Thermokarst initiation process after wildfire: case studies of the 2018, 2019 and 2020 fires near Batagay, Eastern Siberia

Abstract:

In Verkhoyansk, Eastern Siberia, a new temperature record of 38℃ reported on 20 June 2020, and numerous wildfires have been detected in the entire Siberia even before the fire season, according to WMO. Holloway et al. (2020) reviewed recent researches on permafrost degradation caused by wildfire. Notably, thermokarst associated with the Anaktuvuk River fire had been studied in Alaska, using a variety of methods including InSAR, LiDAR, and in situ observation (e.g. Liu et al., 2014; Jones et al., 2015; Iwahana et al., 2016). On the other hand, in Siberia, there were few studies of post-wildfire ground deformation by InSAR, and how much of the permafrost was thawed by recent wildfires is unknown.

In our previous study (Yanagiya and Furuya, 2020), we focused on the post-wildfire area burned in 2014 near Batagay, which is in 55km east to Verkhoyansk. InSAR images revealed the spatio-temporal variation of thawing subsidence over the five years after the fire, and we estimated the amount of melted massive ground ice and interpreted the uplift value by premelting theory. However, we were not able to analyze the deformation immediately after the fire due to the loss of satellite data.

Here, we focused on the 2018, 2019 and 2020 fires near Batagay. We generated InSAR images from two independent SAR satellite data Sentinel-1 and ALOS2. InSAR images detected seasonal uplift signals from October to December and long-term (1year) deformation signals with winter pair. The timing of thermokarst initiation was characteristically different depending on the number of years since the fires. Additionally, we conducted in situ observation in September 2019 and observed thaw depth, ground temperature, moisture, and leveling. The thaw depth depended on the fire year and was consistent with InSAR images. We also examined phase noise on InSAR signals due to soil moisture and combine with field data to validate ground deformation value immediately after the wildfires.