Widespread Slope Movements in Countrywide InSAR Mapping of Iceland

We report on countrywide InSAR deformation mapping of Iceland using all the available Sentinel-1 radar data (Summer/Fall 2015-2021) from three parallel and overlapping descending and three ascending orbit tracks, yielding a complete countrywide coverage for both look directions. The total number of satellite passes for each of the six orbit tracks is about 170, meaning that over 1000 data sets were used, from which we processed around 8700 interferograms (multilooked to 100 m x 100 m pixels). Atmospheric signals in the data were reduced using a two-step correction approach based on global atmospheric model outputs and information about the stochastic characteristics of atmospheric noise. We then solved for the time-series of each of the six data sets and inverted for near-east and near-vertical time-series, assuming that north ground displacements are small. Large-scale displacements in Iceland are dominated by the plate motion and by glacio-isostatic adjustment. The results show how the width of the plate-boundary zone varies from being relatively narrow in Reykjanes to more distributed deformation in the Eastern Volcanic Zone. The glacio-isostatic uplift reaches a maximum of ~3 cm/year in central Iceland and appears to accelerate during the observation period. These large-scale horizontal and vertical displacements can be removed with a model of the plate motion, plate-boundary deformation and glacio-isostatic adjustment, leaving only local deformation signals in the residual displacement rate map, e.g., at central volcanoes and areas of geothermal exploitation. Widespread slope movements are also evident in the residual deformation map. Almost all east-facing slopes are moving eastward and west-facing slopes westward. This deformation is seen all over Iceland and amounts to a few mm/year, with faster rates at some known landslides. Example areas include northwestern and central-north Iceland where 5-10 mm/year movement rate is found on many of slopes, as well as the Western Fjords and Snæfellsnes peninsula. Recent slope failures in North Iceland in 2021 and 2022, which resulted in mudslides with road closures and some structural damage, occurred on slopes that can be seen moving during the years before the failures. However, no anomalous motion is detected at these slopes in the months before the failures; they are just slowly creeping like many other slopes in this area. In Summary, our results show that InSAR data are effective to map country-wide ground velocities and velocity changes as well as local deformation signals and transients at volcanoes and geothermal areas.  The results also show that slopes all over Iceland are subject to steady gravitational soil creep amounting to several mm/year, with higher rates observed in many areas where geomorphologically landslides can be identified in the landscape.