Giant tsunami may have repeatedly flooded the Sanriku coast of the Tohoku region about once a millennium, a possibility indicated by sand and stone deposits discovered by a Hokkaido University researcher.
Giant tsunami may have repeatedly flooded the Sanriku coast of the Tohoku region about once a millennium, a possibility indicated by sand and stone deposits discovered by a Hokkaido University researcher.
Following the Great East Japan Earthquake, the Central Disaster Management Council and other authorities have begun reviewing their emergency policies to prepare for the largest, scientifically possible earthquakes and tsunami.
The evidence of repeated giant tsunami striking the coast may help to determine how large their potential.
Kazuomi Hirakawa, a research professor of physical geography at Hokkaido University, discovered six layers of sand and stones, deposited by tsunami, on the face of a steep cliff, between 1 and 5 meters high, facing the seashore in Kesennuma, Miyagi Prefecture. He also discovered more than one similar geological formation in Miyako, Iwate Prefecture, near the point where the March 11 tsunami rose to a height of 32 meters.
This is the first time that more than one such geological formation was discovered on the faces of cliffs along the Sanriku coast. Only tsunami of enormous sizes could have left traces on the tops of such high cliffs. On the basis of volcanic ash and earthenware contained in those geological formations, Hirakawa determined that tsunami flooded the coast six times during the past 6,000 years.
Studies of deposits from the 869 "Jogan tsunami" in the Sendai and Ishinomaki plains have produced a magnitude estimate of 8.4 for the earthquake that triggered it. Hirakawa believes that part of the geological formations he discovered are from the Jogan earthquake. It may have been a magnitude-9 earthquake, he said, given that the tsunami had traveled as far north as the Sanriku coast.