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The lifting of the Nankai Trough megaquake advisory came after a tense week, which raises fears even greater confusion could ensue in the event a higher-level “alert” becomes necessary.

On Aug. 8, the Japan Meteorological Agency issued the first-ever megaquake advisory after its panel of experts calculated that an earthquake earlier in the day off southern Kyushu had a magnitude of 7.0.

The temblor’s epicenter was in the Hyuganada Sea, which lies in the westernmost part of the estimated hypocentral region of a long-anticipated megaquake along the Nankai Trough, an ocean-floor trench that runs along the Pacific coast from the Tokai to Kyushu regions.

The plate boundary has a history of huge earthquakes occurring in pairs.

While a megaquake advisory, which is officially called “megathrust earthquake attention,” is issued if the magnitude ranges from 7.0 to under 8.0, the JMA issues a higher-level megaquake alert if the magnitude of a Nankai Trough earthquake is 8.0 or greater.

The JMA expert panel issues a megaquake alert or advisory, or concludes its analysis, based on the Nankai Trough Earthquake Extra Information protocol after studying a powerful earthquake and other events in the estimated hypocentral region.

The megaquake advisory was lifted on Aug. 15 after no significant changes were observed in crustal movements for seven days.

It urged residents in 707 pre-designated municipalities in Tokyo and 28 prefectures from Ibaraki to Okinawa to check their quake preparedness and be ready to evacuate immediately.

A megaquake alert, on top of those measures, would call on residents of coastal areas in 139 municipalities to evacuate for a week because of the risk they might not be able to flee in time before a tsunami generated by a subsequent giant quake arrives.

The 139 municipalities are in Tokyo and 13 prefectures from Chiba to Kagoshima in Kyushu.

If a week passes safely after the alert is issued, residents would be asked to take the same precautions under a megaquake advisory for another seven days.

The government’s evacuation request is not mandatory.

It is also left to municipalities to decide areas where evacuations will be recommended and whether to ask residents to flee in advance when an alert is issued.

The Cabinet Office does not know how many people would be asked to evacuate in advance if a megaquake alert is issued.

One reason is that residents would not have to evacuate if reliable safety precautions, such as coastal tsunami towers, are in place.

In addition, 54.6 percent of people surveyed by the Cabinet Office in 2023 said they do not know whether the areas in which they live are covered by a weeklong evacuation request under a megaquake alert.

When the megaquake advisory was issued last week, Central Japan Railway Co. decided to run bullet trains at a slower speed on the Tokaido Shinkansen Line between Mishima Station in Shizuoka Prefecture and Mikawa-Anjo Station in Aichi Prefecture.

In principle, transportation companies will also decide on their own how to respond to a megaquake alert.

There are no standards on actions that utilities are expected to take in the event of a megaquake alert.

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