Landslide Character and Hydrological Thresholds for Slope Failure Initiation on Volcanic Scoria Slopes: Case Study From Mt Hobson/Ōhinerau, Auckland, New Zealand

Rainfall‐induced landslides involving on Auckland Volcanic Field scoria cones are highly unusual, compared with widespread landsliding in the surrounding Auckland clay‐rich residual soils. Indeed, while Auckland can suffer from thousands of rainfall‐induced shallow landslides from severe storms at subdecadal timescales, these are usually limited to residual soils formed above Miocene‐age parent rock. We report a study of six shallow landslides in volcanic soils on Mt Hobson/Ōhinerau, triggered by the Auckland Anniversary Storm (27 January 2023). The storm was significant, causing widespread landslide and flood damage, including fatalities. Mt Hobson/Ōhinerau is a 143 m‐high scoria cone, that formed ≈28,600 years ago, from a typical Strombolian‐type event. The scoria has weathered in situ, on which thin soils have developed, which are more permeable than surrounding clay‐rich residual clays, allowing lateral flow and downslope drainage within the scoria substratum. While the scoriaceous soils have moderate friction angles, cohesion is limited to apparent cohesion in unsaturated conditions. However, this apparent cohesion is eliminated in extremely heavy and/or prolonged rainfall events that occur on soils with prevailing high antecedent moisture contents. Thus, slope failure only occurs in the most extreme hydrological conditions, as underpinned by a global comparison of rainfall intensity–duration thresholds for volcanic and nonvolcanic soils. The identification of a larger palaeo‐landslide signifies the feasibility of deeper landslides with longer runouts that could pose a hazard in future.