Rocks spewed out by the eruption of Mt. Ontake likely reached maximum speeds of 300 kph, according to a survey by the University of Tokyo's Earthquake Research Institute.
Holes ranging from 10 centimeters to dozens of centimeters in diameter were concentrated within a roughly 500-meter range from the main vents, the survey also found.
"Rocks fell densely at high speeds, which likely led to many casualties," said Takayuki Kaneko, an assistant professor at the institute and a volcanic geology expert who led the survey.
Kaneko took aerial photos of the mountain from a helicopter Sunday, the day after the eruption. Examination of the photos showed that the holes created by the rocks were mainly located about 500 meters northeast of the mountain's major vents. There were more than 10 holes on average per 16 square meters, and the rocks were at least 10 centimeters in diameter.
The professor calculated the rocks' velocities based on such factors as the distances between the holes and the vents, and concluded that they likely reached top speeds of about 300 kph.
Kaneko also discovered what appeared to be holes made by rocks measuring dozens of centimeters in diameter about a kilometer away from one of the volcano's vents.