What is a mega earthquake and why did Japan issue warnings about it?

TOKYO (Reuters) – Japan, one of the most earthquake-prone countries on Earth, issued its first “super-quake warning” last week after a powerful earthquake struck the southeastern coast of the southern main island of Kyushu.

The 7.1-magnitude quake caused no deaths or major damage, but the warning led to widespread confusion and a lingering sense of anxiety — in a country accustomed to regular earthquakes — about when the next big one might strike.

The Associated Press explains what the warning means, what people are being asked to do, and what could happen if a major earthquake hits Japan.

What is a major earthquake warning?

The Japan Meteorological Agency issued the warning after concluding that the 7.1-magnitude earthquake that struck on Aug. 8 on the western edge of the Nankai Trough had increased the possibility of another major quake.

The Japan Meteorological Agency says there is a 70% to 80% chance of a magnitude 8 or 9 earthquake linked to the Nankai region occurring in the next 30 years, and that the probability is now “higher than usual” after the latest quake.

But that doesn’t necessarily mean a major quake will happen at a specific time or place, says University of Tokyo seismologist Naoshi Hirata, who heads the Japan Meteorological Agency’s expert panel. He urged people to be careful and prepared.

What is the Nankai Basin?

The Nankai Trench is an underwater trench that extends from Hyoganada, in the waters off the southeastern coast of Kyushu, to Suruga Bay in central Japan. It extends for 800 kilometres (500 mi) along the Pacific coast.

The Japan Meteorological Agency says the Philippine Sea plate is slowly pulling the Eurasian plate and causing it to bounce back from time to time, which could lead to a massive earthquake and tsunami.

The last Nankai Trough earthquake off Shikoku Island in 1946 registered an initial magnitude of 8.0 on the Richter scale, and killed more than 1,300 people.

How destructive can a major earthquake be?

In 2013, a government disaster prevention team said a magnitude 9.1 earthquake in the Nankai Trough could generate a tsunami exceeding 10 meters (33 feet) in a matter of minutes, killing up to 323,000 people, destroying more than 2 million buildings and causing economic damage of more than 220 trillion yen ($1.5 trillion) to large swaths of Japan’s Pacific coast.

What is the government doing to prepare?

As a result of this “huge earthquake warning,” Japanese Prime Minister Fumio Kishida cancelled his scheduled visit to Central Asia from August 9 to 12, and announced that he would lead the government’s response and ensure preventive measures and communication with the public.

The Fire and Disaster Management Agency has instructed 707 municipalities considered at risk from the Nankai Trough earthquake to review response measures and evacuation plans.

Experts and officials urged people to remain calm and continue their daily social and economic activities while securing emergency food and water and discussing evacuation plans with family members.

In a reassuring note on Monday, experts from the Japan Meteorological Agency said they had not yet found any abnormal seismic or tectonic activity that would indicate a major quake.

How do people react?

The “massive earthquake warning,” filled with scientific jargon, has alarmed people across the country. Some cities have closed beaches and canceled annual events, creating challenges for travelers during Japan’s Obon holiday week, when festivals and fireworks are held across the country.

Many postponed planned trips and rushed to stock up on rice, dried noodles, canned food, bottled water, portable toilets and other emergency goods, leaving shelves empty at many supermarkets in western Japan and Tokyo, even though the capital is outside the danger zone.

Supermarket chain Summit said there is a shortage of microwaveable rice, and stores are limiting purchases to one package per customer.

Yoshiko Kudo and her husband, Shinya, said they had difficulty understanding exactly what the warning meant, how concerned they should be and what they should do.

“We try not to overdo it,” Yoshiko Kudo said. “Too much worry is not good.”

“We don’t know how to be prepared and continue living normally as the experts tell us,” said Shinya Kudo, a caregiver in his 60s.

“It’s scary,” said Yoneko Oshima, walking past a major train station in Tokyo. “They say there’s a 70-80 percent chance it will happen in the next 30 years, but it could happen tomorrow.” Her latest purchase is a portable toilet. She says the water is indispensable for her diabetic husband, who needs to take medicine after every meal.

“I plan to take this opportunity to make a list and make sure we have everything on hand,” Oshima said. She hasn’t changed her vacation plans this week, but her daughter has canceled a planned trip to Mount Fuji.

In Matsuyama City on Shikoku Island, home to many hot springs, hotels and resorts have revised their evacuation procedures and emergency equipment and launched a radio communication system for emergency use. Hideki Ochi, director of the Dogo Onsen Ryokan Association, said they have received hundreds of cancellations since the notice was issued.

Rail companies serving the area said their trains were running at slightly reduced speeds as a precaution.

In the coastal city of Kuroshio in Kochi Prefecture, where the government predicted a tsunami of up to 34 meters (111 feet), a crisis management task force set up 30 shelters across the city. But only two shelters remained open after the Japan Meteorological Agency said Monday there were no signs of an imminent major quake.

Osaka’s Higashi urged residents on the city’s website not to make any “unnecessary and non-urgent” travel in the event of a major quake.

The popular coastal city of Shirahama in Wakayama Prefecture announced the closure of its four outdoor hot springs, parks and other facilities for a week. An annual fireworks festival scheduled for Saturday has also been canceled.