Northeast Japan escaped relatively unscathed from the magnitude 7.7 earthquake that struck off its coast on Monday, but residents have been advised to stay alert as it is too early to tell whether the tremor was the main shock or a precursor to a far larger one.

Hours after the quake, the Japan Meteorological Agency (JMA) issued a special advisory warning of a slightly elevated risk of a temblor with a magnitude of 8 or stronger in the coming days.

There was a 1 per cent chance of a megaquake, compared to 0.1 per cent at other times, it said, particularly close to the Chishima Trench, about 150km (93 miles) off the coast of Hokkaido, and the Japan Trench, which is further south and around 300km offshore.

However, the warning was only of increased risk, rather than a specific prediction, it said.

“Although the probability is low, there is a possibility of another major earthquake occurring,” the JMA said in a statement. “Therefore, please review your earthquake preparedness measures in these areas.”

Monday’s quake rattled large parts of eastern Japan and prompted tsunami warnings for coastal areas in Hokkaido, Iwate and Aomori prefectures, regions still marked by the March 2011 magnitude 9 quake and ensuing tsunami.

While initial estimates warned of three-metre (10-foot) tsunami waves, the largest wave to come ashore was only 80cm high and caused no damage, officials said. The warning was later lifted, permitting 182,000 evacuees to return to their homes.

There were also very few injuries. One woman in her eighties broke an arm while evacuating from her home in Hokkaido while a man, also in his eighties, fractured a leg in a fall in a supermarket.

Aftershocks recorded

Residents cannot afford to relax, though, as experts say the danger has not entirely passed, with 10 aftershocks recorded in virtually the same place on a known seismic fault line that runs parallel to the coast off the Tohoku region.

The tremors ranged from magnitude 4.4 to 5.4 offshore, although they were significantly weaker onshore.

In the weeks before March 11, 2011, experts point out, there were a series of foreshocks, including a 7.2-magnitude tremor two days before the most powerful earthquake ever recorded in Japan.

More than 22,000 people were killed or are still listed as missing from the disaster.

“There was some shaking here in the university in Sendai City and we are currently trying to determine the relationship between the quake and recent seismic activity in the region,” said Fumiaki Tomita, an assistant professor at Tohoku University’s research centre for the prediction of earthquakes and volcanic eruptions.

“We are trying to enhance our understanding of the mechanisms of these quakes so we can devise better countermeasures, but that is scientifically very difficult,” he said. “At present, seismologists cannot determine whether a quake is a precursor event or the main tremor.”

Despite the government alert and empirical evidence indicating a slightly higher chance of a major earthquake, “it is very difficult to guarantee that”, he said.

Monday’s quake occurred a day after a study by an urban disaster prevention expert warned that over 1 million people could be without homes for more than six months in the event of a megaquake striking Tokyo.

Keiichi Sato, a professor at Senshu University, used government statistics assuming a major earthquake beneath the capital and called for greater preparations for such a disaster.

The government has advised residents to take precautions over the coming days, including by having emergency stocks of non-perishable food and water, as well as basic first aid equipment, torches and spare batteries.

Those in areas most at risk in northeast Japan, such as coastal communities, are also being instructed to ensure their vehicles have sufficient fuel in case they need to evacuate quickly.

“These steps are really small and simple, but I believe they would be very effective in the event of a natural disaster,” Tomita said.

His own preparations include rechecking emergency supplies at home on Monday evening and verifying that sufficient equipment and supplies were on hand at the university on Tuesday morning.