Photo/Illutration A house that collapsed in an earthquake in Osaki, Kagoshima Prefecture, on Aug. 9, 2024 (Hayato Kaji)

The probability of a magnitude-8 or -9 megaquake occurring in the Nankai Trough within the next 30 years has risen to “around 80 percent,” according to the government’s Earthquake Research Committee.

It previously estimated there was a “70 to 80 percent” probability of a megaquake off Japan’s heavily populated Pacific coastline during this period.

The Nankai Trough stretches from Shizuoka Prefecture to off the coast of Kyushu.

Earthquake probability is calculated on the basis of the length of interval between past earthquakes and the number of years since the most recent earthquake, officials said.

This means the probability increases slightly each year if an expected megaquake has not occurred in the previous year.

Officials said the Jan. 15 recalculation was not due to abnormal phenomena in the possible hypocenter area of the Nankai Trough, but simply because of the automatic annual update.

“We concluded there wasn’t any data that increased the possibility of a large earthquake occurring compared to normal times,” said committee chairman Naoshi Hirata in reference to a temblor centered in the Hyuganada Sea off the coast of Miyazaki Prefecture in the Kyushu region on Jan. 13.

According to the committee, the probability of a Nankai Trough megaquake increases by less than one percent each year.

The probability of one happening, calculated under multiple scenarios, was 75 to 82 percent as of Jan. 1 this year. It was rounded off as usual and revised to about 80 percent this time around.

In 2018, the probability increased to “70 to 80 percent” from the previous “about 70 percent.”

The committee also discussed the status of seismic activity following the magnitude-7.6 earthquake that struck the Noto Peninsula on Jan. 1, 2024.

It said the outlook for future activity was difficult to determine as a series of earthquakes in the magnitude-6 class after the magnitude-7.6 earthquake “has never been observed before in Japan.”

(This article was written by Takahiro Takenouchi and Fumi Yada.)