Japan’s Aichi Prefecture Sees Seismic Intensity 5-Upper for First Time in 30 Years; At Least 24 Injured

【Financial News, Tokyo—Correspondent Seo Hye-jin】A magnitude-5.7 earthquake struck western Shizuoka Prefecture early on the 8th, and shaking of intensity 5-upper was observed in neighboring Aichi Prefecture for the first time in about 30 years. At least 24 people were injured in Aichi Prefecture, and rail services were disrupted. The Japan Meteorological Agency (JMA) judged that the quake was unlikely to have any particular effect on the plate boundary that could generate a Nankai Trough megathrust earthquake, but urged people to remain alert for similar-strength shaking over roughly the next week.
According to NHK and the Yomiuri Shimbun, an earthquake with its epicenter in western Shizuoka Prefecture occurred at around 1:04 a.m. that day. The focal depth was about 40 kilometers. Shaking of up to intensity 5-upper was observed in Chita City, Aichi Prefecture, while intensity 5-lower shaking was recorded in various parts of the Tokai region and in Nagano Prefecture.
Unlike magnitude, which indicates the energy released by an earthquake itself, seismic intensity refers to the strength of shaking in different areas. It was the first time since March 1997 that shaking of intensity 5-upper or higher had been observed in Aichi Prefecture.
As of reports at 9:17 a.m. that day, at least 24 people in Aichi Prefecture had suffered injuries of varying severity in incidents including falls. In Nishio City, a man in his 50s who was asleep was struck by a falling object and injured his face. In Toyota City, two residents also fell and injured their heads.
Damage to commercial facilities was also reported one after another. At a bookstore in Toyota City, books fell from display shelves and fire-resistant glass was shattered. An employee who arrived at the store, which had been operating without staff, about 40 minutes after the earthquake said they had closed it because the damage was worse than expected and did not know when it would reopen.
Safety inspections disrupted rail services from the first trains of the day. JR Central suspended service on some sections of conventional lines, including the Tokaido and Chuo lines.
In Iida City, Nagano Prefecture, a slope beside a road collapsed, covering the road with earth and debris. Fallen trees and rockfalls were also confirmed, and traffic in the surrounding area was restricted.
The earthquake occurred within the expected source region of a Nankai Trough megathrust earthquake. However, the JMA judged that because the quake was relatively small and had a different mechanism, it was unlikely to cause any particular change in the state of the plate boundary where a major earthquake is expected.
The difference lies in where the earthquakes occur. A Nankai Trough megathrust earthquake is expected to occur at the boundary between the landward plate and the Philippine Sea Plate, which subducts beneath it. By contrast, this quake is thought to have occurred within the Philippine Sea Plate, at a depth of about 40 kilometers.
Professor Takuya Nishimura of Kyoto University’s Disaster Prevention Research Institute explained that magnitude-5-class earthquakes also occurred in the area in 1983, 1997 and 2020, and that it would be difficult to directly link this quake to a Nankai Trough megathrust earthquake.
The JMA also said, “There is no need to be excessively afraid of Nankai Trough earthquakes.” However, it urged people to stay alert for earthquakes accompanied by shaking of around intensity 5-upper at most over roughly the next week. Professor Nishimura likewise emphasized that although intraplate earthquakes generally have fewer aftershocks, quakes of similar magnitude could occur for the time being, and that people should also watch out for rockfalls.
Meanwhile, an earthquake of magnitude 4.3 struck the Amakusa-Ashikita region of Kumamoto Prefecture at around 10:53 a.m. that day. Its depth was estimated at about 10 kilometers, and intensity 4 shaking was observed in Minami Ward of Kumamoto City and in Uto and Uki cities. The JMA said there was no tsunami threat from the quake.


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