TOKYO (Reuters) - Japan hiked the volcanic warning level on one southern island to the second highest on Wednesday, mandating that residents should prepare for evacuation, though an actual eruption at the peak had yet to be detected.

Japan’s Meteorological Agency said that volcanic earthquakes and sulfur emissions had increased at a peak on Kuchinoerabujima island, located roughly 1,000 km (621 miles) southwest of Tokyo, and raised its warning level to 4 out of 5.

The mountain erupted explosively three years ago, sending ash and smoke thousands of meters into the sky and releasing potentially deadly pyroclastic flows, flows of super-heated ash and gas, that reached the sea.

At that time, the island’s entire population evacuated but eventually returned. Some 100 people live there at present.

Japan has 110 active volcanoes and monitors 47 constantly. When 63 people were killed in the volcanic eruption of Mount Ontake in September 2014, it was the country’s worst such toll for nearly 90 years.

In January, a member of Japan’s military was struck and killed when rocks from a volcanic eruption rained down on skiers at a central mountain resort.