AUGUST 12, 2024 – Japan’s seismic experts are urging the nation to brace for the possibility of a future “megaquake” that could result in hundreds of thousands of fatalities. However, it’s important to note that they emphasize that this does not mean such a catastrophic event is imminent. The August 9 alert issued by the Japan Meteorological Agency (JMA) makes this the first alert of its kind under new protocols established after the devastating 2011 earthquake, tsunami, and nuclear disaster, which claimed around 18,500 lives. Alerts like these are designed to remind people to be ready for disasters, whatever they may be.
What is a “Megaquake” Advisory?
The JMA’s “megaquake advisory” cautions that a major earthquake in the future could produce intense shaking and trigger large tsunamis. While the likelihood of a significant quake is currently higher than usual, the advisory clarifies that this does not guarantee a major tremor will occur within any specific timeframe.
The focus of this warning is the Nankai Trough, a subduction zone in the Pacific Ocean where two tectonic plates converge. This area has historically been the site of massive earthquakes.
Understanding the Nankai Trough and Earthquakes
The Nankai Trough stretches 800 kilometers from Shizuoka, Japan (west of Tokyo), to the southern tip of Kyushu just below the main island of Honshu. It has been the origin of devastating earthquakes, often measuring magnitude eight or nine, occurring roughly every century or two. These so-called “megathrust quakes” traditionally strike in pairs and have a history of unleashing dangerous tsunamis along Japan’s southern coastline.
Notably, in 1707, the entire Nankai Trough ruptured simultaneously, causing the nation’s second-most powerful recorded earthquake and triggering the last eruption of Mount Fuji. Subsequent significant quakes in this region occurred in 1854, 1944, and 1946.
According to Japan’s government, there is an estimated 70 percent chance of another magnitude 8-9 megaquake striking the Nankai Trough within the next 30 years. In the worst-case scenario, experts estimate that 300,000 lives could be lost, with potential damage reaching $13 trillion as infrastructure crumbles.
“The history of great earthquakes at Nankai is convincingly scary,” geologists Kyle Bradley and Judith A. Hubbard wrote in their Earthquake Insights newsletter, as reported by the Associated Foreign Press. While it is impossible to predict earthquakes, they noted that one earthquake often increases the likelihood of another. That’s why we are where we are now with all the “megaquake” talk.
Should you be concerned?
Japan is urging residents in earthquake-prone areas to take standard precautions, such as securing furniture and knowing the location of the nearest evacuation shelter. It’s important to note that MOST of Japan has some kind of earthquake activity from time to time.
Many households also maintain disaster kits with essentials like bottled water, long-life food, a flashlight, and a radio. These are all excellent practices regardless of any advisory.
Despite all the buzz, experts say there is absolutely no reason whatsoever to panic. Bradley and Hubbard explain that there is only a “small probability” that a recent magnitude 7.1 earthquake is a foreshock. “One of the challenges is that even when the risk of a second earthquake is elevated, it is still always low,” they said. For instance, in California, the rule of thumb is that any given earthquake has around a five percent chance of being a foreshock.
Still, it makes for a great reminder to have a disaster kit ready and waiting, just in case.
Story by Charlie Maib
U.S. Army Corps of Engineers, Japan District