On December 9, 2025, Japan activated its newly established large-scale offshore earthquake alert system following a significant magnitude 7.5 tremor that struck off Aomori Prefecture the previous night. The Japan Meteorological Agency (JMA) initiated the Hokkaido–Sanriku offshore earthquake alert, a category introduced in 2022, signaling a heightened potential for a subsequent, more devastating event along the convergence of the Japan and Kuril (Chishima) oceanic trenches. The warning prompted immediate calls for preparedness across 182 municipalities facing the Pacific, spanning seven prefectures including Hokkaido, Aomori, Iwate, Miyagi, Fukushima, Ibaraki, and Chiba.
Background and Context
The powerful quake, registered at approximately 11:15 p.m. on Sunday at a depth of 54 kilometers, produced intense long-period ground motions, specialized, slow-moving waves capable of intensely swaying tall structures. The town of Hachinohe reported an intensity reading of upper 6 on Japan’s 0-7 seismic scale. Following the event, authorities reported limited initial damage: 33 minor injuries across Hokkaido, Aomori, and Iwate, minor structural damage at seven schools in Aomori, and some localized water outages. Transportation impacts were minimal, with a brief suspension of the Tohoku Shinkansen line between Morioka and Shin-Aomori that was quickly resolved. Initial tsunami warnings were issued for portions of the northern coast, though these were swiftly downgraded to advisories and lifted after only small waves, up to 70 centimeters, were observed. Nuclear facilities across the affected regions reported no anomalies.
The subsequent alert demonstrates a shift in governmental strategy to address the catastrophic risk posed by the major trench systems. Citing lessons learned from the 2011 Great East Japan Earthquake, the Cabinet Office employed the latest scientific models to assess potential mega-quake and tsunami impacts. The Sunday quake’s location and intensity confirmed it fell within the parameters designated for triggering the alert under the Basic Plan to Promote Countermeasures for Earthquakes along the Japan and Kuril Trenches.
Understanding the Catastrophic Projection
The new alert is a critical early warning signal, not a specific prediction, indicating the statistical possibility of a magnitude-8-class or greater event in the coming week. The JMA stressed that the statistical probability of such a massive quake occurring within a seven-day window is approximately one percent.
However, government modeling released by domestic media underscores the extreme potential consequences of a future mega-quake scenario. Worst-case mortality projections suggest fatalities could reach approximately 300,000—comprising roughly 199,000 for a Japan Trench event and 100,000 for a Kuril Trench event. Tsunami waves could potentially reach heights of 30 meters.
Economically, the impact is estimated at nearly 48 trillion yen (approximately HK$2.4 trillion), reflecting both infrastructural damage and considerable output losses from suspended production and supply chain disruptions. Structural damage projections estimate roughly 220,000 buildings could be affected in a Japan Trench scenario. Officials highlighted that early evacuation and rigorous preparedness are the primary mitigants for significantly reducing these fatality and damage estimates.
Actionable Preparedness Measures
Chief Cabinet Secretary Kihara Minoru urged residents in affected areas to remain vigilant and rely solely on verified government and trusted media sources, advising strongly against misinformation.
Key Preparedness Actions:
- Review Evacuation Plans: Residents should immediately verify and confirm primary and secondary evacuation routes.
- Secure Homes: Heavy furniture must be secured to prevent injury during renewed shaking.
- Update Emergency Kits: Kits should be stocked with essential food, water, portable toilets, and necessary medication for at least three days.
- Stay Informed: Follow official updates from the JMA and the government task force established at the Prime Minister’s Office.
Experts, including Professor Naoya Sekiya of the University of Tokyo, emphasized that the alert should be viewed as a vital prompt to confirm household and community preparedness, not as a source of panic. This first activation of the new alert system marks a significant milestone in Japan’s proactive approach to mitigating the risk of future seismic catastrophes.