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

TOKYO– Japan issued a megaquake warning on Tuesday after a 7.5-magnitude earthquake struck the eastern coast of Aomori, the northernmost prefecture of Japan’s main island of Honshu and just south of the northern island of Hokkaido. The damage caused by this earthquake was modest: 34 mostly minor injuries and some damage to roads and buildings.

Officials said the advisory was not a prediction and that the chance of an earthquake of magnitude 8 or greater was only about 1 percent. But we hope that this notice will serve as a wake-up call in the face of an earthquake which could have consequences as devastating as the 2011 disaster which killed nearly 20,000 people and destroyed a nuclear power plant.

There would be an increased risk of a subsequent earthquake, magnitude 8 or greater, over the next week. Authorities are urging residents, especially along coastal areas, to be well prepared so they can grab an emergency bag and run as soon as possible in the event of a larger earthquake.

This review seemed sound compared to another review from last year. The southern half of Japan’s Pacific coast received a “Nankai Trough” mega-earthquake warning in the summer of 2024, but the ambiguity of this warning led to panic buying of emergency food, event cancellations and business closures.

The Japan Meteorological Agency says Monday’s powerful earthquake temporarily increased potential risks in the Hokkaido and Sanriku Coast regions. This is where the Pacific Plate beneath Japan forms the two trenches – the Japan Trench and the Chishima Trench – which have caused many large earthquakes in the past.

Experts say the deadly 2011 earthquake and tsunami were caused by movements linked to the Japan Trench. It extends from the east coast of Chiba to Aomori, and the Chishima Trench extends from the east coast of Hokkaido to the northern islands and the Kuril Islands.

In explaining the advisory, the JMA said the 9.0 magnitude earthquake of March 11, 2011, which devastated large areas of Japan’s northern coast, occurred two days after a 7.3 magnitude earthquake that occurred in the Japan Trench off the east coast of Iwate, one of the areas hardest hit by that disaster as well as Monday’s quake.

The 2011 earthquake caused a tsunami that hit northern coastal towns in Iwate, Miyagi and Fukushima prefectures. The tsunami, which reached 15 meters (50 feet) in some areas, hit and destroyed the Fukushima Daiichi nuclear power plant. This created deep radiation fears that still persist today.

Another mega-earthquake in the Hokkaido-Sanriku region could cause a tsunami of up to 30 meters (98 feet) in the region, kill up to 199,000 people, destroy up to 220,000 homes and buildings and cause economic damage estimated at 31 trillion yen ($198 billion), according to a government estimate. According to the report, up to 42,000 people could suffer from hypothermia in winter.

The areas covered by the notice span 182 municipalities, from Hokkaido to Chiba Prefecture.

Japan’s separate advisory for an even more devastating megaquake originating from the Nankai Basin, which affects the southern half of Japan’s Pacific coast, was first activated last August after a magnitude 7.1 quake struck off the eastern coast of Miyazaki.

In a 2013 damage estimate for a possible mega-earthquake in the Nankai Trench, the government said a 9.1 magnitude earthquake could generate a tsunami of more than 10 meters (33 feet) within minutes, killing up to 323,000 people, destroying more than 2 million buildings and causing economic damage exceeding 200 trillion yen ($1.28 trillion) in the region.

Officials stress that the latest advisory does not include any prediction of a megaquake at any specific time or location, Tsukasa Morikubo, cabinet official for disaster prevention, said at a news conference Tuesday morning. He called on residents to be careful and prepared while continuing their daily activities and work.

Officials are urging people to keep an emergency bag with a few days’ worth of essentials, as well as shoes and helmets. Residents in the area are also advised to discuss evacuation procedures with their family members and to sleep in day clothes, not pajamas, so that they can escape immediately. Furniture must also be attached to the floor or wall.

Designated municipalities explained the warning on their websites and began inspecting stocks of relief goods and equipment for use in evacuation centers.

The town of Iwaki in Fukushima urged residents to sign up to receive emergency emails, while authorities in the town of Oarai in Ibaraki Prefecture, northeast of Tokyo, inspected wireless communications devices.

The first mega-quake warning issued by Japan in August last year contained a lot of scientific jargon. This has worried and confused many people across the country. Some cities have closed beaches and canceled annual events, disappointing many travelers during Japan’s Buddhist holidays.

Many people postponed planned trips and rushed to stock up on rice, dried noodles, bottled water and portable toilets, leaving shelves empty in many supermarkets in western Japan and even in Tokyo, which is outside the risk zone.