Post-wildfire ground deformation in Eastern Siberian permafrost areas detected by InSAR

Permafrost thaw by recent global warming can liberate the frozen organic carbon, which is decomposed into greenhouse gasses. As the released greenhouse gasses may lead to a positive feedback of global warming, it is important to monitor where and how permafrost thawing has been occurring. However, it is infeasible to perform direct field observations over such wide areas where permafrost is distributed. Instead, it is technically possible to monitor the subsidence and uplift of the ground over the wide areas, which could make a significant contribution to monitoring thawing processes of permafrost. In this study, we observe quantitatively the amount of ground deformation in the vast permafrost area with InSAR (Interferometric Synthetic Aperture Radar) image analysis. In particular, we focus on post-wildfire areas where remarkable deformation is occurred and reveal the spatiotemporal change of that.

From the InSAR images of ALOS2 taken in 2016, we detect significant displacement signals in the northwest hills from the Batagay, Eastern Siberia. At maximum 10 cm subsidence signal is detected in the area of 36.6 km². Notably, the subsidence signals are observed only during summer, indicating seasonal phenomenon. Examining the optical remote sensing images taken by Landsat and MODIS, we notice that the subsiding area is identical to the area that underwent wildfire in 2014. In addition, from the NDVI calculated based on Landsat data, we confirm that the subsidence amount slowed down due to the recovery of vegetation. To our knowledge, this is the second report that detected ground subsidence associated with arctic wildfire, following the report at North Slope, Alaska. Thawing subsidence is also detected in InSAR images of ALOS2 at 6 sites of other post-wildfire sites. The C-band Sentinel-1 images are also used to detect the same signals as ALOS2 by stacking short-term InSAR images, because the C-band InSAR images often lose coherence in long-term pairs. We interpret that the thawing subsidence occurs due to the increase of heat absorption in the summer season, because the ground surface vegetation plays a role to insulate the shallow frozen soils. In our future work, we will perform a field observation to clarify a thawing-recovery process after a wildfire and develop a physical model to reproduce the observed subsidence.