Landslides and rock falls
Acceleration due to ground shaking can destabilise cliffs and steep slopes, triggering landslides and rock falls.
Any incline where relatively large masses of material are supported by soil that is likely to soften under strain is prone to landslide hazards. The risk increases in areas where the ground is steep, weak or fractured, is saturated by heavy rain or is compromised by historical ground movements.
Rock falls occur on inclines where rock is weakly supported by the surrounding ground or is part of a larger brittle mass that is likely to shatter or fragment when subjected to seismic ground accelerations.
The hazard due to rock fall is primarily at locations on, and some distance radiating away from, the incline where the fall occurs. This can affect a very wide area, and rock fall hazards are therefore difficult to contain and control.
Rock fall was a major problem in Christchurch’s Port Hills following the 2010–11 Darfield and Christchurch earthquakes and aftershocks, as this video from GNS Science shows.
The simplest way to reduce the risk of rock fall hazards to structures is often to avoid building too close to high-risk areas, such as near the base of cliffs or on steep rocky slopes.