TOKYO (AP) - A powerful 7.5-magnitude earthquake struck off northern Japan late Monday, injuring more than 20 people and triggering a tsunami of up to 70 centimeters (28 inches) in Pacific coast communities, officials said. The Japan Meteorological Agency is warning of possible aftershocks and increased risk of a megaquake.

The Japanese government was still assessing damages from the tsunami and late-evening quake, which struck at about 11:15 p.m. (1415 GMT) in the Pacific Ocean around 80 kilometers (50 miles) off the coast of Aomori, the northernmost prefecture of Japan’s main Honshu island. 

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

A tsunami of 70 centimeters was measured in Kuji port in Iwate prefecture, just south of Aomori, and tsunami levels of up to 50 centimeters struck other coastal communities in the region, the Japan Meteorological Agency said. 

The Fire and Disaster Management Agency said 23 people were injured, including one seriously. Most of the victims were hit by falling objects, NHK reported, adding that several people were injured in a hotel in Hachinohe and a man in Tohoku was slightly hurt when his car fell into a hole.

The meteorological agency reported the quake's magnitude as 7.5, down from its earlier estimate of 7.6. It issued an alert for potential tsunami surges of up to 3 meters (10 feet) in some areas and later downgraded to an advisory.

Chief Cabinet Secretary Minoru Kihara urged residents to go to higher ground or seek shelter until advisories were lifted. He said about 800 homes were without electricity, and that the Shinkansen bullet trains and some local lines were suspended in parts of the region.

Nuclear power plants in the region were conducting safety checks, Kihara said. The Nuclear Regulation Authority said about 450 liters (118 gallons) of water spilled from a spent fuel cooling area at the Rokkasho fuel reprocessing plant in Aomori, but that its water level remained within the normal range and there was no safety concern. 

About 480 residents were taking shelter at the Hachinohe Air Base, and 18 defense helicopters were mobilized for a damage assessment, Defense Minister Shinjiro Koizumi said.

About 200 passengers were stranded for the night at New Chitose Airport in Hokkaido, NHK reported. 

The meteorological agency issued a caution about possible aftershocks in the coming days. It said there is a slight increase in risk of a magnitude 8-level quake and possible tsunami occurring along Japan's northeastern coast from Chiba, just east of Tokyo, to Hokkaido. The agency urged residents in 182 municipalities in the area to monitor their emergency preparedness in the coming week.

Prime Minister Sanae Takaichi said in brief comments to reporters that the government set up an emergency task force to urgently assess the extent of damage. “We are putting people’s lives first and doing everything we can,” she said. 

Later, she urged residents in the region to pay attention to the latest information from local municipalities. “Please be prepared so you can immediately evacuate as soon as you feel a tremor."

The quake struck about 80 kilometers (50 miles) northeast of Hachinohe, and about 50 kilometers (30 miles) below the sea surface, the meteorological agency said. 

It was just north of the Japanese coast that suffered the magnitude 9.0 quake and tsunami in 2011 that killed nearly 20,000 people and destroyed the Fukushima Daiichi nuclear power plant. 

“You need to prepare, assuming that a disaster like that could happen again," the meteorological agency's earthquake and volcano division official Satoshi Harada said.