Japan issues first-ever megaquake warning after M7.1 Kyushu earthquake
Image credit: TW/SAM, ESRI
The Japan Meteorological Agency (JMA) issued its first megaquake advisory on Thursday, August 8, 2024, several hours after the M7.1 earthquake hit near the coast of Kyushu. The advisory warns that if a major earthquake were to occur in the future, strong shaking and large tsunamis would be generated.
- This is the first time the JMA has issued the Nankai Trough Earthquake Extra Information as part of efforts to assess the probability of a subsequent megaquake in the Nankai Trough area.
- Nankai Trough is an 800 km (500 miles) long subduction zone between two tectonic plates in the Pacific Ocean, where massive earthquakes have hit in the past.
- While the likelihood of a new major earthquake is higher than normal, this is not an indication that a major quake will definitely occur during a specific period of time.
- The warning comes under new protocols adopted in 2019, drawn after the devastating 2011 M9.1 Tohoku-Oki earthquake and tsunami in which around 18 500 people were killed. The 2011 earthquake was preceded by a magnitude 7.2 foreshock which went largely unnoticed.
The M7.1 earthquake that hit near the coast of Kyushu at 07:43 UTC on August 8, 2024, prompted the Japan Meteorological Agency (JMA) to issue its first-ever warning for a megaquake along the Nankai Trough subduction zone — running along the country’s eastern coast.
“As a result of examining the relationship between this earthquake and the Nankai Trough earthquake, it is believed that the possibility of a large-scale earthquake occurring in the hypothetical source area of the Nankai Trough earthquake is relatively higher than usual,” JMA said. “Please take disaster prevention measures in accordance with future appeals from the government and local governments.”
This earthquake was a reverse fault type with a pressure axis in the west-northwest and east-southeast direction, and was evaluated as a moment magnitude 7.0 earthquake caused by a part of the boundary between the land plate and the Philippine Sea plate in the assumed focal area of the Nankai Trough earthquake, according to the JMA.
Statistical data on past large earthquakes around the world shows that of the 1 437 earthquakes with moment magnitude 7.0 or more that occurred between 1904 and 2014, there were 6 cases of earthquakes with moment magnitude 8 or more occurring in the same area within seven days of the first earthquake, and the frequency of occurrence thereafter decreases over time.
This data includes a case where a moment magnitude 7 earthquake occurred two days before the 2011 Tohoku Pacific Ocean Earthquake (moment magnitude 9.0).
In the world, the frequency of a moment magnitude 8 or higher earthquake occurring within seven days of a moment magnitude 7.0 or higher earthquake in the same area is about once in several hundred.
“From these findings, it is believed that the possibility of a large-scale earthquake occurring in the expected focal area of the Nankai Trough earthquake is relatively higher than usual,” JMA said.
There is diversity in Nankai Trough earthquakes, and the focal area of a large-scale earthquake may be limited to the area around this earthquake, or it may extend to the entire Nankai Trough.
If the largest earthquake occurs, strong shaking is expected over a wide area from the Kanto region to the Kyushu region, and a large tsunami is expected along the Pacific coast from the Kanto region to the Okinawa region, JMA said.
While the likelihood of a new major earthquake is higher than normal, this is not an indication that a major quake will definitely occur during a specific period of time, JMA added.
The agency is advising residents to continue their daily lives while staying prepared for potential disasters. A voluntary evacuation is recommended for vulnerable groups, including people with physical disabilities, the elderly, and children. The duration of this recommendation remains uncertain.
In previous estimates, JMA said the next M8 to 9 megaquake along the Nankai Trough has a roughly 70% chance of happening within the next 30 years.
The Nankai Trough, an 800 km (500 miles) long undersea trench extending from Shizuoka, west of Tokyo, to the southern tip of Kyushu island, has been the site of significant seismic activity throughout history.
This region is notorious for producing destructive earthquakes, often with magnitudes of eight or nine, typically occurring every century or two. These “megathrust quakes” are particularly concerning due to their potential to trigger massive tsunamis along Japan’s southern coast.
Historically, the Nankai Trough has produced some of Japan’s most devastating seismic events. In 1707, all segments of the trough ruptured simultaneously, resulting in an earthquake that stands as the nation’s second-most powerful on record. This earthquake also triggered the last known eruption of Mount Fuji. Subsequent powerful quakes followed, with notable Nankai megathrust events occurring in pairs in 1854 and again in 1944 and 1946.
The warning comes under new protocols adopted in 2019, drawn after the devastating 2011 M9.1 Tohoku-Oki earthquake and tsunami in which around 18 500 people were killed.
The 2011 M9.1 Tohoku-Oki earthquake, which caused widespread devastation in Japan, was preceded by a magnitude 7.2 foreshock that went largely unnoticed.
This earlier quake was not considered particularly significant, leading to minimal commentary and no advisory being issued in the two days before the catastrophic mainshock, seismologists Kyle Bradley and Judith A. Hubbard said.
In hindsight, had an advisory been issued, it is likely that many lives could have been saved, as people would have had time to prepare. Additionally, some of the severe infrastructural damage that occurred in the aftermath might have been mitigated with the advance warning.
While the current earthquake setting differs slightly, this historical lesson shows the importance of the recent advisory in a similar seismic context.
Prime Minister Fumio Kishida called off a planned trip to Central Asia as a precautionary measure following the recent warning, reflecting the seriousness of the situation and the government’s commitment to ensuring the safety of its citizens.
In response to the warning, bullet trains across the country were operating at reduced speeds to minimize the risk of accidents. The national broadcaster, NHK, has also been displaying cautionary graphics to keep the public informed about potential dangers. Additionally, several semiconductor-related factories temporarily halted production as a safety measure.
Local reports indicate that some beaches in the affected regions have been closed off for the time being. This step is likely intended to prevent any potential incidents and ensure that residents and visitors remain safe during this period of heightened alert.
References:
1 Nankai Trough Earthquake Emergency Information (Massive Earthquake Warning) – JMA – August 8, 2024
2 M7.1 earthquake strikes southern Japan; megaquake advisory issued – Earthquake Insights – August 8, 2024
I'm a dedicated researcher, journalist, and editor at The Watchers. With over 20 years of experience in the media industry, I specialize in hard science news, focusing on extreme weather, seismic and volcanic activity, space weather, and astronomy, including near-Earth objects and planetary defense strategies. You can reach me at teo /at/ watchers.news.
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