Japan Just Issued Its First ‘Megaquake’ Alert. Here’s What You Should Know

Japanese scientists issued their first ever massive earthquake alert after a significant quake struck parts of the country on Thursday.

Although the threat isn’t imminent or guaranteed, the Japanese Meteorological Agency said in a press release that the possibility of a “large-scale earthquake” is “relatively higher than usual,” but “this does not mean that a large-scale earthquake will definitely occur during a specific period.”

If such a quake were to occur, it would likely cause tsunamis and strong shaking, the agency warned. Residents have been advised to take disaster prevention measures as issued by local governments and to be on alert for the next seven days. The warning prompted Japanese Prime Minister Fumio Kishida to cancel a trip to several Central Asia countries.

The megaquake advisory was prompted by Thursday’s 7.1-magnitude quake, which originated in the Nankai Trough in the Pacific Ocean off Japan’s south coast. The JMA issued a tsunami warning, which was later dismissed. On Friday, a weaker quake, measuring 5.3, struck the Kanagawa Prefecture south of Tokyo. The Japan Times reported that the quakes didn’t result in any fatalities and caused only 16 mostly minor injuries.

The Nankai Trough is an underwater basin that lies over a major fault line and has been the point of origin for several major earthquakes, including a pair of 1854 quakes that resulted in tsunamis that caused around 3,000 deaths. Another pair of sister quakes in 1944 and 1946 claimed 1,200 and 1,300 lives, respectively. In 2022, Japan’s Earthquake Research Committee estimated a 70% to 80% chance of a major earthquake occurring along the fault line within the next 30 years.

This fault line was not the source of the 2011 earthquake that killed nearly 20,000 people and triggered the devastating tsunami responsible for the Fukushima nuclear disaster. That quake was caused by seismic activity along the boundary between the Pacific and North American tectonic plates.

A 2022 study estimated a major earthquake originating on the Nankai Trough fault line could cause $8.4 trillion (1,240 trillion yen) in damages.