Japanese seismologists say the country should prepare for a possible future earthquake that could kill hundreds of thousands of people, although they stress that the warning does not mean a major quake is imminent.

*The warning from the Japan Meteorological Agency (JMA) is the first issued under new rules established after the 2011 earthquake, tsunami and nuclear disaster, which killed about 18,500 people.

The JMA’s “mega-earthquake” warning states that “if a large earthquake occurs in the future, it will result in strong tremors and large tsunamis.”

“The likelihood of a new large earthquake is higher than usual, but this does not mean that a large earthquake will definitely occur within a specific time period,” the warning added. The warning is related to the Nankai Trench “subduction zone” between two tectonic plates in the Pacific Ocean, where large earthquakes have occurred in the past.

How dangerous is the situation?
The Japanese government has previously said there is a 70% chance of the next magnitude 8-9 quake in the Nankai Trench occurring within the next 30 years. In a worst-case scenario, experts estimate that 300,000 people could die, with some engineers saying the damage could be as high as $13 trillion due to infrastructure being wiped out.

“The history of large earthquakes in the Nankai area is truly terrifying,” geologists Kyle Bradley and Judith A Hubbard write in their Earthquake Insights newsletter.

And “although earthquakes are impossible to predict, the occurrence of one often increases the likelihood of another. A large future Nankai earthquake is certainly the most anticipated earthquake in history—it is the original definition of the ‘Big One.’”

How should people prepare?
Japan is reminding people living in earthquake zones to take general precautions, from securing belongings to knowing where the nearest shelter is. Many households in the country also keep a disaster kit stocked with bottled water, non-perishable food, flashlights, radios and other practical items.

What to do if a big earthquake suddenly strikes?

But there’s no need to panic—there’s only a “small chance” that the 7.1 magnitude quake that struck Miyazaki on the 8th was a foreshock, according to Bradley and Hubbard. “One of the challenges is that even as the risk of a second earthquake increases, it’s still low. In California, for example, the general rule is that any earthquake has about a 5 percent chance of being a foreshock.”