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Ayşe Defne Orhan

Japan’s First Mega-Earthquake Alert Raises Concerns

For the first time, the Japan Meteorological Agency (JMA) has announced a mega-earthquake alert after the 7.1 magnitude earthquake on Thursday, 8th August 2024.


As a country in the “Ring of Fire” and among the most earthquake-prone countries, Japan is used to earthquakes with the majority of them causing little damage although some had more severe effects such as the 2011 Tōhoku earthquake and tsunami with 9 magnitude, resulting in more than 18,000 deaths and rendering Fukushima nuclear disaster on March 2011.


The Nuclear Regulation Authority stated all 12 nuclear reactors in Kyushu and Shikoku are secured.


The mega earthquake alert led Japan's Prime Minister Fumio Kishida to cancel his visit to a summit in Central Asia that was planned to happen on August 9-12 in order to "ensure our preparations and communications are in order". Furthermore, the Fire and Disaster Management Agency required 707 municipalities at high risk to check their measures and evacuation plans while officials advised people to secure emergency food and water and make evacuation plans meanwhile staying calm. In fact, in Kochi Prefecture, 10 municipalities already opened a minimum of 75 evacuation shelters by Friday morning. Moreover, trains started to operate at slower speeds as a preventive measure.


Many people in Japan are now wondering if the predicted earthquake is the long-anticipated "big one," which is said to occur once in a century.


Officials warned the public to be on alert since the risk of a major earthquake is detected to be higher than usual. The advisory was right after a 7.1 magnitude earthquake on the southern coast of Japan that left neither major damage nor caused death. According to the Japan Meteorological Agency (JMA), the earthquake occurred on Kyushu island at 4:42 p.m. local time at around 30 kilometers depth.


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A collapsed house in Japan, after the 7.1-magnitude earthquake on August 8, 2024.



Previously, officials had issued tsunami warnings estimating 1-meter waves, urging residents to immediately vacate the coastal areas of Kochi and Miyazaki until a second call. Around half an hour later, tsunami waves reached 1.6 feet, approximately 50 centimeters, in parts of Kyushu’s southern coast and the nearby island of Shikoku.


When seismologists met to analyze whether the recent earthquake impacted seismic activity in Nankai Trough, an underwater subduction zone having the potential to produce magnitude 8 or 9 quakes, they concluded recent earthquake on the western edge of the Nankai Trough heightened the risk of a mega earthquake.


The Pacific “Ring of Fire” region contains a group of subduction zones. For example, in the U.S., the Cascadia subduction zone off the West Coast reaches from Vancouver Island, Canada, to Cape Mendocino, California.


Stress is accumulated in subduction zone faults and when a locked fault slips to release the stress, megathrust earthquakes, also known by the shortened form “megaquakes” occur.


The source of concern, Nankai Trough is an 800-kilometer undersea trench located where The Philippine Sea Plate and Eurasian Plate collide in the Pacific Ocean, which witnessed massive earthquakes in the past. It starts from Shizuoka, west of Tokyo, stretching to the southern tip of Kyushu island.


Nankai Trough fault has several segments, but if all segments were to slip together, scientists think that the trough could generate an earthquake of up to magnitude 9.1.

In 1707, all segments ruptured at once, causing the second-biggest earthquake recorded in Japan, with approximately 8.6 or 8.7 magnitude, followed by the eruption of Mount Fiji.


According to research, Nankai Trough yields mega-earthquakes once every 100-150 years, which tend to be in pairs. The last megathrust pairs were in 1944 and 1946 both measuring 8 magnitude and killing at least 2500 people in total. By calculating the intervals between significant earthquakes, scientists have found out that there is a 70%-80% risk of mega earthquakes between magnitude 8 and 9 epicenter in Nankai Trough within 30 years.


Therefore, in the first-ever mega earthquake advisory, experts highlighted an elevated risk of a mega earthquake on the Nankai Trough and forewarned area residents to be prepared for such a case. According to officials, a megaquake with 8-9 magnitude near the Nankai Trough holds a 70%-80% chance of hitting within the next 30 years, and the risk now is greater.


In the worst-case scenario, experts anticipate more than 300,000 loss of lives and damage that costs $13 trillion with potentially 30-meter waves hitting the coast.

















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Nonetheless, since the 2011 earthquake was preceded by a 7.2 magnitude foreshock, experts are alarmed that this 7.1 magnitude earthquake may be a foreshock for a bigger one.


As Shoichi Yoshioka, a professor at Japan’s Kobe University notes “It is true that after one earthquake, a second, larger one can follow – which is why authorities issued the unprecedented warning last Thursday. But even then, the probability of the Nankai Trough earthquake happening the next day is low – perhaps increasing from the typical risk of one in 1,000 to one in a few hundred. That’s still less than a 1% chance”.

Notably, on Monday, JMA experts expressed they have not so far found any abnormal seismic or tectonic activity that would indicate a megaquake.


As University of Tokyo seismologist Naoshi Hirata remarks, the advisory is not a prediction that a megaquake will happen at any specific time or location, but rather a reminder that people should stay cautious. All in all, the most possible outcome is that the recent shaking will not trigger a mega earthquake, even though the probability is now a little higher. Still, there is no harm in being cautious and mindful of the risk.




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