"Impending Magnitude 9 Megaquake" Warning of a "Once-in-400-Years Disaster"...What Is Happening in Japan

Researchers from Tohoku University and Hokkaido University announce findings
"Possibility of an ultra-large earthquake after 400 years"

A study has found that there is a possibility that an ultra-large earthquake and tsunami similar to those of the 17th century could recur along the Pacific coast of Hokkaido, Japan. Based on observations of crustal movements on the seafloor, the researchers analyzed that an amount of energy comparable to that of past giant earthquakes has already accumulated.

A road is split in Abira, Hokkaido, Japan, where a magnitude-6.7 earthquake struck on September 6, 2018. EPA/Yonhap News

According to the Mainichi Shimbun on February 23, a joint research team from Tohoku University, Hokkaido University, and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) published these findings recently in the international journal "Communications Earth & Environment."

The researchers identified the Kuril Trench (Chishima Trench) area to the east of Hokkaido as a strong candidate region for the next giant earthquake. The Kuril Trench is a zone where the Pacific Plate rapidly subducts beneath the Okhotsk Plate, and it is historically known as a region where magnitude-8 to -9 earthquakes and large tsunamis have repeatedly occurred.

Energy buildup similar to a magnitude-8.8 earthquake in the 17th century

The Tohoku University research team estimated that ultra-large earthquakes have repeatedly occurred along the Pacific coast of Hokkaido in cycles of about 400 years. The last major earthquake is believed to have been a magnitude-approximately-8.8 event that occurred sometime between 1611 and 1637, and the tsunami it triggered is said to have inundated areas up to 1 to 4 kilometers inland from the coast.

From 2019 to 2024, the researchers installed three observation instruments on the seafloor in the Nemuro offshore area to precisely measure crustal deformation. As a result, it was found that both the Pacific Plate and the continental plate near the trench are moving toward the west-northwest by about 8 centimeters per year.

If such deformation has continued to accumulate since the 17th century, the total displacement of the Pacific Plate is estimated to reach 20.5 to 30 meters. Considering that the plate-boundary slip during the 17th-century giant earthquake was about 25 meters, the researchers suggest that stress sufficient to trigger an earthquake of similar magnitude may already have accumulated.

On December 9 last year in Aomori Prefecture, Japan, a vehicle lies in a roadway that had subsided due to an earthquake. EPA/Yonhap News

Similar to the 'seismic gap' before the Great East Japan Earthquake

The Mainichi Shimbun pointed out that, prior to the 2011 Great East Japan Earthquake, there had also been a "seismic gap" off the coast of Miyagi Prefecture near the Japan Trench, where no large earthquakes had occurred. The magnitude-9.0 earthquake at that time caused a massive tsunami as the fault slipped on a large scale. The region targeted in this latest study is likewise a segment where no large ruptures have occurred for a long period, and analysts warn that if the fault collapses all at once in the future, a large-scale tsunami could follow.

Fumiaki Tomita, an associate professor at Tohoku University, told the Mainichi, "Along the coast of Hokkaido, we expect a very large tsunami of up to about 20 meters," and urged, "I hope people will live with an awareness of the risk that a giant earthquake is certain to occur in the future."

Japanese government raises probability of a major Nemuro offshore quake to 90%

The Japanese government’s Earthquake Research Committee also recently raised the probability that a magnitude-7.8 to -8.5 earthquake will occur off the eastern coast of Hokkaido near Nemuro within the next 30 years from the previous "around 80%" to "around 90%." In this offshore area, large earthquakes have occurred at average intervals of about 65 years, and more than 50 years have already passed since the last major quake, which was reflected in the revision. Earthquake activity continues in the waters of northeastern Japan, including a magnitude-7.5 earthquake that struck Aomori Prefecture, facing Hokkaido, in December last year.

The interior of a building in Hachinohe, Aomori Prefecture, is shaking due to an earthquake that occurred off the coast of Aomori Prefecture on December 12 of last year. QAB

The projected 30-year probability of a Nankai Trough megaquake, expected to occur from off the coast of Shizuoka Prefecture west of the Tokyo metropolitan area through the southern part of Shikoku to the eastern part of Kyushu, remains unchanged from previous estimates (60% to more than 90%, or 20% to 50%). Historically, magnitude-8 to -9 earthquakes have repeatedly occurred along the Nankai Trough at intervals of 100 to 200 years. Experts noted that both the Kuril Trench and the Nankai Trough are zones of long-term stress accumulation, and stressed the need for disaster-prevention measures and strengthened early-warning systems that take their structural characteristics into account.

This content was produced with the assistance of AI translation services.