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August 8, 2024 -

On the afternoon of August 8, 2024, a magnitude 7.1 earthquake hit southwest Japan, rattling the city of Kyushu and vigorously shaking the city of Nichinan.

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Concern from seismologists -

Triggered at a depth of 37 kilometers (23 miles) below the Earth’s surface, the quake caught the attention of seismologists at the Japan Meteorological Agency (JMA).

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Emergency meeting -

The seismologists called a special meeting, where they discussed whether the earthquake could be an indication of something bigger to come.

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Main concern -

In particular, they were concerned about a massive earthquake occurring along the Nankai Trough, which runs below the sea south of Shikoku and western Honshu.

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Megaquake warning -

Later the same day, the JMA issued its first ever megaquake alert. Residents of risk zones were encouraged to review evacuation procedures and stay vigilant for a week.

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Likelihood of a megaquake -

In the end, a megaquake did not occur. Indeed, according to the JMA, the chance of a bigger earthquake after a magnitude seven tremor is around one in several hundred.

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Earthquakes in Japan -

The event did serve as a reminder, however, that Japan is one of the world’s most earthquake-prone countries.

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Ring of Fire -

Indeed, it sits on an arc of volcanoes and oceanic trenches known as the "Ring of Fire." In 2011, for example, more than 15,000 people were killed in a magnitude 9 quake.

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2011 earthquake -

The 2011 earthquake hit northeast Japan and triggered a tsunami, as well as causing triple reactor meltdowns at a nuclear power plant.

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The Nankai Trough -

One of the highest-risk areas for earthquakes in Japan is the Nankai Trough, a submarine trough located off the country’s southwest coast. It runs for approximately 900 kilometers (600 miles).

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Subduction zone -

The Nankai Trough marks a subduction zone, where the Philippine Sea Plate is subducting under the Eurasian plate.

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Tectonic strains -

As per recorded history, the tectonic strains that accumulate as a result of this subduction result in a megaquake roughly every 100 to 150 years.

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20th-century events -

It has been around 80 years since the last significant shakes in the region. The Tonankai earthquake (1946) and the Nankai earthquake (1946) had magnitudes of 7.9 and 8.0, respectively.

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Looking ahead -

According to the Japanese government, there is an 80-90% likelihood that a megaquake will occur along the Nankai Trough in the next 30 years.

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Potential for damage -

A megaquake along the Nankai Trough has the potential to cause devastating damage. The maximum measurable tremors would be felt in areas from central Shizuoka to southwestern Miyazaki.

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Tsunami risk -

Within minutes after the quake, tsunami waves of up to 30 meters (98 feet) could reach Japan’s Pacific coasts.

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Important variables -

The exact timing and size of the tsunami waves would depend on the epicenter of the earthquake and the tidal situation at the time.

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Projections -

In 2019, the government’s Central Disaster Prevention Council released projections about the number of lives that may be taken by a megaquake and ensuing tsunami.

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Loss of life -

According to the projections, there could be as many as 230,000 people dead or missing as the result of such an event.

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Impact on buildings -

In a worst-case scenario, nearly 2.1 million buildings in the affected region could be burned to the ground or otherwise destroyed.

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Economic damage -

According to Reuters, the economic damage to the country could be as much as 220 trillion yen (US$1.5 trillion), which equals more than a third of Japan’s annual GDP.

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Disruption to supply chains -

There would be a long-lasting impact on infrastructure and long-term disruption to supply chains that are needed to produce key Japanese products, such as cars.

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Earthquake history -

As previously mentioned, Japan has a long history of megaquakes along the Nankai Trough. The earliest recorded megaquake in the region was in 684 CE.

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Kochi to Wakayama -

With an epicenter spanning from today’s Kochi Prefecture to Wakayama, records suggest that this magnitude 8.3 quake caused landslides and overflowing rivers.

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Widespread impact -

There were reports of numerous buildings, temples, and shrines being destroyed, as well as many humans and livestock being losing their lives.

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1944 earthquake -

More recently, there was a pair of significant earthquakes that struck the region in the 1940s. The first had a magnitude of 7.9 and struck in 1944, offshore from the Kii Peninsula to the Enshunada Sea.

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1946 earthquake -

Two years later, the second quake, which had a magnitude of 8.0, hit the Nankai fault to the west, offshore from the Kii Peninsula and along the Shikoku coast.

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Damage caused -

The 1944 earthquake caused severe damage to the Shizuoka, Aichi, Gifu, and Mie prefectures. The 1946 quake caused a huge tsunami that struck shorelines between the Boso Peninsula and Kyushu.

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Twin shakes -

Interestingly, this was not the first time that two megaquakes struck the region in quick succession–the same thing happened in 1096 and 1099.

Sources: (Reuters) (Nippon.com)

See also: Early warning signs of earthquakes, tsunamis, and other disasters

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