Why Japan issued an advisory for a possible megaquake in the country’s north

Japan issued a megaquake advisory Tuesday after a magnitude 7.5 earthquake struck off the eastern coast of Aomori, the northernmost prefecture of Japan>s main island of Honshu, and just south of the northern island of Hokkaido.

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Damage from this quake was modest — 34 mostly mild injuries and some damage to roads and buildings.Officials said the advisory is not a prediction and the probability of amagnitude 8 or larger quake is only about 1%. But there’s hope the advisory will serve as a wake-up call for a quake that could have the devastation of the 2011 disaster that killed nearly 20,000 people and destroyed a nuclear plant.

There’s said to be an increased risk of asubsequent, magnitude-8 or larger quake within the next week. Officials are urging residents, especially along coastal areas, to be well prepared so they can grab an emergency bag and run as soon as possible if a bigger quake hits.

This advisory seemed mindful compared with another advisory last year. The southern half of Japan’s Pacific coastline received a “Nankai Trough” megaquake advisory in the summer of 2024, but the ambiguity of that warning led to panic buying of emergency food, event cancellations and business closures.

Amegaquake advisory for Japan’s northeastern coast

The Japan Meteorological Agency says Monday’s powerful quake temporarily increased potential risks in the regions of Hokkaido and the Sanriku coast. That’s where the Pacific Plate beneath Japan forms the two trenches — the Japan Trench and Chishima Trench — that have caused many large quakes in the past.