ABSTRACT
Many of the 1,172 mapped landslides in the Neogene sedimentary rocks of the lower North Island New Zealand are large, deep-seated translational block slides in rock, where slip occurs along low angle (i.e. inclined <13° from the horizontal), and thin (1–10 cm) layers of montmorillonite-type clay of the smectite group. Movement patterns of many of these slides can be described as post–failure episodic creep, where movement rates vary from slow to extremely slow, between periods at rest. Past researchers have attributed their formation and movement primarily to strong earthquakes. One such landslide is the 18–24 ka, 22 × 106 m3 Utiku landslide. At this landslide, the 10,000 year return period peak ground acceleration calculated from the probabilistic seismic hazard model for New Zealand is 6.9 m/s/s (0.7 g), which should have occurred since the formation of the landslide. However, during a period of high-precision movement monitoring no episodes of landslide movement could be attributed to earthquake-induced ground accelerations, even though accelerations up to 0.94 m/s/s (0.1 g) (having a >20-year return period) were recorded at the site. Although some earthquake-induced landslide movement may have occurred during the longer period that the landslide has been active, our analysis suggests that the amounts of movement initiated by earthquakes—considering a representative set of earthquakes covering the range of severity from small to large—would account for a mean movement rate of 0.005 to 0.05 m/year. In comparison, historical and recent movement rates of the landslide (1972 to 2015), ranging from 0.04 to 0.07 m/year, were primarily controlled by changes in pore-water pressure induced during long wet periods. The historical rates are similar to pre-historical rates (0.05–0.07 m/year) derived from radiocarbon dating and geomorphic indices. These results suggest that earthquake-induced displacements are not the driver of the long-term movement rate of this landslide, and by implication other similar landslides with low-angle basal slide surfaces, formed in similar materials, in the area.
