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An agency of Japan's government recently gave its first-ever "megaquake advisory¹," or warning message.

The Japan Meteorological Agency (JMA) issued the warning after a powerful quake struck on August 8 off the southeastern coast of Japan's main island of Kyushu.

The magnitude² 7.1 quake caused no deaths or severe damage, but the advisory has led to questions and a sense of worry. Japan is among the countries most hit by earthquakes. Now many citizens are wondering about when the next big quake will hit.

The Associated Press recently explained what the advisory means, what people are being told to do:

What is a megaquake advisory?

JMA issued the advisory after deciding that the August 8 quake increased the likelihood³ of another major quake.

The agency said there is a 70 to 80 percent chance of a magnitude 8 or 9 quake associated with an area called the Nankai Trough within the next 30 years. The JMA said the probability⁴ of such an event is now "higher than normal."

Naoshi Hirata is a seismologist at the University of Tokyo and head of the JMA's group of experts. He said that the warning was not a prediction that a megaquake will happen at any specific time or place. He urged people to remain careful and prepared.

What is the Nankai Trough?

The Nankai Trough is an undersea trench⁵, or a deep valley on the ocean floor, that runs from the waters just off the southeastern coast of Kyushu, to Suruga Bay in central Japan. It is about 800 kilometers long.

Two large areas of the Earth's surface, the Philippine Sea Plate and the Eurasian Plate meet there. The Philippine plate slowly pulls down on the Eurasian plate and causes it to release energy from time to time. The JMA said that could lead to a megaquake and tsunami⁶.

The last Nankai Trough quake was off Shikoku in 1946. The U.S. Geological⁷ Survey estimated its magnitude to be 8.3, and more than 1,400 people died.

What is the government doing to prepare?

As a result of the "megaquake advisory," Japanese Prime Minister Fumio Kishida canceled a trip and announced he would lead the government response. Local governments are studying their emergency plans.

Experts and officials have urged people to stay calm and to continue normal life but to prepare for a possible evacuation.

JMA experts said on August 12 they have so far found no abnormal⁸ activity that would suggest a megaquake.

How are people reacting?

The "megaquake advisory" is filled with scientific terms. It has worried people across the country. Some towns closed beaches and canceled yearly events. This has led to problems for travelers during Japan's Obon holiday week, a time for festivals and fireworks across the nation.

Some stores are running out of emergency supplies, even in areas where there is little risk. In the vacation town of Matsuyama city on the island of Shikoku, hotels and resorts studied their evacuation plans and emergency equipment. In another seaside town, Shirahama, outdoor hot springs were closed for a week and a fireworks festival was cancelled.

Words in This Story

magnitude - n. a number that shows the power of an earthquake

seismologist -n. a person who studies the movements of the Earth such as earthquakes

trench - n. a long, narrow hole in the ocean floor

plate - n. one of the very large sections of the Earth's surface that are believed to move and cause earthquakes where they touch each other

evacuate⁹ - v. to remove (someone) from a dangerous place

resort -n. a place where people can stay on vacation that includes a hotel, restaurants and activities