From Hokkaido in the north of Japan to Kyushu at the other end of the archipelago, the shaking started at 8:23 pm on Saturday. It began as a slight shaking of the ground before quickly building up to more violent movements. For anyone who experienced the magnitude 8.1 earthquake, it seemed to last a lot longer than the three minutes that were recorded by the Japan Meteorological Agency's instruments.

The tremor was the sixth-largest to strike Japan in the last 100 years and instantaneously brought back images of the devastation caused the length of the coastline of north-east Japan in March 2011 by the Great East Japan Earthquake.

This time, however, Japan got off very lightly. Emergency services reported a dozen minor injuries, hundreds of elevators halted, and people had to walk down staircases to leave buildings, trains were halted until track inspections could be carried out, while similar checks were carried out at Narita and Haneda airports.

No tsunami

With a focus 682 km below the surface, damage was negligible and the quake did not trigger a tsunami - but Japan is jittery because it struck the day after a volcano off the south coast erupted, and parts of the mountain town of Hakone are still off-limits due to volcanic activity. On May 25, an earthquake with a magnitude of 5.5 struck just north of Tokyo, shaking tall buildings in the city and disrupting train services.

For many people, four significant seismic events in the space of one week is cause for serious concern.

"After the 2011 Tohoku earthquake, the whole of the Japanese archipelago has been activated," Naoshi Hirata, a professor of earthquake science at The University of Tokyo, and an advisor to the government on the impact of major tremors, told DW.

"Immediately after the quake there was a lot of seismic activity, but that appeared to taper off," he said. "But we must remember that Japan is one of the most seismically active parts of the world, and we need to expect this degree of activity."

Experts admit that the sciences of both earthquakes and volcanoes are still imprecise, but the connections between the two are similarly poorly understood.

"The recent event could be partly connected," agreed Professor Hirata. "Immediately after March 2011, we saw increased seismic activity directly beneath the craters of several volcanoes. Fortunately, none of them erupted, but this does support the theory of a connection."

Warning levels raised

In recent months, the alert level for Mount Zao, on the borders of Yamagata and Miyagi prefectures in central Japan, has been raised and the meteorological agency is closely watching developments.

Infamously, Mount Ontake erupted without warning in September last year, killing 63 people. Authorities are about to resume the search for 12 people still listed as missing after their bodies were covered in as much as 1 meter of volcanic ash. The peak appears to have calmed down once again, but it remains off-limits to hikers.

The latest volcano to roar back into life is Mount Shindake, the peak that dominates the island of Kuchinoerabu, off southern Japan. The volcano experienced a minor eruption last year, but Friday's event was a Level 5 on Japan's five-grade scale and triggered the immediate evacuation of the 137 residents.

The plume of ash rose some nine km into the sky and lava flowed from the upper reaches of the 626-meter volcano, covering as much as 20 percent of the 36-square-kilometer island.

Thirty residents of the island were permitted to return to their homes on Monday for a couple of hours, accompanied by police and emergency teams aboard a Coast Guard patrol vessel and helicopters. They ensured electrical appliances were unplugged to prevent accidental fires and fed livestock that had been left behind.

'No undue alarm'

Professor Shigeo Aramaki, an authority on volcanoes who has retired from The University of Tokyo but still advises the government on natural disasters, concurs on the need to be vigilant in areas close to the 100 volcanoes across the country, which are listed by the government as active, but says there is no need for undue alarm.

"The point is that the Japanese islands have been extraordinarily quiet for the last half-century or so, meaning this could very well simply be a return to a normal situation," he told DW. "I wouldn't say I have no concerns, but we can't be too concerned."

"The problem is that the science of volcanoes and earthquakes is still not well understood, so it is impossible at the moment to say whether these separate incidents are connected or if it is merely coincidence," he added.