Rainfall‐triggered landslides from the January 2023 1‐in‐200 year storm at Maungakiekie/One Tree Hill, Auckland, New Zealand

ABSTRACT We describe engineering geological characteristics and failure mechanisms of shallow soil landslides activated on Maungakiekie/One Tree Hill in central Auckland, New Zealand on 27 January 2023. The hill is a scoria cone in the young Auckland Volcanic Field (AVF), and formed ∼60 ka BP. Subsequently, much of the volcano was blanked by 3–5 m of fine volcanic ash, erupted from neighbouring Three Kings volcano ∼28.5 ka BP, on which residual soils have formed. These slopes were then terraced using cut‐and‐fill earthworks by pre‐European Māori until abandoned in 1822. The terraces remained largely intact until 27 January 2023 when the ‘Auckland Anniversary Storm’ (a > 1‐in‐200‐year event), crossed Auckland, triggering 19 shallow failures on the terraced slopes. Accordingly, we investigated the geotechnical characteristics of the volcanic soils associated with these landslides. An important destabilising factor is the presence of thin (∼20 mm) layers of weathered Three Kings ash exposed within the 1–2 m high headscarps. These dip unfavourably out of the slope, also acting as an aquiclude, leading to increased porewater pressure in overlying soils. The occurrence of landslides only on ‘filled' slopes supports emerging evidence that human disturbance may be a more important future landslide conditioning factor than climate.