Increased Probability of Megaquakes in the Nankai Trough

A recent assessment by the Earthquake Research Committee of the Japanese government has determined that the likelihood of a significant earthquake, rated between magnitude 8 and 9, occurring within the Nankai Trough area in the next three decades has risen to approximately 80 percent. This figure marks an increase from the previous estimate of 70 to 80 percent and is a result of the annual recalibration of earthquake probabilities based on historical data concerning previous seismic events.

The Nankai Trough, which runs from Shizuoka Prefecture to the waters off Kyushu, is closely monitored due to its seismic activity. Government officials explain that the probability calculations stem from the intervals between past earthquakes and the duration since the last significant event, leading to a gradual annual increase in estimates. The committee clarified that their updated probability on January 15 was not triggered by any unusual seismic phenomena, but rather a standard update routine, emphasizing that there was no substantial new data indicating an increased risk for major earthquakes outside of normal expectations.

Notably, while the probability of a megaquake happening is on the rise, this increment is typically less than one percent per year, based on various forecasting scenarios. As of January 1, the estimated risk ranged from 75 to 82 percent, which was rounded to the now updated figure of about 80 percent. Furthermore, the committee has been analyzing the seismic activity following a significant 7.6 magnitude earthquake that hit Japan's Noto Peninsula on January 1, 2024, noting that the subsequent series of magnitude-6 quakes are unprecedented in Japan’s seismic history, making future activity forecasts particularly challenging.