Waipoapoa Landslide: A deep‐seated complex block slide in Tertiary weak‐rock flysch, Southern Hawke's Bay, New Zealand

Abstract The Waipoapoa Landslide (February 1976) in Southern Hawke's Bay is a complex failure in Upper Miocene weak alternating sandstone and mudstone (flysch). The landslide extends over approximately 18 ha and forms a reactivated portion of a larger ancient landslide complex. The Waipoapoa Landslide has a calculated volume of 8.35 × 10 6 m 3 . Morphologically the landslide is comprised of a head zone, front face, and debris‐flow complex. Rock‐mass defects have controlled slide block geometry. The crown escarpment of the head zone has propagated on nearly vertical intersecting fractures (joints and faults). The intersecting subhorizontal basal shear surface is coincident with bedding contact(s) in an alternating succession of friable porous sandstone and weakly consolidated slake‐prone mudstone. Bedding attitude is inclined into the slope, with dips ranging up to 13°. Block movement has occurred against dip direction and has imparted a back‐tilted geometry to the head zone depression. A multiple basal shear surface, and basal shear‐gouge zone are inferred. The front face is postulated to have bulged outward during the main movement phase and successively collapsed, generating large, rapidly moving debris flows with an estimated volume of 1.75 × 10 6 m 3 . These flows stretch is excess of 900 m downvalley from the landslide toe. Failure mechanisms included both sliding and flowage. The failure involves several large blocks and is of multiple block geometry. Principal factors contributing to failure could have included: the presence of fractures (joints and faults) and other defects (e.g., bedding) along which block movement was facilitated; rapid ground‐water infiltration along open fractures within the rock mass; high pore‐water pressures in one or more porous sandstone beds; and the local steep relief.