by Mary Caperton Morton Tuesday, December 20, 2016
Landslides aren’t always fast-moving disasters. Slow landslides creep downhill at rates up to a few meters a year, which might not sound dramatic, but slow slides can still damage roads, pipelines and communities. Slow-moving slides are most commonly triggered by increased pore pressure in the soils due to rainfall or snowmelt, but in some places, according to a new study, temperature may also play a role. The new study looking at slow slides in Japan found that cold underground temperatures — independent of increased rainfall — may lubricate slow-moving slides.
Slow-moving landslides most often occur in areas underlain by smectite clay. “Smectite is a unique clay mineral that shows high plasticity and high viscosity characteristics,” says Tatsuya Shibasaki, a geologist at Kyoto University in Uji, Japan, and lead author of the new study, published in Geophysical Research Letters. That means that once a landslide begins, the movement of the slide doesn’t usually accelerate, it just creeps along slowly.
These unique slides occur throughout Japan and many other places in the world where volcanic ash is mixed in with sedimentary layers, creating the smectite clay deposits. “Smectite clay is so weak that it only takes a few centimeters of it to lubricate entire mountainsides,” says Chris Massey, an engineering geologist at GNS Science in New Zealand who was not involved in the new study.
Shibasaki and colleagues focused on Niigata Prefecture in north-central Japan, where hundreds of slow-moving slides have been monitored for several decades. These slides seem to be most active during the winter, regardless of the amount of rain or snowfall the area receives. To determine how ground temperatures might be affecting the slides, the team took soil samples from the Busuno-Touge landslide, which has been monitored since the 1980s and shows a pattern of increased activity in late autumn to early winter. This landslide is 400 meters long, extends down to a depth of 4 to 6 meters and moves downslope at a rate of 1 to 2.5 meters a year.
A series of experiments conducted on the smectite-rich soils from the Busuno-Touge landslide showed the shear strength of the clay-based soils decreased at colder temperatures. “Based on our experiments, we infer that landslides occurring in smectite-bearing rocks can be activated by a decrease in ground temperature,” Shibasaki says. “This effect may be limited to shallow, small landslides, because seasonal fluctuation in ground temperature usually occurs in the near-surface ground.”
The relationship between ground temperature and slow slides has not been thoroughly explored, Massey says. “I’ve not seen this connection laid out at this level of detail before. [This study] is really a nice piece of work.” Whether temperature could be playing a role in slow-moving slides in other locations remains to be seen, he says. In New Zealand, where slow-moving slides are common, ground temperature fluctuations are almost negligible, he says, whereas in the study area in Japan, temperatures dropped from 25 degrees Celsius down to 9 degrees in the most active area of the slide.
The new information may help predict where cold-triggered, slow-moving slides may occur: in areas with clay-rich soils and strong seasonal ground temperature fluctuations, Massey says. “Although behaviors of landslides are monitored in many places around the world, in most cases, ground temperature is not usually monitored,” Shibasaki says. “I hope that this new knowledge could help people living in landslide-prone areas to predict when landslide movement will occur and help them judge how best to mitigate the damages.”
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