INSAR APPLICATION FOR THE DETECTION OF PRECURSORS OF THE ACHOMA LANDSLIDE, PERU

Purpose: In the last few decades, InSAR has been used to identify ground deformation related to slope instability and to retrieve time series of landslide displacements. In some cases, retrospective retrieval of time series revealed acceleration patterns precursory to failure. Although the higher temporal and spatial resolution of new-generation satellites may offer the opportunity to detect precursory motion with viable lead time, to rely entirely on the possibility of retrieving continuous time series of displacements over landslides is a limiting strategy. This is because successful phase unwrapping is impaired by factors such as unfavourable orientation, landcover and high deformation gradients over relatively small areas, all common on landslides. Methods: We generated and analysed 112 Sentinel-1 interferograms, covering the period between April 2015 and June 2020, at medium spatial resolution (8 and 2 looks in range and azimuth respectively) over the Achoma landslide in the Colca valley, Peru. This large, deep seated landslide, covering an area of about 40 hectares, previously unidentified, failed catastrophically on 18th June 2020, damming the Rio Colca and giving origin to a lake. We also generated a time series of displacements based on optical Planet Lab images covering the three years prior to failure. We explored a methodology to retrieve precursory signs of destabilisation of landslides with characteristics unfavourable to unwrapping and time series inversion based on the investigation of spatial and temporal patterns of coherence loss within the landslide and in the surrounding area and on the extraction of a relative measure of incremental displacements through time obtained from the wrapped phase. Results: We observed significant, local interferometric coherence loss outlining the scarp and the southeastern flank of the landslide, intermittently in the years before failure. Moreover, we observe a sharp decrease in the ratio between the coherence within the landslide and in the surrounding area, roughly six months before the failure which is interpreted as a sign of critical landslide activity and a precursor. The latter is concomitant to increasing displacement rates observed with optical images. The wrapped interferometric phase also revealed a sequence of acceleration phases, each characterised by increasing annual rates. We observe a behaviour that recalls progressive failure, with no clear evidence for response to one particular trigger and two acceleration phases followed by a more stable period and the last leading to failure. Conclusions: This type of approach is promising with respect to the extraction of relevant information from interferometric data when the generation of accurate and continuous time series of displacements is hindered by the nature of landcover or of the landslide studied, such in the case of the Achoma landslide. The combination of key, relevant parameters and their changes through time obtained with this methodology may prove necessary for the identification of precursors over a wider range of landslides than with InSAR time series generation alone.