A house is seen collapsed in Oosaki town, Kagoshima prefecture, southern Japan Friday, August 9, 2024, following Thursday’s powerful earthquake.
| Photo Credit: AP

Japan’s earthquake scientists say the country should prepare for a possible “megaquake” one day that could kill hundreds of thousands of people – although they stress the warning does not mean a colossal tremor is imminent.

The Japan Meteorological Association (JMA) warning is the first issued under new rules drawn up after a 2011 earthquake, tsunami and nuclear disaster killed around 18,500 people.

What does the warning say?

The JMA’s “megaquake advisory” warns that “if a major earthquake were to occur in the future, strong shaking and large tsunamis would be generated. The likelihood of a new major earthquake is higher than normal, but this is not an indication that a major earthquake will definitely occur during a specific period of time,” it added.

The advisory concerns the Nankai Trough “subduction zone” between two tectonic plates in the Pacific Ocean, where massive earthquakes have hit in the past.

What is the Nankai Trough?

The 800 km undersea trough runs from Shizuoka, west of Tokyo, to the southern tip of Kyushu Island. It has been the site of destructive quakes of magnitude eight or nine every century or two. These so-called “megathrust quakes”, which often occur in pairs, have been known to unleash dangerous tsunamis along Japan’s southern coast.

In 1707, all segments of the Nankai Trough ruptured at once, unleashing an earthquake that remains the nation’s second-most powerful on record. That quake – which also triggered the last eruption of Mount Fuji – was followed by two powerful Nankai megathrusts in 1854, and then a pair in 1944 and 1946.

How much is at stake?

Japan’s government has previously said the next magnitude 8-9 megaquake along the Nankai Trough has a roughly 70% probability of striking within the next 30 years. In the worst-case scenario 300,000 lives could be lost, experts estimate, with some engineers saying the damage could reach $13 trillion with infrastructure wiped out.

“The history of great earthquakes at Nankai is convincingly scary,” geologists Kyle Bradley and Judith A. Hubbard wrote in their Earthquake Insights newsletter. And “while earthquake prediction is impossible, the occurrence of one earthquake usually does raise the likelihood of another”, they explained.

“A future great Nankai earthquake is surely the most long-anticipated earthquake in history – it is the original definition of the ‘Big One’.”

How worried should people be?

Japan is reminding people living in quake zones to take general precautions, from securing furniture to knowing the location of their nearest evacuation shelter. Many households in the country also keep a disaster kit handy with bottled water, long-life food, a torch, radio and other practical items.

But there’s no need to panic – there is only a “small probability” that Thursday’s (August 8, 2024) magnitude 7.1 earthquake is a foreshock, according to Bradley and Hubbard.

“One of the challenges is that even when the risk of a second earthquake is elevated, it is still always low. For instance, in California, the rule of thumb is that any given earthquake has around 5% chance of being a foreshock,“ they said.

People take shelter outside building following an earthquake in Miyazaki, western Japan on August 8, 2024. Photo: Kyodo News via AP

A powerful earthquake struck off Japan’s southern coast on August 8, triggering a tsunami advisory. Residents were urged to stay away from the coastline, but there were no immediate reports of injuries or serious damage.

The Japan Meteorological Agency said the quake registered magnitude 7.1 and was centered in waters off the eastern coast of Japan’s southern main island of Kyushu at a depth of about 30 kilometres (18.6 miles).

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The quake most strongly shook Nichinan city and nearby areas in Miyazaki prefecture on Kyushu island.

The agency said tsunami waves of up 50 centimetres (1.6 feet) were detected along parts of Kyushu’s southern coast and the nearby island of Shikoku about a half hour after the quake struck.

Seismologists were holding an emergency meeting to analyse whether the quake had affected the nearby Nankai Trough, the source of past devastating earthquakes.

Chief Cabinet Secretary Yoshimasa Hayashi said officials were assessing possible injuries or serious damage, though none were immediately reported. He urged residents of the affected region to stay away from the coastline.

Seismology Department official Shigeki Aoki warned that strong aftershocks could occur for about a week.

Japan’s NHK public television said windows were broken at the Miyazaki airport near the epicentre. The airport’s runway was temporarily closed for safety checks.

NHK showed dozens of people gathering at a designated hilltop evacuation area.

In Osaki in neighbouring Kagoshima prefecture, concrete walls collapsed and a wooden house was damaged, but no injuries were reported.

The Nuclear Regulation Authority said all 12 nuclear reactors, including three that are currently operating, on Kyushu and Shikoku remained safe.

Earthquakes in areas with nuclear power plants have been a major concern since a massive earthquake and tsunami in March 2011 triggered the Fukushima nuclear disaster.

Japan sits on the Pacific “Ring of Fire,” a line of seismic faults encircling the Pacific Ocean, and is one of the world’s most earthquake-prone countries.

An earthquake on Jan. 1 in Japan’s north-central region of Noto left more than 240 people dead.