Japan’s ‘megaquake’ warning explained: What did experts, PM Fumio Kishida say?

Japan, in a first-of-its-kind advisory, has warned of a “major earthquake” or “megaquake” in the future that could result in the loss of hundreds of thousands of lives, and the possibility of its happening is “higher than normal”.

The advisory was issued by the Japan Meteorological Agency (JMA) just hours after a magnitude 7.1 earthquake occurred in southwestern Japan, with its focus located in waters off Miyazaki Prefecture, on the western edge of the Nankai Trough, a report in Kyodo News said.

In the worst-case scenario, a powerful temblor could shake a wide area of Japan – from the Kanto region centring on Tokyo to the southwestern Kyushu region – and high tsunami waves could engulf the coastal areas of Kanto to Okinawa, according to the JMA, the report added.

What is the ‘megaquake’ advisory?

The JMA warning has been issued under a new set of rules that came into effect after a 2011 earthquake, tsunami and nuclear disaster killed around 18,500 people.

According to JMA’s advisory, “If a major earthquake were to occur in the future, strong shaking and large tsunamis would be generated.”

It further stated that the possibility of megaquake happening is “higher than normal”. “The likelihood of a new major earthquake is higher than normal, but this is not an indication that a major earthquake will definitely occur during a specific period of time.”

What experts say

Addressing a press conference, Naoshi Hirata, professor emeritus at the University of Tokyo and chief of the JMA panel, said, “The possibility of a (Nankai Trough) earthquake occurring is now several times higher than usual.”

However, he added that earthquake scientist cannot exactly tell when or where megaquake will occur, and urged people to be prepared.

“We can’t tell exactly when or where a megaquake will occur, but we want people to reaffirm their preparedness,” he said.

Geologists Kyle Bradley and Judith A Hubbard wrote in their Earthquake Insights newsletter that Nankai earthquake is “the most long-anticipated earthquake in history”, and the history of great earthquakes at Nankai is “convincingly scary”.

“A future great Nankai earthquake is surely the most long-anticipated earthquake in history — it is the original definition of the ‘Big One’. Previous estimates of potential fatalities from such an event – without any warning or evacuation – sometimes exceed 240,000. So, it’s a big deal,” they said.

“While earthquake prediction is impossible, the occurrence of one earthquake usually does raise the likelihood of another. One of the challenges is that even when the risk of a second earthquake is elevated, it is still always low – for instance, in California the rule of thumb is that any given earthquake has around five per cent chance of being a foreshock,” they added.

The duo asked people not to panic. “The advisory was issued because there is a small probability that the M7.1 earthquake is a foreshock to a much larger and more dangerous earthquake.”

What is Nankai Trough?

A trough is a long hollow in the seabed, like a trench. The Nankai Trough runs along the seafloor off Shizuoka prefecture, southwest of Tokyo, to Miyazaki prefecture on Kyushu. It is the meeting point of the Philippine Sea Plate and the Eurasian Plate, which are two of about 10 rock-hard tectonic plates that cover the Earth’s surface, according to a report on Nikkei Asia.

Along the Nankai Trough, megathrust earthquakes with a magnitude eight or more occur repeatedly. These are generally referred to as Nankai Trough earthquakes. Although Nankai Trough earthquakes occurred with a roughly 100-150-year cycle, they have various repetition intervals and source regions. They are difficult to accurately predict in terms of timing, location and scale, JMA said.

Among previous megathrust earthquakes in the trough, the Ansei Tokai earthquake of 1854 and the Showa Tonankai earthquake of 1944 (occurring in the eastern part of the trough) were followed 32 hours later by the Ansei Nankai earthquake of 1854 and two years later by the Showa Nankai Earthquake of 1946 (occurring in the western part of the trough), respectively.

Accordingly, the occurrence of a megathrust earthquake along the trough may be associated with elevated potential for a future occurrence elsewhere in the trough.

In addition, numerous slow slips are observed in periods of normal activity on the plate boundary around the assumed source region of Nankai Trough earthquakes. These phenomena are seen as indicators of change in the coupling conditions of the boundary. A slow slip in an unusual region of the trough or an irregular occurrence mode (such as a high change rate) can also be considered to indicate elevated potential for a Nankai Trough earthquake.

“An earthquake could occur in the Nankai Trough at any time without the opportunity for issuance of related information in advance. Accordingly, it is important to prepare for major earthquakes as part of daily life. It should be noted that the issuance of information indicating elevated potential for a Nankai Trough earthquake does not necessarily mean that one will actually strike,” JMA said.

As per a government estimate in 2012, a Nankai Trough earthquake could lead to a tsunami of over 30 metres and can cause up to 3.2 lakh deaths and economic losses of $1.5 trillion.

Japan PM cancels foreign trip over megaquake advisory

According to local media reports, Japan Prime Minister Fumio Kishida on Friday (August 9) cancelled his trip to Kazakhstan, Uzbekistan and Mongolia following JMA’s megaquake advisory.

“As the top official in charge of the nation’s crisis management, as an extra precaution, I should remain in Japan for at least about a week,” Kishida said and urged people to be prepared for such a situation.