National / Science & Health

Traces of previously unknown Mount Fuji eruptions uncovered at bottom of nearby lake

JIJI

Traces of two past unknown eruptions of Mount Fuji have been found in a layer at the bottom of Lake Motosu, northwest of the country’s tallest mountain, according to a research group that included Akita University.

The finding about the 3,776-meter volcano, a UNESCO World Heritage site, was recently reported in the online edition of a Dutch journal.

“Characteristics of past eruptions and precise data on intervals between them are useful for drawing up hazard maps,” Akita University professor Stephen Obrochta said.

Municipalities near the mountain that straddles Yamanashi and Shizuoka prefectures are working on revising hazard maps that will be utilized for evacuation plans in the event of an eruption.

Sediment at the bottom of deep lakes is relatively insulated from the effects of wind and waves and tends to contain plankton that can be used to estimate age.

The group, which also includes researchers from Yamagata University and the University of Tokyo, collected 4-meter-thick sediment from the bottom of the lake, covering some 8,000 years of geological layers.

The group found ash layers indicating two eruptions on the western flank of the mountain some 2,500 years ago. The eruptions, which had been unknown, happened about 20 years apart.

The group also found that eruptions believed to have occurred 3,400 and 3,200 years ago actually happened 200 to 300 years later. The areas hit by volcanic ash were wider than previously estimated, according to the group.