Landslide and Lahar at Ontake Volcano, Japan on September 14, 1984
Earthquake and Rainfall Triggers LaharA magnitude 6.8 earthquake struck 1.1 km beneath Mt. Ontake, a dormant stratovolcano in central Honshu, Japan, on September 14, 1984, after several days of heavy rain (16.9 cm total rainfall). The earthquake at 8:48 a.m. triggered a landslide with a volume of 32-36 million m3 on the volcano's southeast flank. The landslide traveled down a steep narrow valley as a wet but unsaturated mass for about 8 km and then transformed into a lahar and continued flowing downstream at least another 4 km.
A deep scar in the side of Mt. Ontake marks the site of the landslide. The scar is about 1,300 m long and 400 m wide. As the landslide swept into the Denjo River, its maximum depth was about 170 m; the average depth was about 80 m.
Landslide Source Area
Looking into the deep landslide scar, old layers of volcanic deposits from earlier eruptions of Mt. Ontake show the layer-cake construction typical of volcanoes. This source area consisted of layers of weakly consolidated pyroclastic-flow and tephra deposits. Scientists studying the source area found a layer of pumice about 10 cm thick that seems to mark the actual failure or rupture surface.
Landslide Scours Vegetation
Moving down the Denjo River at an average speed of at least 75 km/hour, the wet landslide scoured sediment and vegetation from along the channel. Part of the wet flow had enough momentum to surge up and over the small ridge in the center of the image. Fifteen people were buried by the landslide -- the only warning they may have had that a landslide broke loose upstream was the strong ground vibration caused by the earthquake.
When the landslide swept out of the steep Denjo River and
encountered the relatively flat Otaki-gawa River about 8 km
from the volcano, it was thoroughly saturated with water, which indicated
it had transformed into a lahar. Most of the lahar stopped flowing
within 4 km and formed a deposit 30 to 60 m thick. Many
small hills that scientists refer to as "hummocks" were
scattered across the lahar deposit, like the one shown here.