What does a ‘subsequent earthquake advisory’ mean?ーNHK WORLD-JAPAN NEWS

Key Concepts

• Mega Quake Advisory: A formal warning system introduced by the Japan Meteorological Agency (JMA) in 2022 to alert the public to an increased probability of a large-scale earthquake.
• Seismic Intensity Scale: The Japanese scale (0–7) used to measure the strength of ground shaking at specific locations.
• Trench Seismology: The study of seismic activity along the Japan Trench and Kuril Trench, where magnitude 7–9 earthquakes frequently occur.
• Disaster Preparedness: Proactive measures including evacuation planning, securing furniture, and maintaining emergency stockpiles.

1. Termination of the Mega Quake Advisory

The Japan Meteorological Agency (JMA) officially lifted the advisory for a potential mega quake in northeastern Japan as of 5:00 p.m. on Monday. This advisory was originally triggered by a magnitude 7.7 earthquake that occurred off the coast of the Sanriku region on April 20th.

While the specific period for "special caution" has concluded, officials from the Cabinet Office emphasized that the risk of a large-scale earthquake has not been eliminated. A major seismic event can still occur suddenly without any foreshocks or preceding tremors.

2. Historical Context and Statistical Frequency

• Advisory Frequency: The JMA introduced the "subsequent earthquake advisory" system in 2022. This recent event marks only the second time the advisory has been issued, the first being in December of the previous year following a quake off the coast of Aomori Prefecture.
• Future Projections: Data suggests that such advisories may be issued approximately once every two years. The JMA maintains a policy to issue a new week-long advisory if another earthquake of magnitude 7.0 or greater occurs in the region.
• Geological Risk: The Japan Trench and the Kuril Trench are identified as high-risk zones where magnitude 7 to 9 earthquakes have historically occurred repeatedly.

3. Case Study: The Importance of Sustained Vigilance

Correspondent Yako Kento highlighted the 2025 Kamchatka Peninsula earthquake as a critical case study:

• Sequence of Events: On July 20, 2025, a magnitude 7.5 earthquake struck the region.
• Escalation: Ten days later, a significantly larger magnitude 8.8 earthquake occurred, triggering widespread tsunami warnings across the Pacific.
• Key Takeaway: While the probability of a major earthquake may decrease over time following an initial tremor, the risk remains constant. Furthermore, many magnitude 8+ earthquakes occur without any prior seismic activity, underscoring the necessity of perpetual readiness.

4. Disaster Preparedness Framework

Even though the advisory has ended, the JMA and Cabinet Office advise residents to maintain normal social and economic activities while adhering to a strict preparedness checklist:

• Evacuation Planning: Identify and verify evacuation routes and the locations of designated shelters.
• Home Safety: Secure heavy furniture to prevent injury during shaking.
• Emergency Supplies: Regularly check and replenish stockpiles of essential items, specifically food and water.
• Situational Awareness: During the spring holiday season, travelers are urged to research the specific disaster risks and evacuation protocols for the areas they are visiting.

5. Synthesis and Conclusion

The lifting of the advisory does not signal a return to safety, but rather a transition from a state of "special caution" to "daily preparedness." The JMA’s advisory system serves as both a targeted warning for high-risk regions and a broader reminder for the entire nation to remain vigilant. The core message is that because the timing and location of mega quakes are unpredictable, proactive planning—such as knowing evacuation routes and maintaining emergency supplies—is the only effective strategy for mitigating the impact of a potential disaster.