Ancient tsunami traces up to 15 m above sea level

September 12, 2015

Trace of higher-than-estimated tsunami found near nuclear plant

http://mainichi.jp/english/english/newsselect/news/20150912p2g00m0dm026000c.html

TOKYO (Kyodo) -- A Tohoku University research team said Friday it has found what are believed to be traces of tsunamis that occurred up to 2,000 years ago near Tohoku Electric Power Co.'s nuclear plant in northeastern Japan.

Two traces have been found around 10 to 15 meters above sea level beneath a sand hill located about 10 kilometers north of the Higashidori nuclear complex in Aomori Prefecture, the team said. That suggests the area may have been hit by tsunamis possibly higher than the utility's estimate of 11.7 meters used for designing a new breakwater near the plant.

Tohoku Electric, which seeks to reactivate the nuclear plant, is constructing a breakwater as high as 16 meters above sea level.

The latest findings may force the utility to review its estimate and safety measures against tsunami hazards at the Higashidori nuclear plant.

According to Daisuke Sugawara, assistant professor of geology at the university, the team found strata -- consisting of accumulating mud and sand brought by strong tides -- in the soil underneath a sand hill. One believed to have been formed 1,500 to 2,000 years ago was found at a height of around 10 meters above sea level, and another believed formed 500 to 800 years ago was found about 15 meters above sea level.

The team said it is unclear whether the traces are related to huge past tsunamis that are already known, and it is difficult to make a direct comparison of their finding with Tohoku Electric's tsunami estimate that has been calculated based on different conditions.

The team needs to conduct further research on when the tsunamis occurred and how they affected the formation of the sand hill, said Sugawara.

The findings were announced at a meeting of the Geological Society of Japan held in Nagano Prefecture, central Japan.

Last March, a panel under the Nuclear Regulation Authority compiled a report saying at least two key geological faults running under the premises of the Higashidori plant may be active, clouding the prospect of the facility's restart.