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Japan raised an eruption warning for a volcano on the southern island of Kyushu to the second-highest level, days after restarting a nuclear reactor 50 kilometers (31 miles) away.

The Japan Meteorological Agency increased the alert level for the volcano Sakurajima to 4 from 3, advising people within 3 kilometers of the crater to prepare to evacuate. It detected multiple earthquakes in the area on Saturday, the agency said on its website.

Kyushu Electric Power Co. began bringing online the No. 1 reactor at its Sendai power station on Aug. 11, the first nuclear facility to restart in Japan under new safety rules implemented following the 2011 Fukushima disaster.

The area around the volcano had registered more than 800 tremors as of 2:15 p.m. local time, according to the meteorological agency’s website. Some 77 residents were advised to evacuate, and 24 had done so as of 3:50 p.m., the Fire and Disaster Management Agency said in a statement.

Kyushu Electric’s Sendai plant is operating normally, a company spokeswoman said by phone Saturday, asking not to be named in accordance with the Fukuoka-based company’s policy.

Volcanic rock and ash could cut off transport routes and prompt workers at Sendai to flee the nuclear site in the event of an eruption, putting operations at risk, engineering consultant John Large wrote in a February report commissioned by Greenpeace.

Japan lies on the so-called “Ring of Fire,” an arc of volcanoes and fault lines surrounding the Pacific Basin, and it sits at the three-way meeting point of the North American, Eurasian and Philippine Sea tectonic plates.

Authorities ordered the complete evacuation of Kuchinoerabu island off Kyushu in May. More than 50 people, mostly hikers, were killed in an eruption at Mount Ontake in Nagano in central Japan last September.