Assessment of ground surface displacement in Taihape landslide, New Zealand, with C- and X-band SAR interferometry

In this study, we assess the capability of C-band advanced synthetic aperture radar (ASAR) data from Envisat and X-band Stripmap SAR from TerraSAR-X to analyse the extremely slow displacement rate of Taihape landslide in the central North Island of New Zealand. The small-baseline subset (SBAS) technique is applied to obtain displacement time series from SAR images between 2003 and 2011. Time-series results suggest that the landslide is active near its toe with maximum line-of-sight (LOS) surface velocity of 6 mm a(-1) between 2003 and 2010, derived from Envisat, and 8 mm a(-1) in the year 2010 to 2011, derived from TerraSAR-X. Comparing TerraSAR-X and Envisat results, the former detects approximately 15 times more coherent pixels for deformation analysis. Interferometric SAR (InSAR) displacement time series show a good agreement with field measurement from prisms installed on the surface of the landslide (maximum root-mean-square error or RMSE <8 mm). In addition to the long-term gravitational slope motion we also observe seasonal uplift and subsidence at the toe, which is attributed to surficial swelling and shrinkage of ground surface in response to cyclic variations in groundwater level.