OSAKA – Researchers have found sediment of tsunami that hit what is now Shizuoka Prefecture at the same time as the Hakuho earthquake of 684, the oldest recorded major temblor to hit the Nankai region on the Pacific coast of western Japan.
The new findings in the Tokai region by the National Institute of Advanced Industrial Science and Technology raise the possibility that three temblors occurred simultaneously at that time in wide areas in the Tokai, Tonankai and Nankai regions, which are considered to be at particularly high risk of major seismic activity.
Evidence of tsunami sedimentation at the time of the quake in the late seventh century had so far been found in the Tonankai and Nankai regions.
Until now, the Hoei quake of 1707 had been considered the oldest such triple disaster.
The research team, headed by Osamu Fujiwara at the institution, found four layers of sandy sediment in two locations, about 2.5 km and 3 km each from the mouth of the Ota River in Iwata, Shizuoka Prefecture.
It concluded that the four layers were created by tsunami instead of river flooding because they contained the fossils of seashells and minerals found in the sea.
The team said Monday it believes one of the layers accumulated at the time of the Hakuho quake and the three others were created as a result of major quakes in 887, 1096 and 1498.
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