Megaquake Advisory Issued After M7.5 Quake Hits Aomori

A magnitude 7.5 earthquake struck the Aomori region at around 11:15 PM on December 8. The quake registered a lower 6 on the Japanese seismic intensity scale in the Sanpachi and Kamikita area in the southeastern part of the prefecture. According to the Japan Meteorological Agency (JMA), the epicenter was located offshore east of Aomori. The depth was about 50 kilometers.

The agency issued tsunami warnings for the central Pacific coast of Hokkaido, the Pacific coast of Aomori Prefecture, and Iwate Prefecture. Estimated wave heights were up to 3 meters.

By midnight on December 9, tsunami waves of up to 40 centimeters had been observed in parts of Hokkaido and Aomori. As of Tuesday morning, all tsunami advisories for the Pacific coastline in northern Japan were lifted, according to NHK.

*A road that collapsed in Tohoku Town, Aomori Prefecture, after the earthquake — 10:41 AM on December 9.*

Nuclear Plants Report No Problems

According to the Nuclear Regulation Authority and other agencies, no abnormalities were reported at following nuclear plants:

Tomari Nuclear Power Plant in Hokkaido
Higashidori Nuclear Power Station in Aomori
Onagawa Nuclear Power Station in Miyagi
Fukushima Daiichi Nuclear Power Station in Fukushima

JR East said services in both directions on the Tohoku Shinkansen were suspended between Fukushima and Shin-Aomori.

Risk of Megaquake Rises

Early on December 9, the government issued a Hokkaido-Sanriku Offshore Subsequent Earthquake Advisory. It said that the relative likelihood of a major earthquake had increased compared with normal conditions. The Sanriku Coast refers to coastal areas spanning parts of Aomori, Iwate, and Miyagi prefectures.

*Japan Meteorological Agency and Cabinet Office officials announce the Hokkaido–Sanriku Offshore Subsequent Earthquake Advisory at a news conference at 2:07 AM on December 9, at the JMA.*

JMA's advisory warns of the possible occurrence of a massive earthquake of magnitude 8 or higher along the Japan Trench and the Kuril Trench, offshore of northern Japan. The advisory is equivalent to the Major Earthquake Advisory issued in August 2024 under the Nankai Trough Earthquake Temporary Information framework.

Based on global earthquake statistics, JMA said the probability of a magnitude 8 class or larger earthquake occurring within seven days has increased from about one in 1,000 (0.01%) to about one in 100 (1%).

Over the next week, residents are urged to review their disaster preparedness. They are also asked to be ready to evacuate quickly if necessary.

Risk Along Major Offshore Trenches

Both trenches covered by the advisory lie along zones where the Pacific Plate is subducting beneath the continental plate. The Japanese archipelago rests on the continental plate. As strain builds up through this process, it is periodically released through massive earthquakes of magnitude 8 or higher. These quakes occur either at the plate boundary or along faults within the plate.

Historically, between 1904 and 2017, about one out of every 25 earthquakes of magnitude 7.0 or greater in this region was followed by a massive earthquake of magnitude 8 or higher. One example is the 2011 Great East Japan Earthquake. It struck two days after a magnitude 7.3 quake occurred nearby.

The section of the Japan Trench off the Sanriku coast includes areas where the fault did not slip during the 2011 disaster. Estimates by the government suggest that if a worst case major earthquake were to occur in this region, it could reach a magnitude of 9.1. This would exceed the scale of the 2011 earthquake.

Past Patterns of Major Quakes

Along the Kuril Trench, massive M8-plus earthquakes have occurred repeatedly. These include the Tokachi oki earthquakes of 1952 and 2003.

In recent years, tsunami deposits discovered along the Pacific coast of Hokkaido have shown that enormous earthquakes of around magnitude 8.8 or greater have occurred at average intervals of 340 to 380 years. The most recent of these occurred in the 17th century. Nearly 400 years have now passed, and the risk is considered increasingly urgent.

Based on past cases, the JMA has said that such advisories are expected to be issued about once every two years on average. It also emphasized that these advisories do not predict when a massive earthquake will actually occur.

(Read the article in Japanese.)

Author: The Sankei Shimbun