Mega-Quake Risk In Japan

⏱ 7 minutes

Due to its location on the Pacific Ring of Fire 🔥, the Japanese archipelago is one of the countries with the highest yearly seismic activity in the world. Most of the earthquakes occurring on a daily basis are imperceptible or very slight and not too serious, but Japan is aware of the possibility it can be hit at any moment by a large scale earthquake called a "mega-quake."

A mega-quake is defined by:

A magnitude superior to 8,5 on the Richter scale; and,
A shallow depth: less than 50 kilometers beneath the ground’s surface.

On March 11, 2011, the Sanriku coast was hit by the Great Tohoku earthquake, with a 9,0 to 9,1 magnitude, which is to this day the most recent mega-quake recorded in Japan. Most of us remember how the area was devastated on the long term, as the strong quake triggered a mega-tsunami (a tidal wave of more than 20 meters high) which caused Fukushima’s nuclear power plant accident.

Nankai Through: a careful surveillance

The Japan Meteorological Agency (JMA) is consequently permanently monitoring the Japanese territory, and especially the most sensitive zones sitting at the boundaries of tectonic plates, which are prone to frequent giant earthquakes.

The Nankai Through in particular, is in the media spotlight. The Through is spanning over 800 kilometers along Japan’s Pacific Coast in the south of the main islands: from the Hyuga-nada Sea in Kyushu to Suruga Bay in Shizuoka Prefecture, between Tokyo and Nagoya. The long trench is divided into 3 large geographic areas and 5 segments:

The "Nankai" area (segments A and B), Nankai meaning "Southern Sea", that begins in the east of Kyushu and extends to the south of Shikoku;
The "Tonankai" area (segments C and D) is the central part of the Through, below the Kii Peninsula (Wakayama and Mie prefectures, in the south of Kyoto and Osaka);
The "Tokai" area (segment E) is the north-eastern end of the fault, spanning from the south of Shizuoka to Suruga Bay; it is the area closest to Tokyo.

The Through is a subduction zone, where the Philippine Sea Plate dives under the Eurasian Plate, causing several big earthquakes throughout Japan’s history, to name a few:

Hoei earthquake in 1707, in the south of Kii Peninsula, with an estimated 8,6 magnitude and that induced the most recent eruption of Mount Fuji 🗻;
2 of the 3 Ansei era earthquakes, in 1854, in Suruga, of a magnitude 8,9 and that stroke less than 48 hours apart; and,
1946 Nankai earthquake (at the beginning of the Post-War period) in the south of Shirahama, of an 8,1 magnitude, the latest to date in the area.

Scientists assessed that a big earthquake along the Through, usually involving 1 to 2 different segments, occurs about every 90 to 150 years, sometimes every 200 years. It can also be mentioned that the "Tokai" area’s segment E has almost always broken simultaneous with one of the segments of the "(To)Nankai" area.

The Japanese government announced an estimated probability of 70% to 80% that a mega-quake hits the area in the next 30 years. The "Big One" that would hit Tokyo has indeed been predicted for years and could theoretically happen in the coming days / months / years / decades… but no one can tell precisely when.

The consequences of such an earthquake in this south-central part of Japan might be even worse than March 2011’s, especially as:

The tremor would hit a densely populated area, especially in the north-east of the Through with cities like Tokyo, Kawasaki and Yokohama; and more broadly Shizuoka, Hamamatsu, Nagoya, and up to Osaka Bay;
There is a high risk it triggers a tsunami that would reach the coastlines in less than 5 minutes, especially the Izu and Kii Peninsulas, and Tokushima, Kochi and Miyazaki prefectures; and;
One of the volcanoes close to the epicenter may also be induced to wake up.

1rst alert for major earthquake risk

On Thursday August 8, 2024 at 4:43 p.m., a magnitude 7,1 earthquake occurred at a 30 kilometers depth off Miyazaki City, in Hyuga-nada Sea at the south-western end of the Nankai Through. Registered at a 6 – intensity on the 10-intensity levels Shindo scale used in Japan, it did not cause major damages:

The tsunami risk alert (systematically issued at this magnitude) was quickly lifted;
The only few damaged buildings were not earthquake resistant structures; and,
About 15 persons were injured but there was no fatality.

The anti-seismic construction standards in Japan, as well as the residents’ behavior, who are trained from childhood on how to react in case of an earthquake, certainly helped on limiting casualties.

However, shortly after the tremor, the JMA issued a warning regarding "a heightened risk of a possible mega-quake hitting in the next days" in the epicenter’s area. Seismologists indeed think there is a 5% probability that a relatively big tremor located in a sensitive area, such as the Nankai Through, would be followed by a larger scale event in the next days. The phenomenon was observed in 2011, when a magnitude 7,3 fore-shock hit the Tohoku’s Pacific coast 2 days before March 11.

Reminder of the basic safety tips

Its is the 1rst ever mega-quake warning issued by the Japan Meteorological Agency, and it actually doesn’t predict the occurrence of a mega-quake but is rather intended to remind the population of the basic safety tips, such as:

Frequently checking information about the situation;
Following the safety instructions provided by the authorities, such as how to best prepare one’s home to the tremors;
Having an emergency kit at the ready with food and essential items for 3 days;
Preparing a plan to evacuate, especially if help is required (families with young children, elderly people) and knowing where the nearest evacuation shelter is located;
Being able to communicate one’s location if necessary (spare batteries, small radio device, etc.).

This alert was issued just before mid-August, a national holidays period (Obon) for the Japanese and therefore of inter-regional travels. It impacted the country in various ways:

Japanese Primer Minister Fumio Kishida postponed his diplomatic trip to Central Asia. It might have also been an opportunistic move as he announced his resignation a few days later (in stepping down from September’s LDP presidential election);
Many cautious Obon vacationists have canceled their holiday plans in places located in the areas encompassed in the possible mega-quake zone;
Stocks of water, rice and toilet paper have been partially depleted in supermarkets and online shops;
Train traffic has been slowed down is some areas and events that should have taken place in coastal areas have been canceled (such as the Shirahama Fireworks Festival).

What about temporary visitors in Japan?

As for tourists already traveling in Japan or those who are planning a trip in a near future, we recommend to not give into the panic, as no one can tell the date nor the exact location of the next large earthquake in Japan.

The sole warning of a heightened risk of earthquake doesn’t justify canceling a trip. The most worried may postpone their stay or change their itinerary, however a mega-quake can occur either tomorrow or in 30 years. Moreover, due to its seismic nature, there is no place safe from earthquakes in Japan.

Booking a trip with a Japan-specialized travel agent, like Keikaku, is an additional guarantee to get some support in case of a natural disaster. As a matter of fact, if you have bought your trip (flights ✈️ and accommodations) through a travel agency, said professional is responsible for your safety in case of a force majeure event, whereas when planning your trip alone, you will be on your own.

In any case, it is important to check the latest information before and during a trip, especially thanks to:

Advice to travelers provided by the Embassy of Japan, your country of origin’s Embassy in Japan, and the various expat networks to be warned in case of a major crisis in the country;
Apps providing real time information on earthquake such as:
- Safety tips by the JNTO (Japan National Tourism Organization);
- Yurekuru Call (ゆれくるコール) available on Android and Apple; as well as,
- NERV Disaster Prevention to download and set up on your smartphone.

The 1rst mega-quake alert lasted 1 week and was lifted on Thursday August 14, 2024: seismologists have not made unusual observation or seen abnormal seismic activity along the Nankai Through, such as an abnormal move of the Earth’s crust. Considering the long history and records of large scale earthquakes in Japan, other mega-quake alerts will certainly be issued by the JMA in the future, and they should not be ground to panic.

By Kanpai