Japan Issues First-Ever Nankai Trough Megaquake Alert

On August 8, at around 4:43 PM, a magnitude 7.1 earthquake was detected in Miyazaki Prefecture, southeastern Kyushu. In response, the Japan Meteorological Agency (JMA) issued an alert on the same day, warning about a possible large earthquake around the Nankai Trough. This alert, called the "Nankai Trough Earthquake Extra Information," was issued for the first time since the system began in May 2019.

At 5:30 PM that same day, the JMA convened to assess the risks of a Nankai Trough earthquake. The agency urged residents in areas likely to be affected to take appropriate safety measures based on their individual circumstances.

A Nankai Trough megaquake refers to a potentially massive earthquake that could occur along the Nankai Trough, where an underlying fault system has historically caused catastrophic earthquakes.

The Nankai Trough is a shallow ocean trench extending from the Tokai region to Kyushu along the Pacific coast. It is where the Philippine Sea Plate subducts beneath the continental plate on which the Japanese archipelago lies. The boundary between these two plates has repeatedly slipped, causing megaquakes of M8.0 or greater.

The Megaquake Alert System

In the "Nankai Trough Earthquake Extra Information" alert system, the JMA issues the first "extra information" alert when an earthquake of M6.8 or greater is detected along the Nankai Trough. This alert is also triggered when anomalies requiring evaluation are detected by crustal deformation monitoring equipment.

Following this initial alert, the JMA convenes a meeting of experts to assess the situation. Based on this evaluation, the JMA issues one of the following emergency information statuses.

• "Megathurst Earthquake Alert" for an earthquake of roughly M8.0 or greater.
• "Megathurst Earthquake Attention" for an earthquake of M7.0 or greater, or when an unusual "slow slip" (a gradual movement at the plate boundary that doesn't cause noticeable shaking) is detected.
• "Analysis Complete" if no further action is needed.

The M7.1 Miyazaki Earthquake

Regarding the Miyazaki earthquake, the JMA issued a "Megathurst Earthquake Attention" after the preliminary alert. This advisory urges the public to review their evacuation preparations and be ready to evacuate immediately if necessary, although it does not yet mandate evacuation. This system is particularly intended to help vulnerable populations, such as the elderly and disabled, evacuate to safer locations in advance.

According to the JMA, this earthquake's epicenter was in the Hyuga-nada region, an area of the Pacific Ocean off the eastern coast of Kyushu. The quake originated at a depth of approximately 30 kilometers.

Future Risks

The Japanese government predicts a 70–80% probability of a Nankai Trough megaquake occurring within the next 30 years, based on historical intervals. Estimates under a worst-case scenario suggest a megaquake could result in as many as 323,000 deaths and missing persons.

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Author: JAPAN Forward