Spatial Variation in Thermokarst Subsidence after the Anaktuvuk River Fire on the North Slope, Alaska

The development of thermokarst in ice-rich permafrost regions is a natural hazard, causing irreversible geomorphic changes. The formation of large depressions and lakes or swamps produced by thermokarst processes is observed in discontinuous and continuous permafrost zones, especially in Alaska and Northeastern Siberia. Despite the recognition of uncertainty about the fate of Arctic regions and global climate change due to permafrost degradation information about the spatial extent and rates of thermokarst processes are limited. We investigated thermokarst development triggered by the Anaktuvuk River Fire (ARF), which combusted a vast tundra area in 2007 in the North Slope, Alaska, using both optical and microwave remote sensing as well as in situ fieldwork measurements and observations. The two-pass differential InSAR technique using ALOS-PALSAR (L-band microwave) has been shown capable of capturing thermokarst subsidence at a spatial resolution of tens of meters, with supporting evidence from field data and optical satellite images. Significantly large amounts of subsidence (up to 6.2 cm/year spatial average) were measured by the InSAR within burned areas relative to unburned nearby in the first three years after the fire. Relatively small spatial variation (less than 0.5 cm in spatial average) was observed from two independent InSAR pairs during the pre-fire period. The obtained interferograms did not show sub-meter scale depressions along the troughs of the depression network developed by thermokarst, though they could distinguish small land areas with stable and subsiding land surface at smaller than tens of meters scale and smaller-scale detailed spatial variation of thermokarst subsidence. Post-fire interferograms were decorrelated along fire boundaries where rapid surface changes due to lateral erosion can be expected and clearly separated subsiding burned areas from stable areas of intact environment. Inside the burned areas there are some gradual changes in phase values (e.g. slopes changes) while there are relatively uniform phase values in unburned areas. Despite the topography of the studied area being flat or showing only gentle slopes (95% of the area shows slope angles of less than 5°), the magnitude of subsidence seems to depend on the slope. There was a tendency for areas with larger slopes to experience larger subsidence. It is also worth noting that large subsidence was calculated for fragmented unburned areas, as they were small patches (most of them smaller than 1 m²) surrounded by burned surfaces in which thermokarst had been active. This fact seems to show that thermokarst areas tend to propagate into adjacent areas by the lateral influence of thermal and/or hydrological regime shifts in the ground.