Japan’s seismologists say the country must prepare for a possible “megaquake” one day that could kill millions – though they stressed the warning does not mean a massive quake is imminent.
The warning by the Japan Meteorological Association (JMA) is the first issued under new rules drawn up after the 2011 earthquake, tsunami and nuclear disaster that killed nearly 18,500 people.
What does the warning say?
The JMA’s “Great Earthquake Advisory” warned that “if a major earthquake occurs in the future, strong shaking and a large tsunami will occur”.
“The probability of a new major earthquake occurring is higher than normal, but this is not an indication that a major earthquake will certainly occur during a specific time period,” it said.
The advisory relates to the Nankai Trough “subduction zone” between two tectonic plates in the Pacific Ocean, where major earthquakes have occurred in the past.
What is the Nankai Trough?
The 800 kilometer (500 mile) long undersea trench stretches from Shizuoka, west of Tokyo, to the southern tip of Kyushu Island.
It has been the site of devastating earthquakes of magnitude eight or nine every century or two.
These so-called “megathrust earthquakes”, which often occur in pairs, have been known to trigger dangerous tsunamis along Japan’s southern coast.
In 1707, all segments of the Nankai Trough ruptured simultaneously, causing an earthquake that is the second most powerful ever recorded in the country.
Following that earthquake – which also triggered the last eruption of Mount Fuji – two powerful Nankai megathrusts occurred in 1854, followed by two more in 1944 and 1946.
How much is at stake?
The Japanese government previously said there was about a 70 percent chance the next major earthquake of magnitude 8-9 in the Nankai Trough would occur within the next 30 years.
Experts estimate that in the worst-case scenario, up to 300,000 people could die, and some engineers say damages could reach $13 trillion and infrastructure could be destroyed.
“The history of major earthquakes in Nankai is utterly terrifying,” geologists Kyle Bradley and Judith A. Hubbard wrote in their Earthquake Insights newsletter.
“Although it is impossible to predict earthquakes, the occurrence of one earthquake usually increases the likelihood of another one,” he said.
“The future Great Nankai Earthquake will definitely be the most long-awaited earthquake in history – this is the original definition of a ‘major earthquake’.”
How worried should people be?
Japan is reminding people living in quake-hit areas 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 containing bottled water, long-lasting food, a flashlight, radio, and other practical items.
But there’s no need to panic — according to Bradley and Hubbard, there’s only a “small chance” that Thursday’s magnitude 7.1 quake was a foreshock.
“One of the challenges is that even though the risk of another earthquake has increased, it is always low,” he said.
“For example, in California, the general rule of thumb is that any given earthquake has about a five percent chance of being a foreshock.”
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)