Tiny changes in atmospheric pressure between day and night can trigger landslides. The same phenomenon could be a final straw that sets off earthquakes and volcanic eruptions waiting to happen.
For the past four years, at the United States Geological Survey in Denver, Colorado, and his colleagues have been studying the movement of an enormous ongoing landslide in south-west Colorado called 鈥� so called because the yellowish soil reminded early European settlers of the eponymous stew.
The landslide has been continually slipping for 700聽years and contains over 20聽million cubic metres of material. It is moving down the mountain at an average rate of 1聽centimetre per day for most of the year.
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Daily cycle
To investigate what is causing Slumgullion to slide, aside from gravity, Schulz鈥檚 team measured its movement hour by hour with paired spools of wire that unwind when moved apart.
They also mapped the daily cycle of atmospheric pressure 鈥渢ides鈥� over the landslide. Atmospheric tides are small variations in pressure that occur as air warms during the day.
Schulz found that the landslide鈥檚 movement was not continuous but closely correlated with atmospheric tides. 鈥淭he landslide mainly moves when the pressure drops [at night],鈥� says Schulz.
Upward friction
Landslides are usually triggered by rain or snowmelt flowing into the base of a 鈥渟lump鈥� of loose rock and soil.
Schulz and his colleagues suspect that an atmospheric low tide releases a tiny amount of pressure on the air and water in the soil at the surface. As a result, fluids deeper in the soil move up towards this region of lower pressure.
鈥淭his upward movement pulls the soil structure upward, thereby reducing the frictional strength along the base of the landslide, which is essentially a solid block sliding on a surface,鈥� says Schulz. 鈥淭he reduced frictional strength is sufficient to trigger landslide movement.鈥�
鈥淭his is the first time that such an effect has been noted,鈥� says , who researches landslide mechanics and hazards at Durham University, UK.
The research suggests that certain storms may have unexpectedly dangerous effects, says Petley. The rapid pressure variations associated with fast-moving storm systems such as the may be triggering landslides, he says.
Last straws
Schulz says that atmospheric shifts could also trigger earthquakes and volcanic eruptionsthat are already about to go off, he says.
Previous research by of the Academica Sinica in Taipei, Taiwan, has linked the passage of typhoons with the triggering of 鈥渟low鈥� earthquakes.
Slumgullion itself is unlikely to pose a threat. 鈥淭here鈥檚 a state highway around 100聽metres below the landslide,鈥� says Schulz. 鈥淎t the rate the landslide鈥檚 moving it鈥檒l only reach it in 200 years.鈥�
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