© KyodoAn aerial view shows Mt. Sakurajima in Kagoshima, southwestern Japan.
Level 4 is the highest ever for Sakurajima, located just 4 kilometres away from the centre of the major southern port city of Kagoshima, since the current volcanic alert system was launched in 2007.
Kyushu Electric Power's Sendai nuclear power plant, which Tuesday became the first to be rebooted under tighter safety rules adopted in the wake of the 2011 Fukushima nuclear plant disaster, is located just 50km from the volcano. No abnormality has been reported at the plant.
The alert was raised to 4, meaning people living in the area should prepare to evacuate, from 3, which restricts entry to the mountain area. In response, the Kagoshima city government issued an evacuation advisory for areas near the mountain, affecting 77 residents, all of whom left the areas by evening.
Many earthquakes originating from the volcano have been recorded since 7am Saturday and an increase in crustal movement was also observed, the Japan Meteorological Agency said, adding those are signs indicating magma has risen to near the volcanic vent.
The agency warned of the possibility of an eruption emitting large amounts of ash and pyroclastic flows within 3 km from Sakurajima's Showa vent and the Minamidake summit.
"It would be no surprise if it were to erupt at any moment," an agency official said.
The meteorological agency believes an eruption of similar magnitude to one in 1986 may occur. Six people were injured by the 1986 eruption which emitted large amounts of volcanic ash that fell on residential areas up to 3km away.
But the agency has not observed signs indicating a surge of magma equivalent to an eruption in 1914, in which 58 people died or went missing. Sakurajima was an island until the 1914 eruption, which emitted so much material as to join the volcano to Kyushu.
There have been some 690 eruptions from the Showa vent of the 1,117-metre volcano since the start of this year, with the plume reaching an altitude of some 4,300 metres on May 21.
The volcano last had a major eruption in August 2013, spewing volcanic ash 5,000 metres into the sky.
Evacuees moved to evacuation centres set up by the city government or other places including relatives' homes.
Residents carrying luggage were seen arrived at the centres after the evacuation advisory was issued.
"I'm concerned if an eruption may damage my home," said Yoshiko Ikeda, 87.
"I have lived in Sakurajima for more than 50 years but have not imagined we would have to evacuate," Emiko Miyashita, 80, said.
The city government displayed the alert in Japanese and English in electric boards at the ferry landing as foreigners in the area may not have reached the information.
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