At least three seismic source areas may have been generating magnitude-9 class temblors about once a millennium off eastern Japan between Hokkaido and Ibaraki Prefecture, new research suggests.
One site is off the Shimokita Peninsula in Aomori Prefecture, a location that has drawn little attention so far.
Kazuomi Hirakawa, a research professor of geography at Hokkaido University, published his finding in the February issue of Kagaku (Science) magazine, put out Jan. 26 by Iwanami Shoten Publishers.
Until now, the government's Central Disaster Management Council had only considered a potential M9-class earthquake striking off southern Japan between Shizuoka Prefecture and Kyushu.
The Headquarters for Earthquake Research Promotion, another government-affiliated body, also anticipates a shock of a similar scale striking beneath the seabed off eastern Japan near the source of the March 11, 2011, Great East Japan Earthquake.
After that disaster, Hirakawa reviewed deposits of sand and other substances carried ashore by tsunami in the distant past.
One lesson of the March 11 disaster was that tsunami can swamp areas farther inland than conventionally thought.
Hirakawa compiled data from areas ranging from Hokkaido to Miyagi Prefecture and evaluated the locations of seismic source areas that can generate giant tsunami.
The source area off eastern Japan between the Rikuchu region and the Shimokita Peninsula, one of the three sites he hypothesized about, was estimated from traces of tsunami that traveled as far as the innermost part of Uchiura Bay in southwest Hokkaido about 3,000 years ago and another in the 12th or 13th century.
The time that has elapsed since the last event in the area could mean another disaster is imminent, Hirakawa said.
Another source area off eastern Japan between the Joban and Rikuchu regions corresponds to the source of the Great East Japan Earthquake and the 869 Jogan earthquake.
The other source area off Hokkaido between Cape Erimo and Nemuro contains the source of an M8-class earthquake expected to hit off southern Hokkaido. It may have been the origin of the Keicho Sanriku tsunami that devastated the Tohoku region in 1611, Hirakawa said.
"Aside from 'giant' tsunami that recur once a millennium, there are also 'great' tsunami that hit once every several centuries, so more studies are needed," Hirakawa said.
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