InSAR monitoring of solifluction and permafrost evolution in the Northeastern Tibetan Plateau

The Tibetan Plateau is characterized by a high periglacial landscape. The average annual surface temperature is below 0°C over most of the Plateau, so permafrost (permanently frozen ground) is present over most of its extent. Excess ice in the frost-sensitive materials of the active layer, above the permafrost, thaws during the summer and autumn months and freezes during the winter and spring months. These freezing and thawing phenomena lead to cyclic vertical movements of the surface. On the slopes, solifluction phenomena also related to freeze-thaw activity take place, associated to permanent horizontal displacements. Both movements can be measured by spatial geodesy techniques such as Synthetic Aperture Radar Interferometry (InSAR).The high-altitude Tibetan Plateau, like the Arctic regions, is particularly sensitive to global warming, and recent studies  have documented an apparent acceleration of solifluction processes as well as permanent ground subsidence likely due to ice loss in permafrost. Here, we developed a methodology to analyse hillslope processes from multi-temporal InSAR data to further document this worrying trend, and confirm or not its relation with global and regional temperature increase or with glacier ice loss in Tibet. InSAR time series of surface deformation from 16 yr of Envisat (2003-2011) and Sentinel-1 (2014-2020) ESA satellite radar measurements have been built over an 80,000km2 area to study the permafrost evolution of the northeastern Tibetan Plateau. Time series exhibit three trends, (1) a linear trend of continuous deformation, (2) an annual cyclical deformation whose amplitude appears to (3) increase over time. We conducted an analysis of the annual cyclic and cumulative deformation from the InSAR time series and projected those three trends parallel and normal to the line of the greatest slope. Areas with poor constraints were masked based on hillslope aspect from synthetic tests. The measurements (seasonal cycles and cumulative deformation in the slope and normal) were correlated to the lithology, the nature of the surface formations (moraines, alluvial fans, etc.), the altitude, the slope, the curvature, and the orientation of the slopes, in order to characterize the distribution of these processes.Our change of reference frame strategy proved effective in automatically extracting hillslope processes and quantifying their dynamics. Downward movements affect nearly all terrains, with velocities increasing in line with slope angles. Steeper displacements are observes in unconsolidated, frost-susceptible, and fine-grained sediments, exhibiting higher seasonal amplitude perpendicular to the slope. In contrast, for sedimentary and magmatic rocks that display lower seasonal amplitude, continuous creeping appears to be the primary downward displacement process. Permafrost degradation (long-term subsidence) appears more pronounced at higher altitudes (above 3800m) where one would expect to find the coldest annual average temperatures. We also illustrate significant increases in seasonal amplitude, potentially doubling within 5 years in intermontain basin. These findings suggest a recent degradation of the permafrost and a deepening of the active layer in the northeastern Tibetan Plateau, likely induced by global warming.