Japanese authorities fear "megaseism" could occur soon

Two powerful earthquakes of magnitude 6.9 and 7.1 struck Japan on 8 August. If these have fortunately caused very little damage, the Japanese authorities fear that they will cause a "mega-quakage" on the country in the near future.

Two successive earthquakes on August 8

This Thursday, August 8, two major earthquakes shook Japan at the end of the day. The first, with a magnitude of 6.9 on the Richter scale, occurred at 4:42 p.m. local time off the island of Kyushu in the south of the archipelago and the second, with a magnitude of 7.1 just 25 kilometres away.

Despite these two notable tremors, the damage fortunately remained very limited on the Japanese archipelago, fortunately well prepared to deal with earthquakes. No building has collapsed, and the nuclear power plants in the sector are still operating normally according to the authorities.

If not a few falls of objects and a good fright for the inhabitants, these two successive tremors have therefore not really disrupted the lives of the Japanese, even if 8 people were still slightly injured according to the Japan Fire and Disaster Management Agency.

A tsunami alert had also been issued shortly after these earthquakes, waves of about 1m that could affect the south and southeast coast of the country. Fortunately, once again, these did not happen and the alert was quickly lifted in the evening. However, concern remains present this weekend, the government having warned the population that one earthquake could hide another.

A risk of a mega-quake soon on Japan?

Japan is particularly affected by earthquakes compared to the rest of the world. The country is indeed located on the Pacific Belt of Fire, a very active area at the tectonic level where 90% of earthquakes occur around the world but also at the crossroads of six tectonic plates.

The Nankai Pit, where the two earthquakes occurred this Thursday, is an underwater depression that extends for 800 kilometres, from the city of Shizuoka to the island of Kyushu. This is the starting point for devastating earthquakes of record magnitudes, between 8 and 9 every 100 or 200 years according to Japanese experts. These mega-quakes, which tend to occur in pairs, are also known to generate formidable tsunamis along the southern coast of Japan.

Scientists fear that the two earthquakes of August 8 have destabilised this area of faults, which could favour the occurrence of this type of megaquake in the more or less near future. According to the Japanese Meteorological Agency (JMA), the probability of a new powerful earthquake occurring is higher than normal following these earthquakes, even if this does not indicate that it will occur with certainty.

Despite these uncertainties, the earthquakes being almost impossible to anticipate, the Japanese authorities issued a warning about a large-scale earthquake risk on August 9 and asked the population to remain vigilant. This is the first time that the maximum alert has been issued since the implementation of the new alert system following the disaster that occurred in 2011 on the country.

Scientists and the population are therefore on the alert this weekend even if the outbreak of this famous mega-quake is far from assured. Even if this does not happen, the Japanese government has nevertheless recalled that there is a high probability (70%) that this devastating earthquake will hit Japan in the next 30 years, an earthquake that could affect a significant part of the Japanese Pacific coast, threatening some 300,000 people and could cause $13,000 in damage according to the models carried out by the experts.