Coseismic landsliding during the M w 7.1 Darfield (Canterbury) earthquake: Implications for paleoseismic studies of landslides

The head scarp of the Harper Hills landslide consists of ground cracks with vertical displacement and extension that opened during the 2010 Darfield (Canterbury) M w 7.1 earthquake. The geomorphology of the cracks, regional geology and ground penetrating radar indicate that the landslide formed by bedding-controlled translation and joint-controlled toppling, and suggest incipient deep-seated movement. Crack depth and displacement along the head scarp vary along the ridge; maximum values are located where the head scarp is closest to the local ridge line. Increased seismic shaking due to topographic and geometric amplification of seismic waves is suggested as an explanation for this relationship. An excavation across the head scarp revealed no evidence of prior slip events over a time period that is likely to exceed the return period (1000-2500years) of peak ground accelerations experienced at this location in the Darfield earthquake. We suggest that specific seismologic attributes of the Darfield earthquake may have influenced the location of landsliding in this instance. Studies of paleo-landslides must consider crack preservation potential as well as complex source/site effects that may complicate estimates of acceleration return periods from the subsurface investigation of individual landslide head scarps.