A powerful 7.5-magnitude earthquake has shaken north-eastern Japan, injuring more than 20 people and triggering a tsunami of up to 70cm in Pacific coast communities.

The earthquake and tsunami warnings prompted orders for about 90,000 residents to evacuate, although the warnings were later downgraded to advisories.

Tsunami warnings were issued for the prefectures of Hokkaido, Aomori and Iwate, and tsunamis from 20 to 70cm (7 to 27in) high were observed at several ports, the Japan Meteorological Society (JMA) said.

A tsunami of 70cm was measured in Kuji port in Iwate prefecture, just south of Aomori, and tsunami levels of up to 50cm struck other coastal communities in the region, they added.

The fire and disaster management agency said 23 people were injured, including one seriously. Most of the victims were hit by falling objects, public broadcaster 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.

Satoshi Kato, a vice-principal of a high school in Hachinohe, told the public broadcaster NHK that he was at home when the quake struck, and that glasses and bowls fell and smashed into shards on the floor.

Kato said he drove to the school because it was designated an evacuation centre, and on the way he encountered traffic jams and car accidents as panicked people tried to flee. Nobody had yet come to the school to take shelter, he said.

About 480 residents were taking shelter at the Hachinohe airbase, and 18 defence helicopters were mobilised for a damage assessment, defense minister Shinjirō Koizumi said.

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

The epicentre of the quake was 50 miles (80km) off the coast of Aomori prefecture, at a depth of 30 miles, the agency added.

On Japan’s one-to-seven scale of seismic intensity, the tremor registered as an “upper six” in Aomori prefecture – a quake strong enough to make it impossible to keep standing or move without crawling. In such tremors, most heavy furniture can collapse and wall tiles and windowpanes are damaged in many buildings.

East Japan Railway suspended some services in the area, which was also hit by the massive 9.0-magnitude quake in March 2011.

No irregularities were reported at nuclear power plants in the region run by Tohoku Electric Power and Hokkaido Electric Power, the utilities said. Tohoku Electric did say, however, that thousands were without power.

Japan is one of the world’s most earthquake-prone countries, with a tremor occurring at least every five minutes. Located in the “Ring of Fire” arc of volcanoes and oceanic trenches partly encircling the Pacific Basin, Japan accounts for about 20% of the world’s earthquakes of magnitude 6.0 or greater.