the short term, officials said.

The latest forecasts, however, are only provisional evaluations based on data and methods that are currently available. The forecasts could be subject to another round of reviews and revisions.

Apart from the seas off northeastern Japan, the earthquake research headquarters is also reviewing probabilities of earthquakes along the Nankai Trough off the southern coast of Japan between the Tokai region and Shikoku. The research body may take possible magnitude-9-class earthquakes into consideration.

The current forecasts say that a Tokai earthquake has an 87-percent chance of occurring within 30 years.

By SHIGEKO SEGAWA / Staff Writer
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