Mount Fuji may have erupted more frequently than previously thought, a joint research team from the University of Tokyo and the Mount Fuji Research Institute announced Friday, which could potentially impact disaster preparedness in eastern and central Japan.

“This is an important discovery, since understanding the frequency of volcano eruption can contribute to disaster prevention,” said Yusuke Yokoyama, a professor at The University of Tokyo and one of the team's researchers.

The team, led by Shinya Yamamoto from the Mount Fuji Research Institute, discovered for the first time that there were six eruptions between 5,050 and 3,900 years ago — a period of time that was previously thought to have seen little volcanic activity at Japan's highest peak.

Major eruptions in the past can be detected by studying a layer of volcanic ash, called a tephra layer, found in sediment. The team decided to focus on studying the sediment from Lake Yamanaka in Yamanashi Prefecture, since sediment found in lakes is less disrupted by erosion and thus yields more reliable information.

Using 8,000 years' worth of sediment that was collected from the bottom of Lake Yamanaka in 1998, the team studied the 29 tephra layers present in the sample, while also comparing it to the layers found in the 20-meter hole they dug beside the lake.

Using world-class radiocarbon dating technology, the team managed to date six of the previously undated tephra layers as being from the period when Mount Fuji was previously thought to be dormant.

“If there was a (dormant) period, we can understand Mount Fuji as having periods of rest and periods of activity that it fluctuates between,” Yokoyama said. “Finding out that it has erupted more frequently, we must work on disaster prevention plans accordingly.”

Mount Fuji has been dormant since 1707, but experts say it could erupt again at any time.

The team began researching lakes around Mount Fuji over five years ago and conducted their first study in Lake Motosu. With this new data from Lake Yamanaka, they are closer to better understanding Mount Fuji’s volcanic history — but they’re still just getting started.

Yokoyama says that the team will further their research in Lakes Kawaguchi, Sai and Shoji, the three other lakes that make up the Fuji Five Lakes. By expanding the coverage and knowledge of the area, they believe that they can get a better understanding of Mount Fuji’s eruption tendencies.

“For example, maybe there is an event where it only erupts on the west side, in which case we can evacuate to the east side,” said Yokoyama.

With more research, “We can devise a more intricate disaster prevention plan and understand the evacuation route.”