Experts Warn of Looming Super Nankai Earthquake in Echo of Past Disasters

The “eerie tremors caused by a slow-slip earthquake off the coast of Chiba Prefecture have been occurring one after another. Even before the Great East Japan Earthquake in March 2011, there were many slow-slip earthquakes near the epicenter. There is a possibility that this could be the fuse for a huge disaster.

Dr. Yoshinobu Tsuji, former associate professor at the Seismological Research Institute of the University of Tokyo and visiting researcher at the Fukada Geological Research Institute, sounded the alarm.

Slow slip is the slow displacement of a fault at a plate boundary. If small displacements occur repeatedly, they can cause major earthquakes. Off the coast of Chiba Prefecture, more than 25 earthquakes of intensity 1 or higher have occurred since late February, and it is estimated that the seafloor has moved 2 cm to the southeast. Mr. Toji continues.

If the epicenter moves further to the southeast, we need to be careful. It will stimulate the Sagami Trough (extending from off Kanagawa Prefecture to the Pacific Ocean), which can cause an earthquake directly under the Tokyo metropolitan area.

It is not only off the coast of Chiba Prefecture that should be watched closely. In January this year, the Noto Peninsula was hit by a major earthquake with a maximum intensity of 7 on the Japanese seismic scale, and in February, Hiroshima and Ehime prefectures were hit by major tremors with intensity 4 on the Japanese seismic scale. Manabu Takahashi, an expert on natural disasters and a specially-appointed professor at Ritsumeikan University’s Research Center for Pacific Rim Civilizations, discusses the causes of these earthquakes.

Since the Great East Japan Earthquake, the movement of the Pacific Plate to the east of Japan has become more active. The Pacific Plate is putting pressure on the North American Plate and the Philippine Sea Plate, which are right next to it. These two plates are putting further pressure on the Eurasian plate, creating large strains in various regions. The reason why earthquakes are occurring so frequently in Japan is because we can no longer withstand the strain.

Tsunami speed is several tens of kilometers per hour

The Nankai Trough earthquake, with a magnitude of 8-9 on the Richter scale, is a cause for concern, with the government putting the probability of occurrence at 70-80% over the next 30 years. This is said to occur when the strain accumulated at the boundary between the Philippine Sea Plate and the Eurasian Plate reaches its limit.

Since the Great East Japan Earthquake, strain has been accumulating at the boundary between the Philippine Sea Plate and the Eurasian Plate, where the Nankai Trough is located, and there is no doubt that the danger of a massive earthquake is approaching,” said Takahashi.

The three maps shown here are “critical realities” based on the damage assumptions of the Nankai Trough earthquake calculated by the Cabinet Office. The three maps show the “critical reality” based on the Cabinet Office’s calculations of damage from a Nankai Trough earthquake: a tremor with a maximum intensity of 7 on the Japanese seismic scale, a huge tsunami with a height of over 20 meters, and raging fires that could start from damaged appliances and wiring. In the worst case scenario, nearly 110,000 people are expected to die in Shizuoka Prefecture alone.

Particularly alarming is the tsunami.’ In the Hokkaido Southwest Offshore Earthquake of July 1993, a huge tsunami with a maximum run-up height of 30.6 meters reached the west coast of Okushiri Island only three minutes after the earthquake. Yoshiaki Kawata, a specially appointed professor at Kansai University who specializes in natural disasters, said.

Tsunamis from a Nankai earthquake will strike at several tens of kilometers per hour, even if the sea becomes shallow. And not just once. It is known to hit several times within a six-hour period, at intervals of about 50 minutes. Especially at high tide, the tsunami will be higher, so you need to be careful. It is dangerous for people living in urban centers such as Osaka to think that this is a coastal disaster and has nothing to do with them. About two hours after the earthquake, a tsunami reached the center of Osaka City. The underground shopping center will be submerged, and up to 40,000 people may die if they do not evacuate in advance.

The aforementioned Mr. Takahashi also urges caution about tsunamis.

The Sumatra earthquake that hit western Indonesia in December 2004 caused a tsunami that circled the globe three times. And since tsunamis occur in an ocean that has been swollen by the first tsunami, the second and subsequent tsunamis tend to be higher.

In the Noto earthquake, many villages were isolated due to the severing of roads. The situation will be even more dire in the event of a Nankai Trough earthquake.

There are many villages in the wide area affected by the disaster that are located in the mountains or have poor access to public transportation. Many of them will become isolated islands on land due to landslides, and there will be a large number of victims who cannot be immediately assisted by the Self-Defense Forces or other means,” Kawata said.

The maximum death toll from a Nankai Trough earthquake, as assumed by the government, is more than 320,000. The damage, however, is likely to far exceed that number.

Furthermore, as mentioned at the beginning of this article, the Sagami Trough in the Kanto region is also in danger due to a series of earthquakes off the coast of Chiba Prefecture.

If an earthquake occurs in the Sagami Trough, it will stimulate the Nankai Trough, which is right next to it. There is a possibility of a ‘Super Nankai Earthquake,’ in which huge tremors will occur in tandem from the Kanto region to the Kyushu and Okinawa regions. The damage is not limited to the government’s assumptions. The death toll could exceed 500,000. People living in the Tokyo metropolitan area are no strangers to this kind of thing.

The probability of the Great Hanshin-Awaji Earthquake of January 1995 was said to be 0.02-8%, and that of the Noto Earthquake about 5%. Compare this to the approximately 80% probability of a Nankai Trough earthquake occurring within 30 years, and you can see that a crisis is imminent.

There is a lot that we don’t know about huge earthquakes from current research,” he said. Because we don’t know, we should not be optimistic. We must always exercise the utmost caution and precaution,” Kawata said.

It is too late to panic after an earthquake has occurred. Check evacuation routes and stockpile emergency supplies at ……. We must not neglect our daily earthquake preparedness.

From the March 22, 2024 issue of FRIDAY