Geological experts say Wednesday's deadly landslide in southwestern Japan was likely caused by weathering of the slope's bedrock.
The massive landslide buried 4 houses at the foot of the slope in Nakatsu City, Oita Prefecture.
At the land ministry's request, a team of researchers from a Kyushu University graduate school inspected the site later in the day.
The researchers found that the bedrock on the upper-most part of the slope has turned into whitish sand, due to weathering.
The team says on top of a relatively solid layer of andesite is a softer layer of ignimbrite near the top of the slope. The upper layer is a pyroclastic flow deposit caused by a volcanic eruption between 900,000 and 1,000,000 years ago.
The team says the slope collapsed because the weathering of the bedrock extended more than 10 meters below the surface.
Dozens of boulders measuring 2 to 3 meters in diameter remain on the slope. Experts warn that rain or other conditions may cause the rocks to fall or trigger another landslide.
One of the researchers, Professor Noriyuki Yasufuku, says extra caution is needed to ensure the safety of the rescue workers.