An integrated approach for mapping slow-moving hillslopes and characterizing their activity using InSAR, slope units and a novel 2-D deformation scheme

Strong earthquakes are not only able to change the earth's surface processes by triggering a large population of co-seismic landslides but also by influencing hillslope deformation rates in post-seismic periods. An increase in post-seismic hillslope deformation rates could also be linked to a change in post-seismic landslide hazard level and, thus, could be exploited to better assess post-seismic landslide risk in a given area. However, variations in hillslope deformations from pre- to post-seismic phases have rarely been examined for strong earthquakes. This paper examines pre- and post-seismic hillslope deformations, from 2014 to 2018, for an area (~ 2300 km2) affected by the 2016 Mw7.8 Kaikōura earthquake using time series Interferometric Synthetic Aperture Radar (InSAR) technique. To consistently analyse the entirety of the area from pre- to post-seismic phases, we aggregate InSAR-derived deformations for geomorphologically meaningful landscape partitions called Slope Units (SUs). We further examine the aggregated data through a 2-D hillslope deformation scheme, which we utilise as a method to systematically identify the variations in post-seismic hillslope deformation trends. In this context, we label newly activated, uninterruptedly deforming, and stabilized hillslopes in the post-seismic phase. We found 243 (4.76%) SUs out of 5104 SUs located in the study area to be active in the post-seismic phase. In addition to SUs, which may contain multiple landslides, we also analysed co-seismic landslides, in particular, showing active deformation in the post-seismic period. Results showed that 368 (4.69%) co-seismic landslides out of 7831 are actively deforming in the post-seismic phase. Overall, the areas affected by larger ground shaking show higher post-seismic deformations, which highlights the importance of the earthquake legacy effect as a factor controlling post-seismic hillslope deformations.