The landslide left one dead and 5 missing as of Wednesday afternoon. Emergency workers are searching for them.
It occurred on a mountain near a small cluster of houses. Police say a 200-meter stretch of land slid from a height of about 100 meters. It buried 3 houses.
Following a request from the prefecture, Ground Self-Defense Force personnel joined police officers and firefighters in the rescue operation.
The land ministry says that Oita Prefecture last year designated the affected area as being at high risk of sediment disasters.
Experts warn of secondary disasters
Kyushu University Graduate School Professor Yasuhiro Mitani, who specializes in rock engineering, says the mountain slope is composed largely of andesite, and mud is believed to have slid along the vertical cracks in it.
He says it will be difficult to conduct a rescue operation without heavy machinery because the mud contains lots of boulders. He also warns of secondary disasters, pointing out that the higher ground is at risk of collapsing.
Hiroshi Ikeya, an expert on soil erosion and landslides, says footage shows the land collapsed mainly in 3 locations. He says mud and trees remain on the slope in unstable condition, and a secondary disaster could strike.
He says earthquakes and heavy rain are usually blamed for landslides, but the affected area has had little rain and no quakes as of late. He says that's why it will be very difficult to determine the cause of the disaster.
But he points out groundwater is gushing out along the collapsed slope and the river turns muddy downstream. He says this suggests groundwater had seeped into the slope, and calls for a close analysis.
Ikeya says that as the landslide occurred before sunrise, people in the area may have failed to detect any signs of it. He also says the mud reached the rooftops of houses, so even if the residents had evacuated to the 2nd floor, they would still have been unable to escape the disaster.