Understanding selective triggering of landslide movement in response to rainfall, Gisborne, New Zealand

ABSTRACT Northeastern New Zealand has a landscape susceptible to landslides, on account of (1) its proximity to an active plate boundary, (2) steep slopes, (3) young, soft, sedimentary rocks and soils, (4) landuse changes, and (5) rainfall from ex‐tropical cyclones. The interplay of these led to widespread rainfall‐induced landsliding in the first half of 2023, particularly following Cyclone Gabrielle. Suburban Gisborne was particularly affected, where residential housing has expanded onto steeper terrain surrounding the city. Utilising InSAR, LiDAR, ground investigations and laboratory analysis, we report on an ongoing landslide above Russell Street, Gisborne. The slope appeared stable until February 2023, when slumping of the upper area initiated. Movement accelerated in June 2023, forming a prominent headscarp, and bulging of the slope mid‐section and toe, involving weathered Miocene Tolaga Group weak rock and residual soil. The landslide affected and tilted houses on the slope, which are now abandoned. CPTs indicate a shallow water table, with weaker material <9 m depth. The slope had remained stable during recent decades, but record‐breaking rainfall in 2023 on slopes already exhibiting high antecedent moisture content, was enough to lower effective stress and shear strength, initiating failure.