(BSS/AFP) - A big quake off northern Japan left at least 30 injured, authorities said Tuesday, damaging roads and leaving thousands without power in freezing temperatures.

The Japan Meteorological Agency said the magnitude 7.5 quake at 11:15 pm on Monday (1415 GMT) -- downgraded from 7.6 initially -- raised the chances of similar or larger tremors in the coming days.

Prime Minister Sanae Takaichi said 30 people were injured in the quake off the coast of the Aamori region, which triggered tsunami waves up to 70 centimetres (28 inches) high.

The injured included one person seriously hurt in the main northern island of Hokkaido, according to the Fire and Disaster Management Agency.

Footage showed several crevasses in roads and at least one car in a hole, with broken glass from windows scattered on roads and pavements.

Initially there were reports of several fires. Government spokesman Minoru Kihara said Tuesday that was one confirmed blaze at a house.

In Hokkaido, an AFP reporter said the ground shook violently for around 30 seconds as smartphone alarms rang to alert residents.

Daiki Shimohata, 33, a civil servant in Hashikami in the Aomori region on Honshu island, told AFP that he and his family rushed outside their home.

"The tremor was something that we`ve never experienced. It lasted maybe for about 20 seconds," Shimohata said by phone.

"We were holding our children -- a two-year-old girl and a one-year-old boy -- in our arms. The shaking reminded me of the disaster (in 2011)," he said.

Some 28,000 people were advised after the quake to evacuate from their homes, emergency services said, and media reports said some makeshift shelters were full.

With temperatures around freezing point, some 2,700 homes in Aomori were without power, Kyodo News reported in the hours after the quake.

But by Tuesday morning electricity had been restored to most areas, with fewer than 40 homes still without power, according to utility providers.

At first the JMA warned of tsunamis up to three metres (10 feet), which could have caused major damage, and thousands of residents were urged to go to safe places.

In the end the biggest waves recorded measured up to 70 centimetres and after several hours the warnings were lifted.

Shinkansen bullet-train service was suspended in some areas while engineers checked for any damage to the tracks.

No abnormalities were detected at the Higashidori nuclear power plant in Aomori or the Onagawa facility in nearby Miyagi, operator Tohoku Electric Power said.

Takaichi early Tuesday urged residents to be careful.

"Please listen to information from the JMA or local governments for about a week and check if furniture is fixed .... and be prepared to evacuate when you feel shaking," she said.

- `Megaquake` -

In 2011, a magnitude-9.0 quake triggered a tsunami that left 18,500 people dead or missing and caused a devastating meltdown at the Fukushima nuclear plant.

Japan sits on top of four major tectonic plates along the western edge of the Pacific "Ring of Fire" and is one of the world`s most tectonically active countries.

The archipelago, home to around 125 million people, experiences around 1,500 jolts every year.

The vast majority are mild, although the damage they cause varies according to their location and depth below the Earth`s surface.

Quakes are extremely hard to predict, but in January a government panel marginally increased the probability of a major jolt in the Nankai Trough off Japan in the next 30 years to 75-82 percent.

The government then released a new estimate in March saying that such a "megaquake" and subsequent tsunami could cause as many as 298,000 deaths and damages of up to $2 trillion.