Last week, Japan experienced a massive 7.1 magnitude earthquake off its southern main island of Kyushu. Though the earthquake didn’t cause any casualties or severe damage, afterward it did prompt the nation’s first-ever megaquake advisory.
According to the United States Geological Survey, even the best experts don’t know how—and don’t expect to know how—to precisely predict earthquakes. However, that doesn’t mean they can’t estimate the probability of one occurring.
Such is the case for Japan, one of the most earthquake-prone countries in the world. According to the Japan Meteorological Agency, the country’s original earthquake last week increased its likelihood of another one, which is why experts found it plausible to issue the advisory.
“Megaquake” isn’t a technical scientific term, but more of a general category. Seismologists typically use “great earthquake” for quakes of magnitude 8.0 or higher, or “giant earthquake” for those exceeding magnitude 9.0—both of which are relatively rare. Japan’s last megaquake was the 2011 Tohoku earthquake, a magnitude 9.0 that triggered devastating tsunamis, the meltdown of nuclear power plants, and killed more than 18,000 people.
Due to Japan’s history with huge earthquakes, they estimate that there is a 70-80% chance of a magnitude 8 or 9 earthquake within the next 30 years. After the most recent quake, that chance is even “higher than normal.”
University of Tokyo seismologist Naoshi Hirata specified that the advisory is not a warning or prediction for a megaquake occurring at a certain time or in a certain location, but rather a reminder to remain prepared. Not only could the quake destroy buildings and kill people, but it could also trigger a devastating tsunami within minutes. Of course, this would only lead to more tragedy.
The AP reported that receiving the new megaquake advisory caused many people to feel anxious and uneasy, stocking up on food and essential items in case of an emergency. Even Japanese Prime Minister Fumio Kishida found the situation to be dire enough to cancel a planned trip to Central Asia over the weekend. Instead of traveling, he focused on coordinating the government response to put the right preventative measures in place, while the rest of the area shut down outdoor parks and attractions.
In a widely cited post on their Earthquake Insights newsletter, geologists Kyle Bradley and Judith A Hubbard wrote, “A future great Nankai earthquake is surely the most long-anticipated earthquake in history—it is the original definition of the ‘Big One.’”
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