Signs of 6 massive tsunami over past 6,000 years found in disaster-hit city
Kazuomi Hirakawa provides an explanation on a section of terrain believed to show traces of six massive tsunami, on Aug. 20 in Kesennuma, Miyagi Prefecture. It is believed that the tsunami carried layers of stones and sand from the sea into the area. (Mainichi)
SENDAI -- Evidence of at least six massive tsunami having occurred over the past 6,000 years has been found on the coast of Kesennuma, Miyagi Prefecture -- an area pounded by the devastating March 11 tsunami this year.
The discovery was made by a team of researchers including Kazuomi Hirakawa, a specially appointed professor at Hokkaido University. It is believed that the finding will shed light on tracking the frequency of major tsunamis along Japan's Sanriku Coast.
The team examined a three-meter-high piece of exposed land in a coastal area of the city, and between the peat layers formed by rotted plants, they found six layers in which sand and stones from the shore had apparently been carried into the area by tsunami. The site was discovered while researchers were examining the effects of the latest tsunami.
Hirakawa said that above the lowest layer of rock and sand, was a layer of volcanic ash thought to have been left by an eruption of the Towada volcano some 5,400 years ago. Judging by the thickness of the layers of peat and historical records, Hirakawa concluded that three of the tsunami were caused by the Keicho Sanriku offshore earthquake of 1611, the Jogan Earthquake of 869, and another quake that occurred about 2,000 years ago. The researchers plan to apply data from each layer in estimating the period in which tsunami occurred.
The exposed area examined by the researchers is estimated to have been pushed back nearly 500 meters by tsunami over the course of the past 6,000 years. Many tsunami have struck the Sanriku coast, but it appeared that only very large tsunami that surged over the exposed piece of land created layers in the soil.
"It is possible that each of the quakes triggering these tsunami was a major temblor in the range of magnitude 9," Hirakawa said. "We can conclude that to search for the traces of massive tsunami, it's important to examine high places like this. We should examine coastlines around the nation."