TOKYO -- Japan, one of the most earthquake-prone nations on earth, issued its first-ever “megaquake advisory” last week after a powerful quake struck off the southeastern coast of the southern main island of Kyushu.

The magnitude 7.1 earthquake caused no fatalities or significant damage, but the advisory has generated widespread confusion and a persistent sense of concern — in a nation accustomed to regular seismic activity — about the timing of the next major earthquake.

The Associated Press explains the implications of the advisory, what actions people are advised to take, and the potential consequences if a massive earthquake were to hit Japan.

The Japan Meteorological Agency issued the advisory after concluding that the magnitude 7.1 earthquake that struck on Aug. 8 on the western edge of the Nankai Trough increased the likelihood of a more powerful earthquake.

The JMA estimates that there is a 70-80% chance of an earthquake of magnitude 8 or 9 occurring in the Nankai Trough within the next 30 years. Following the recent earthquake, the probability of such an event is now considered “higher than normal”.

However, this latest earthquake does not mean that a megaquake is imminent at any specific time or place, says Naoshi Hirata, a seismologist at the University of Tokyo and head of the JMA's expert panel. He emphasized the importance of continued vigilance and preparedness.

The Nankai Trough is an underwater trench that extends from Hyuganada, off the southeastern coast of Kyushu, to Suruga Bay in central Japan. This trench stretches approximately 800 kilometers (500 miles) along the Pacific coast.

The Philippine Sea Plate in that area slowly descends beneath the Eurasian Plate, causing occasional sudden movements. This activity could potentially lead to a massive earthquake and tsunami, according to the Japan Meteorological Agency (JMA).

The last major earthquake in the Nankai Trough, off the coast of Shikoku in 1946, had an initial magnitude of 8.0 and resulted in the loss of over 1,300 lives.

In 2013, a government disaster preparedness team warned that a magnitude 9.1 Nankai Trough earthquake could trigger a tsunami exceeding 10 meters (33 feet) within minutes, causing a devastating loss of life, extensive property damage, and significant economic repercussions for Japan’s Pacific coast.

Following the issuance of a "megaquake advisory," Japanese Prime Minister Fumio Kishida canceled his planned visit to Central Asia from August 9th to 12th and declared he would oversee the government's response, ensuring preventative measures and public communication.

The Fire and Disaster Management Agency instructed 707 municipalities identified as vulnerable to a Nankai Trough earthquake to reassess their emergency protocols and evacuation strategies.

Authorities and specialists have advised the public to remain calm and continue their usual social and economic activities, while also taking steps to secure emergency supplies of food and water and discussing evacuation plans with family members.

In a calming statement on Monday, JMA experts stated that they have not detected any unusual seismic or tectonic activity that would suggest an impending major earthquake.

The “megaquake advisory,” filled with technical language, has caused concern and confusion among people nationwide. Some towns have closed beaches and canceled annual events, creating difficulties for travelers during Japan’s Obon holiday week, traditionally a time for festivals and fireworks across the nation.

Many individuals have postponed planned journeys and rushed to stockpile rice, dried noodles, canned goods, bottled water, portable toilets, and other emergency items, resulting in empty shelves at numerous supermarkets in western Japan and Tokyo, even though the capital is outside the region considered at risk.

The Summit supermarket chain reported a shortage of microwavable rice and has implemented a purchase limit of one pack per customer.

Yoshiko Kudo and her spouse, Shinya, expressed confusion regarding the specific implications of the advisory, the appropriate level of concern, and the necessary actions to take.

“We are trying to stay balanced. Excessive worry isn't helpful,” Yoshiko Kudo said.

“We aren’t sure how to prepare and still maintain a normal life as the experts advise,” said Shinya Kudo, a caregiver in his 60s.

Yoneko Oshima, walking near a major train station in Tokyo, said: “It’s frightening ... They say there's a (70-80%) chance in the next 30 years, but it could happen tomorrow.” Her recent purchase is a portable toilet. She explains that water is essential for her diabetic husband, who requires medication after every meal.

“I'm planning to make a list and ensure we have everything ready,” Oshima said. She hasn’t changed her vacation plans this week, but her daughter canceled a trip to Mount Fuji.

In Matsuyama city on Shikoku Island, renowned for its numerous hot springs, hotels, and resorts, conducted a review of their evacuation procedures and emergency equipment, and activated a radio communication system for emergency purposes. They have experienced hundreds of cancellations since the advisory was issued, according to Hideki Ochi, director of the Dogo Onsen Ryokan Association.

Rail companies serving the region reported their trains are operating at slightly reduced speeds as a safety measure.

A crisis management team in the coastal town of Kuroshio in Kochi prefecture, where government risk analysis predicted a tsunami reaching 34 meters (111 feet), initially established 30 evacuation centers throughout the town. However, only two remain open after Monday’s JMA announcement stating no signs of an imminent massive earthquake.

Higashi Osaka officials advised residents on the town's website to refrain from “unnecessary and non-essential” travel in the event of a significant earthquake.

The popular seaside town of Shirahama in Wakayama Prefecture announced that its four outdoor hot springs, parks, and other facilities would be closed for a week. The annual fireworks festival scheduled for Saturday was also canceled.