The Western European Alps display measurable surface deformation rates from leveling and GNSS data. Based on the time‐series analysis of 4 years of Sentinel‐1 data, we propose for the first time an InSAR‐based mapping of the uplift pattern affecting the Western Alps on a ∼350 × 175‐km‐wide area. This approach provides a denser spatial distribution of vertical motion despite the high noise level inherent to mountainous areas and the low expected deformation signal. Our results show consistency with other geodetic measurements at the regional scale, and reveal smaller‐scale spatial variations in the uplift pattern. Higher uplift rates are found within the external crystalline massifs compared to surrounding areas, in agreement with the variations expected from recent deglaciation and long‐term exhumation data. This work brings the first InSAR‐based geodetic clue of differential uplift within the Alpine belt in response to the surface and deep processes affecting the belt.

Within the low-deforming western European Alpine belt, GNSS measurements show that uplift is the main signal characterizing current surface deformation in the range, reaching up to 2 mm/yr, while no shortening is observed across the belt. Based on the huge amount of satellite data available today, it now appears possible to constrain new high resolution surface velocities in the western Alps, which is of primary importance to better understand the links between surface deformation and neotectonics processes in this region. Relying on ~ 170 radar acquisitions from Sentinel-1 satellite over four years, we propose for the first time an InSAR-based mapping of the uplift pattern affecting the Western Alps on a ~350x175 km-wide area. Their processing is challenging due to the high noise level inherent to mountainous areas and the low expected deformation signal. We thus use in this study the NSBAS small baseline approach (Doin et al., 2011) for interferograms corrections, unwrapping, and time-series inversion. Atmospheric corrections are made using ERA5 reanalysis model (Hersbach et al., 2020). We estimate regional line-of-sight (LOS) velocities by correcting the resulting time-series from outliers and by separating seasonal and linear signals through different approaches which all yield similar results, thus highlighting the robustness of the obtained LOS velocity field. Based on several assumptions, we finally convert LOS velocities to uplift rates using local incidence angles. The corresponding InSAR-derived velocity field is validated by the comparison with GNSS solutions. They both show uplift in the core of the belt, with higher rates in its northern part, and subsidence at its periphery. Our approach however provides a denser spatial distribution of vertical motions compared with GNSS. Higher uplift rates are found within the external crystalline massifs compared with surrounding areas, in agreement with the variations expected from recent deglaciation and long-term exhumation data. These results bring new insights into active tectonics in the Western Alps. While several distinct wavelength patterns can be identified within the uplift signal throughout the western Alps, we suggest that they may originate from common geodynamic processes, with differential surficial responses explaining their localization. These processes may involve glacial isostatic adjustment, erosion, and/or slab break-off.

Earth and Space Science Open Archive This work has been accepted for publication in Geophysical Research Letters. Version of RecordESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary. Learn more about preprints. preprintOpen AccessYou are viewing an older version [v1]Go to new versionSpatial Heterogeneity of Uplift Pattern in the Western European Alps Revealed by InSAR Time Series AnalysisAuthorsMargueriteMatheyiDMarie-PierreDoinPaulineAndréiDAndreaWalpersdorfiDStéphaneBaizeiDChristianSueiDSee all authors Marguerite MatheyiDCorresponding Author• Submitting AuthorUniversité Grenoble AlpesiDhttps://orcid.org/0000-0001-5738-9027view email addressThe email was not providedcopy email addressMarie-Pierre DoinInstitut des Sciences de la Terreview email addressThe email was not providedcopy email addressPauline AndréiDUniversité Grenobles AlpesiDhttps://orcid.org/0000-0002-7048-0201view email addressThe email was not providedcopy email addressAndrea WalpersdorfiDISTERREiDhttps://orcid.org/0000-0003-1777-9172view email addressThe email was not providedcopy email addressStéphane BaizeiDIRSNiDhttps://orcid.org/0000-0002-7656-1790view email addressThe email was not providedcopy email addressChristian SueiDUniversité Grenoble AlpesiDhttps://orcid.org/0000-0002-2472-5001view email addressThe email was not providedcopy email address

Based on geomorphological criteria, large-scale slow gravitational deformation affecting entire mountain flank, often being referred as Deep-Seated Gravitational Slope Deformation (DSGSD), have been shown to affect most of the reliefs worldwide. For instance in the European Alps, these deformation patterns were identified in several areas such as the Aosta Valley (Martinotti et al., 2011) or the Mercantour massif (Jomard, 2006). DSGSD inventories based on visual interpretation of scarps and field mapping were then compiled (e.g. Crosta et al., 2013) revealing the widespread occurrence of DSGSD. However, many aspects of these large-scale gravitational processes remain unclear and in particular their present-day activity and temporal evolution remain largely unknown. The present study aims at characterizing the spatial extent of DSGSD, and their velocity, at the scale of Western Alps through InSAR time series analysis using NSBAS processing chain (Doin et al., 2001). We used the whole SAR Sentinel-1 archive, between 2014 and 2018, with an acquisition every 6 days, on an ascending track. The processing was adapted to fit the specific conditions of the Alps (seasonal snow cover, strong local relief, vegetation and strong atmospheric heterogeneities). In particular we implemented a correction using the ERA 5 weather model and we used snow masks in winter allowing to select long temporal baseline interferograms with as little snow as possible. As we specifically aim to study deformation patterns at the scale of valley flanks, an average high-pass filter on moving subwindows has been applied to the interferograms prior to the implementation of time-serie inversions. This step strongly reduced the impact of residual atmospheric delays. The resulting velocity map in the line of sight (LOS) of the satellite reveals ubiquitous gravitational deformation patterns over the whole Western Alps, with localized patches of moving slopes showing sharp discontinuities with stable surrounding areas. We used radar geometry and InSAR measurement quality factors as indicators to identify the most trusted areas and to extract an inventory of potential DSGSD with their spatial extent. Doing so, we identified more than two thousands slowly deforming areas characterized by LOS velocities from 4 to 20 mm/year. We then compared the geometries of our “InSAR-detected-deforming-slopes” with previously published DSGSD inventories. Good agreements were found for example in the Aosta valley where most of the deforming areas from our velocity map are falling into the DSGSD outlines of Crosta et al. (2013). Currently, we continue to investigate the potential of this large-scale velocity map for DSGSD understanding and we plan to use artificial intelligence to search for possible generic properties between the detected sites.