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Japan issues megaquake advisory following 7.5 tremor
Officials warn residents across seven prefectures to prepare for a possible magnitude 8+ earthquake in the Nankai Trough region, though no evacuation orders have been issued.
What is the 'Big One'?
The term refers to a catastrophic earthquake expected to strike Japan's Pacific coast along the Nankai Trough, a seismically active zone. Historical records show such events occur roughly once every century, often in pairs, with the last major quakes hitting in 1944 and 1946.
Geologists Kyle Bradley and Judith A Hubbard describe the Nankai Trough as "the original definition of the 'Big One,'" noting its history of devastating tremors.
Potential impact and risks
In April, authorities projected a megaquake could trigger a tsunami exceeding 20 meters (66 feet), inundating parts of Tokyo and surrounding prefectures. Estimates suggest up to 300,000 deaths and trillions of dollars in economic damage.
The Japan Meteorological Agency (JMA) estimates a 60-90% chance of such an event within the next 30 years, though the likelihood of an imminent quake remains low-about one in 100.
Preparedness measures urged
Officials have advised residents in Hokkaido, Chiba, and five other prefectures to review evacuation routes, secure furniture, and stock emergency supplies, including food, water, and portable toilets. However, no mandatory evacuations have been ordered.
"A large-scale earthquake with a magnitude of 8 or higher could occur as a follow-up," a government official stated, emphasizing the need for vigilance.
Can megaquakes be predicted?
Experts remain divided. Robert Geller, professor emeritus of seismology at the University of Tokyo, criticized last year's advisory as "not a useful piece of information," arguing that earthquakes cannot be reliably forecast as foreshocks or aftershocks.
Bradley and Hubbard countered that while only 5% of quakes are foreshocks, the 2011 magnitude 9.0 disaster was preceded by a largely ignored 7.2 tremor. The JMA's warning system, introduced after 2011, aims to mitigate such risks but stops short of predicting specific events.
"The likelihood of a new major earthquake is higher than normal, but this is not an indication that a major earthquake will definitely occur," the JMA clarified in August 2024.
Japan's seismic vulnerability
Located on the Pacific Ring of Fire, Japan experiences roughly 1,500 earthquakes annually. While most cause minimal damage, the 2011 disaster-Japan's strongest recorded quake-killed over 18,000 people and triggered a catastrophic tsunami.
Historical records, including a 1707 quake that ruptured the entire 600-kilometer Nankai Trough and triggered Mount Fuji's eruption, underscore the region's seismic volatility.
