A powerful earthquake struck in waters off northeastern Japan on Monday afternoon, the authorities said, prompting tsunami warnings along the coastline that were later downgraded.
While there were no reports of casualties or significant damage, officials warned that there was a slightly elevated risk of a so-called megaquake — a far larger and more destructive earthquake — over the next week.
The earthquake on Monday, which occurred at 4:52 p.m. local time, had a magnitude of 7.7, according to the Japan Meteorological Agency, and could be felt several hundred miles away in Tokyo. The authorities in Japan initially issued tsunami warnings for waves as high as three meters, or about 10 feet, in parts of Iwate, the neighboring prefecture of Aomori, and the northern island of Hokkaido. By Monday night, those warnings were downgraded to advisories, with waves of up to one meter, or three feet, expected.
Videos posted on social media showed electrical lines swaying and pots and pans rattling inside a kitchen. Only a handful of injuries were reported initially and there were scattered reports of minor damage to buildings.
Prime Minister Sanae Takaichi of Japan said in a post on X that residents in areas with tsunami warnings should “immediately evacuate to higher ground or evacuation buildings and other higher, safer locations.” She said the government had established a crisis management center to monitor potential damage, coordinate emergency measures and provide information to the public.
The chance of a megaquake — defined as those with a magnitude of more than 8 — is typically about 0.1 percent. But in light of Monday’s earthquake, the odds have risen to approximately 1 percent over the next week, the authorities said. While that is a small increase, experts say it is meaningful.
“While it is uncertain whether a large-scale earthquake will actually occur, we ask that you take disaster prevention measures with the understanding that you must protect your own life,” Yugo Masuya, a cabinet official, said in a statement.
Coastal towns in several prefectures issued evacuation orders on Monday affecting tens of thousands of people. Residents were urged to stay out of the water in the affected areas. At the Kuji Port in Iwate, tsunami waves about 31 inches high were detected hitting the shore at 5:34 p.m.
Shinji Kiyomoto, an official with the Japan Meteorological Agency, warned that quakes of a similar scale could occur over the next week. He said at a news conference that people should not return to low-lying areas until evacuation orders are lifted.
Several nuclear power plant operators in the region said there were no abnormalities detected in the aftermath of the quake.
Train service between Tokyo and Aomori was suspended.
The Japan Meteorological Agency initially estimated the earthquake’s magnitude as 7.4, but later revised it to 7.7. (The United States Geological Survey listed the quake’s magnitude as 7.4.) The earthquake’s depth was fairly shallow, at about 20 kilometers, or about 12 miles, according to the Japan Meteorological Agency. Shallow quakes tend to be more destructive than deeper ones.
In December, the area experienced a 7.6 magnitude earthquake, which struck off the coast of Aomori Prefecture. That quake did not cause major damage but left dozens injured.
Kiuko Notoya and Hisako Ueno contributed reporting from Tokyo.
