Tokyo. A powerful magnitude 7.5 earthquake struck off northern Japan late Monday, injuring 23 people and triggering a small tsunami along the Pacific coast, officials said. Authorities warned of possible aftershocks and an increased risk of a larger quake.

The government was still assessing the damage from the late-evening temblor and tsunami. The quake struck at about 11:15 p.m. in the Pacific Ocean, roughly 80 kilometers (50 miles) off the coast of Aomori, the northernmost prefecture on Japan’s main Honshu island.

“I’ve never experienced such a big shaking,” convenience store owner Nobuo Yamada told public broadcaster NHK from Hachinohe in Aomori prefecture, adding that power lines in his area were still functioning.

A tsunami of up to 70 centimeters (2 feet, 4 inches) was recorded at Kuji port in Iwate prefecture, just south of Aomori, while waves of up to 50 centimeters hit other coastal communities, the Japan Meteorological Agency said.

The Fire and Disaster Management Agency said 23 people were injured, including one seriously. NHK reported most injuries were caused by falling objects. Several people were hurt at a hotel in Hachinohe, and a man in Tohoku suffered minor injuries when his car fell into a hole.

The meteorological agency revised the quake’s magnitude to 7.5 from an earlier estimate of 7.6. It initially issued an alert for tsunami waves of up to 3 meters (10 feet) in some areas before downgrading it to an advisory.

Chief Cabinet Secretary Minoru Kihara urged residents to move to higher ground or seek shelter until advisories were lifted. He said about 800 homes lost electricity, and that Shinkansen bullet trains and some local rail lines were suspended in parts of the region.

Nuclear power plants in the area conducted safety checks. The Nuclear Regulation Authority said about 450 liters (118 gallons) of water spilled from a spent fuel cooling pool at the Rokkasho reprocessing plant in Aomori, but water levels remained within normal range and posed no safety threat.

About 480 residents took shelter at the Hachinohe Air Base, and 18 defense helicopters were deployed for damage assessments, Defense Minister Shinjiro Koizumi said.

NHK reported about 200 passengers were stranded overnight at New Chitose Airport in Hokkaido.

The meteorological agency warned that aftershocks could continue for days and said there was a slightly increased risk of a magnitude 8-level quake along Japan’s northeastern coast, from Chiba east of Tokyo to Hokkaido. It urged residents in 182 municipalities to review their emergency preparedness in the coming week.

Satoshi Kato, a vice principal at a high school in Hachinohe, told NHK he was at home when the quake struck, and that cups and bowls shattered on the floor. He said he drove to the school, which serves as an evacuation center, but encountered traffic jams and accidents as panicked residents tried to flee. No evacuees had arrived at the school as of late Monday.

Prime Minister Sanae Takaichi told reporters the government had set up an emergency task force to assess the damage. “We are putting people’s lives first and doing everything we can,” she said. Later, she urged residents to monitor updates from local authorities and be ready to evacuate immediately if they feel strong shaking.

The quake struck about 80 kilometers (50 miles) northeast of Hachinohe at a depth of 50 kilometers (30 miles), the meteorological agency said.

The epicenter was just north of the area devastated by the 2011 magnitude 9.0 quake and tsunami that killed nearly 20,000 people and triggered the Fukushima Daiichi nuclear disaster.

“You need to prepare, assuming that a disaster like that could happen again,” said Satoshi Harada of the agency’s earthquake and volcano division.

Authorities lifted all tsunami advisories for northern Japan’s Pacific coastline at 6:20 a.m. Tuesday, NHK said.

The US Geological Survey reported a separate magnitude 5.1 quake early Tuesday, about 122 kilometers (76 miles) south of Honcho, at a depth of 35 kilometers. No additional details were immediately available.