Warning of potential magnitude-9 mega quake in trench off Hokkaido

Researchers are warning that unreleased stress energy at an undersea trench off northern Japan has become large enough to possibly trigger a magnitude-9 mega quake, according to reports.

Researchers from Tohoku and Hokkaido universities and the Japan Agency for Marine-Earth Science and Technology released the results of their five-year study of the Chishima Trench off Hokkaido, NHK reported.

A massive quake is believed to have occurred near the zone in the 17th century. The government's earthquake panel has said the chances of a quake with a magnitude of at least 8.8 occurring there within 30 years are between 7 and 40 percent, the report added.

The team installed GPS observation stations on the seabed in 2019 around the point where an ocean plate sinks beneath a continental plate.

Stations on the ocean plate moved about eight centimeters per year toward the continental side. But a station on the continental plate where the two plates meet also moved about eight centimeters in the same direction.

Researchers, according to NHK, said this indicates that parts of the plates are firmly joined together and strain may be accumulating.

Assuming that the plates have been joined in a similar way since the 17th-century quake, the team says the built-up strain would be equivalent to the energy released by a mega quake.

The team plans to conduct a study at another location off Hokkaido, the report added.

Tomita Fumiaki, assistant professor of Tohoku University International Research Institute of Disaster Science, said memories of the 2011 Fukushima earthquake and tsunami have been fading.

"We need to think again about what we can do to prepare for another mega quake," he told NHK.