Cabinet Office announcement, significantly lower
Tsunami deposits (center layer) found during a survey in Tomakomai City, Hokkaido (provided by Yasuhiro Takashimizu Laboratory at Niigata University)
Regarding the huge tsunami that is said to have hit Hokkaido in the early 17th century during the early Edo period, a research team led by Associate Professor Yasuhiro Takashimizu (geology) of Niigata University will remain in the Yufutsu Plain (Tomakomai City) in southern Hokkaido by the 18th. The distribution range of the traces was clarified. The tsunami deposits underground were only about 2 kilometers inland from the current coastline, which was far below the inundation area predicted by the Cabinet Office. Although there are few historical records of the tsunami and many unknowns, such as the year and cause of the tsunami, Takashimizu points out, “The Cabinet Office’s assumptions may be overstated.”
In April 2020, the Cabinet Office’s Council of Experts announced that Tomakomai City was expected to be flooded up to about 10 kilometers from the coast, or more than 100 square kilometers.
The survey was conducted from 2019 to 2022. They pulled out strata at a depth of about 1.5 to 2 meters from a total of 86 locations and examined sediments from the past 3,000 years. Marine phytoplankton brought from the sea by the tsunami was discovered in the strata dating back to the first half of the 17th century. When we tracked its distribution using computerized tomography (CT), we found that it could no longer be seen beyond about 2 kilometers from the coast.