Photo/IllutrationThis image of an area surrounding Mount Moto-Shiranesan in Gunma Prefecture shows the vents of the Jan. 23 eruption (blue) and likely traces of vents from similar events (purple and green). (Prepared by the Geospatial Information Authority of Japan and Asia Air Survey Co.)

  • Photo/Illustraion
  • Photo/Illustraion

A volcano in Gunma Prefecture that erupted Jan. 23, killing one and injuring 11, likely had many more such events than previously thought, volcanologists said.

Researchers with the Geospatial Information Authority of Japan (GSI) and the National Institute of Advanced Industrial Science and Technology (AIST) identified traces of similar eruptions that occurred repeatedly at Mount Moto-Shiranesan, one of the peaks that constitute the Mount Kusatsu-Shiranesan volcano complex.

The January eruption formed more than one string of volcanic vents, which stretched a maximum of 500 meters.

Vents of that type are relatively small and are covered under trees with the passage of time. That makes them difficult to locate through conventional aerial photography.

The latest eruption prompted the researchers to scrutinize special topographic maps created through laser measurement data taken from up high.

The scientists looked for vents similar to the recently formed ones.

They found a number of areas where likely traces of vents, each measuring 10 to 100 meters across, were arranged in a string stretching several hundred meters.

Four or five similar strings ran parallel to the string of the latest vents within about 1 kilometer of the most recent string, the researchers said.

Based on a study by the University of Toyama and other sources, the previous eruption of Mount Moto-Shiranesan is believed to have occurred 1,500 to 3,000 years ago.

However, some of the recently found vents appear younger than that, indicating the volcano erupted more recently, AIST officials said.

“More eruption events may have recurred than previously known,” said Yoshihisa Kawanabe, a senior researcher with AIST. “Similar traces of eruptions may be left at volcanos across Japan, so they must be properly studied.”