Co-seismic surface rupture of Papatea fault and reactivation mechanism of the Clarence landslide during the 2016 M w 7.8 Kaikoura earthquake, New Zealand

Large earthquake-triggered landslides near a seismogenic fault are usually closely linked to fault slipping. Therefore, it is important to research the relationship between the landslide movement and the deformation pattern of faults to better understand the location and influencing factors related to the large landslides. A large landslide, on the right bank of the Clarence River, New Zealand, was reactivated during the 2016 Mw 7.8 Kaikoura earthquake. The landslide is located in close proximity to the Papatea Fault which ruptured during the earthquake. The special tectonic location of this landslide provided favorable conditions to research the failure mechanism of earthquake-triggered landslides. Field investigation and remote-sensing images revealed that the co-seismic surface rupture zone of the Papatea Fault is relatively complicated and characterized by a wide deformation zone with a width of ~80–170 m. The northern section showed as left-lateral strike-slip and thrust-slip, with a vertical displacement of up to 4–6 m. However, it exhibited thrust motion in the Clarence landslide area. The Clarence landslide was a reactivation of a pre-existing landslide. The thrust faulting of the Papatea Fault produced a wide deformation and fractured zone near the top of the landslide mass, and resulted in over-steepened topography in the landslide area. It should be noted that long-team seismic shaking, especially the main frequency of the seismic acceleration and rapid co-seismic vertical slip of the fault, played the principal role in further accelerating topographic instability during the earthquake. The landslide body thus started to slide along a pre-existing sliding surface under these actions. Our results indicate that the deformation pattern of the seismogenic fault could also play an important role in the occurrence of earthquake-triggered landslides, in addition to geological and geomorphological conditions and local seismic acceleration.