Coastal Oregon is home to a number of slow, recurrent landslides. During bouts of heavy rain, water gets into the soil, reducing friction and causing the ground to slip. Often, these landslides creep along at a barely perceptible rate—less than a centimeter per day. Yet the landslides are a lurking threat, as past events that have damaged infrastructure and cut communities off for months at a time have demonstrated.
Of particular concern is how these landslide regions would respond to the more potent shaking of an earthquake, a process that is not well understood. Based on new laboratory experiments, Schulz and Wang suggest that many of Oregon’s slow creeping coastal areas would fail catastrophically during an earthquake, an added threat in the seismically active region.
Between the smaller recurrent landslide events, the hillslopes are propped up by the constant friction that develops in the soil during the slow movements, known as its “residual shear strength.” A small amount of jostling, however, can actually change the strength of a hillslope, as soil particles are rearranged into either a more shear-resistant or shear-compliant state.
The authors assessed how the soil from two of Oregon’s landslide regions would respond to different kinds of pressure. They tested soil samples from two recurrent landslides under constant and increasing displacement rate and under loading forces that mimicked historical earthquakes.
The researchers found that a small rapid displacement caused a pronounced drop in shear strength. However, if the displacement rate was increasing, rather than constant, the shear strength actually increased—up to a point. Under shaking conditions drawn from historical earthquakes, however, the soils’ residual strength could not hold up. (Journal of Geophysical Research: Earth Surface, doi:10.1002/2014JF003088, 2014
—Colin Schultz, Writer