Because large earthquakes can trigger landslides, the landslide record has potential use for establishing the palaeoseismicity of a region. However, before this potential can be realized, other possible landslide triggering mechanisms must be investigated. In southeastern Taranaki, New Zealand, over 100 deep-seated, complex, rock slumps occur in a region of steeply-dissected, gently-dipping Pliocene marine sediments. Three age populations can be detected by using relative and radiometric dating methods. The youngest group of landslides displays a consistent depth of soil development, lacks an integrated drainage network, dams catchments to produce lakes and has well-defined mass movement features. This group covers an area of 1015 km(2), yields radiocarbon dates of around 1300 BP and is clearly distinguishable from the two older groups of landslides. The synchronicity of landslide formation over a wide area, before New Zealand was populated, rules out human factors and slope undercutting as triggering agents. Comparison with modem analogues also rules out rainstorm as a triggering agent, principally on the basis of depth of movement. The distribution of landslides and the amount of failed surface is shown to be closely related to the location of active fault traces. The increase in landsliding towards the fault cannot be explained by variation in lithology or structure. Limiting equilibrium analysis of one landslide further points to the need for earthquake shaking to induce movement. Together, the lines of evidence investigated provide a strong case for earthquake-triggering of prehistoric landslides in this region.
