A previously unknown trace of a tsunami about 1000 years ago was found at Kujukuri Beach in Chiba Prefecture, and as a result of analysis, it seems that a huge earthquake with an epicenter off the coast of the Boso Peninsula had occurred, such as the National Institute of Advanced Industrial Science and Technology. I found out in the investigation.
An expert in the survey pointed out that "it became clear that huge earthquakes and tsunamis were repeated off the coast of the Boso Peninsula, and we would like you to prepare for the tsunami again in coastal areas."
A research group led by Senior Researcher Yuki Sawai of the National Institute of Advanced Industrial Science and Technology investigated traces of past tsunamis left in the strata near the Kujukuri coast from Sosa City, Sammu City, and Ichinomiya Town in Chiba Prefecture.
As a result, two layers of sea sand and fossils of organisms, which are thought to have been carried by the tsunami in the past, were found in the stratum about 2 km inland from the coast. It was estimated to be.
When simulating multiple cases based on the position of the coastline and the height of the dunes at that time, it was found that a huge earthquake with a magnitude of about 8.5 had occurred with the epicenter off the eastern coast of the Boso Peninsula. I did.
It is believed that the tsunami had reached inland about a kilometer from the coast at that time.
In this area, the land side plate, the Pacific plate on the sea side, and the Philippine Sea plate overlap, and large-scale earthquakes and accompanying tsunamis have occurred in the past.
From records such as ancient documents, there are two known large earthquakes of magnitude 8 class, the 1677 "Enhoboso-oki Earthquake" and the 1703 "Genroku Earthquake". The tsunami was not clear.
Senior Researcher Sawai said, "It has become clear that huge earthquakes and tsunamis have repeatedly occurred off the coast of Boso. There is a possibility that another huge earthquake will occur in the future, and we will prepare for earthquakes and tsunamis in coastal areas. I want you to do it. "
The research results will be published in "Nature Geoscience" announced on the 3rd.