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After a 7.1-magnitude earthquake shook southern Japan on Thursday (August 8), the country’s meteorological agency issued its first-ever “megaquake advisory”.
The warning said the likelihood of strong shaking and large tsunamis is higher than normal on the Nankai Trough, a subduction zone (a region where tectonic plates collide with each other, and the heavier one slides under another) along Japan’s southwest Pacific coast.
However, this does not mean that a major earthquake would definitely happen during a specific period, the advisory said.
The Nankai Trough is an underwater subduction zone (nearly 900 km long) where the Eurasian Plate collides with the Philippine Sea Plate, pushing the latter under the former and into the Earth’s mantle. This accumulates tectonic stress which can cause a megaquake — an earthquake with a magnitude larger than 8.
The trough has produced large earthquakes roughly every 100 to 150 years, according to the 2023 study, ‘High probability of successive occurrence of Nankai megathrust earthquakes’, published in the journal Nature. These tremors usually come in pairs, with the second often rupturing in the subsequent two years — the most recent “twin” earthquakes took place in 1944 and 1946.
Notably, Thursday’s magnitude-7.1 earthquake occurred on or near the Nankai Trough, according to the United States Geological Survey. As a result, experts worry that the next tremor along the trough could be devastating.
In January 2022, Japan’s Earthquake Research Committee said the next magnitude 8-9 megaquake along the trough has a roughly 70% probability of striking within the next 30 years.
Such a megaquake could send tremors to areas from central Shizuoka — about 150 km south of Tokyo — to southwestern Miyazaki, the Reuters report said.
Tsunami waves of up to 98 feet may reach Japan’s Pacific coasts within minutes after the quake.
A 2013 government report found that a major Nankai Trough earthquake could impact an area that covers about a third of Japan and where about half the country’s population of more than 120 million people lives, according to a report by Nikkei Asia magazine.
The economic damage due to the disaster could go up to $1.50 trillion, or more than a third of Japan’s annual gross domestic product.
No. An accurate prediction of an earthquake needs a precursory signal from within the earth, indicating a big quake is on the way. The signal must also occur only before large earthquakes so that it does not indicate every small movement within the earth’s surface. Currently, there is no equipment to find such precursors.
Thursday’s advisory by Japan’s meteorological agency was just a warning not a prediction — it had nothing to do with science, Robert Geller, professor emeritus of seismology at the University of Tokyo, told the BBC. The advisory asked residents to prepare, review evacuation routes, and consider potential future warnings.
