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Strain accumulating at the boundary between tectonic plates under the Hyuganada Sea off Miyazaki Prefecture has caused earthquakes of magnitude 7 or greater five times since 1931, before the latest one on Thursday.
The Hyuganada Sea, where Thursday’s earthquake occurred, is on the western edge of the likely epicenter of a possible Nankai Trough earthquake.
The Nankai Trough lies beneath the Pacific Ocean and runs parallel to the Japanese coast from Shizuoka Prefecture, in the Tokai region, to Kyushu.
The Hyuganada Sea is an area where the Philippine Sea Plate (on the ocean side) subducts beneath the plate on the land side, leading to the accumulation of stress underground. This has caused earthquakes of Thursday’s scale to occur numerous times in the past. The Japan Meteorological Agency announced that Thursday’s earthquake had a magnitude of 7.1 and was caused by a shift in the boundary between the two plates.
It was smaller than the massive magnitude 8 quake that has been predicted for the area.
Since 1931, there have been five earthquakes of magnitude 7 or greater in the Hyuganada Sea, the last of which occurred in 1984, according to the agency.
The government’s Headquarters for Earthquake Research Promotion has determined that earthquakes of about magnitude 7 to 7.5 occur on average every 20 years.
In 2022, it estimated the probability of an earthquake of this scale occurring within the next 10 years to be about 40% and within 30 years to be about 80%.
Prof. Masanao Shinohara of the University of Tokyo, who specializes in undersea seismology, said plate boundary earthquakes like Thursday’s “can affect the surrounding boundary surfaces if the boundary surface shifts.”
