Possibility of Nankai Earthquake in Japan Remains... Experts Say "Energy Accumulated, Occurrence Is a Matter of Time"

NHK "Risk of Massive Earthquake Continues to Rise"
Experts Point Out "Effectively Entered Active Phase"
Magnitude 8-9 or Higher Expected
Japanese Government Conducts Survey for Countermeasures

On the 8th of last month, a magnitude 7.1 earthquake struck Miyazaki Prefecture, and it has been a month since the Japanese government issued a Nankai Trough megaquake advisory (temporary information). Although the temporary information has now been lifted, experts remain concerned about the possibility of a megaquake occurring and warn that it could happen in the near future. The Nankai Trough, an underwater trench located south of Honshu, Japan, is known to produce megaquakes of magnitude 8 to 9 or higher on a cycle of 100 to 150 years.

NHK recently reported that "experts have observed renewed risks of a major earthquake near the area where the Miyazaki Prefecture earthquake occurred," adding that "the risk of a megaquake is steadily increasing." The report also conveyed analyses suggesting that another magnitude 7 or greater earthquake could occur in Miyazaki Prefecture, with potential damage exceeding that of the previous quake.

Yusuke Yamashita, an assistant professor at Kyoto University Disaster Prevention Research Institute, pointed out, "The epicenter of this earthquake is a location where magnitude 7 earthquakes have repeatedly occurred in the past." He explained, "While some crustal deformation was relieved by this earthquake, there remain areas unaffected where deformation has not been released," adding, "It is highly likely that energy accumulation continues in those remaining areas." He further noted, "If a megaquake occurs again at this location, the epicenter will be shallower than this time, which could result in a larger tsunami."

On the 8th of last month, the police are controlling access to buildings damaged by the earthquake that occurred in Miyazaki Prefecture. (Photo by Yonhap News)

Another expert suggested that future earthquakes in Miyazaki Prefecture could accelerate the timing of a Nankai Trough megaquake. Takane Hori, director of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and a member of the government’s Earthquake Research Committee, stated, "Simulations show that if a magnitude 7.5 earthquake occurs in Hyuganada (off the coast of Miyazaki Prefecture) like this recent quake, a megaquake will occur in the Nankai Trough several years later." This indicates the possibility of advancing the timing of a Nankai Trough megaquake. However, he added that the number of earthquakes currently occurring in Hyuganada is decreasing, so this recent quake appears to have had no impact.

There are also opinions that the Nankai Trough megaquake is inevitable and only a matter of time. Takuya Nishimura, a professor at Kyoto University Disaster Prevention Research Institute, said, "Based on satellite observation data analyzing crustal movements, the area predicted to experience a Nankai Trough earthquake continues to undergo deformation of about 4 cm per year." According to Professor Nishimura, energy sufficient to trigger a magnitude 8 or greater earthquake is already accumulating in this area.

Location of Nankai Trough and Expected Earthquake Occurrence Areas. The places marked with lines indicate the location of the Nankai Trough, and the colored areas represent potential epicenter locations. (Photo by NHK)

He emphasized, "The impact of the recent Miyazaki Prefecture earthquake is diminishing, but when considering the entire Nankai Trough, preparations for the next earthquake are underway," adding, "The fact that a megaquake will occur in the near future remains unchanged." He urged, "Megaquakes are more likely to occur during periods when no advisory is issued, so continuous preparedness is necessary."

Director Hori also warned, "Analyzing the timing of past Nankai Trough megaquakes shows that inland earthquakes frequently occurred starting about 50 years before the megaquake." He pointed out, "Recent large inland earthquakes in western Japan, such as the 1995 Hanshin-Awaji earthquake, the 2000 Tottori Prefecture earthquake, the 2016 Kumamoto earthquake, and the 2018 northern Osaka earthquake, indicate that the region is effectively entering an active phase."

The Nankai Trough is an underwater trench located about 4,000 meters deep between Tsuruga Bay in Shizuoka Prefecture and the eastern Pacific coast of Kyushu, where the Eurasian Plate and the Philippine Sea Plate meet. The Japanese government’s Earthquake Research Committee estimates a 70 to 80% probability of a magnitude 8 to 9 earthquake occurring there within the next 30 years, with damage expected to exceed that of the Great East Japan Earthquake.

Meanwhile, the Cabinet Office plans to conduct a survey this month targeting local governments and businesses in sectors such as transportation, tourism, and retail to prepare for a Nankai Trough megaquake. Alongside checking disaster prevention planning status, expert advisory meetings will be held to review methods of communicating earthquake information.