Rock failure and erosion of a fault damage zone as a function of rock properties: Alpine Fault at Waikukupa River

Erosion rates in the hanging wall of the Alpine Fault are high, keeping pace with rock uplift over time frames of 10⁴–10⁶ years. On shorter time frames, prediction of temporal and spatial distribution of erosion is challenging and must account for local conditions and parameters including rock strength, topographic stresses and failure conditions. Constrained by field observations of rock strength, we use 3D mechanical models to predict where and by what mechanism slope failure and erosion are likely to take place along the Waikukupa section of the Alpine Fault. Shear failure is favoured along the base of slopes and where pore pressure is high. Tensile failure is favoured along ridges, higher on slopes and when pore pressure is moderate. A dry material with a high degree of rock strength heterogeneity promotes bedrock gully development, whereas distributed failure is more likely to occur when the material is saturated.