Post-wildfire surface deformation near Batagay, Eastern Siberia, detected by L-band and C-band InSAR
Kazuki Yanagiya1 and Masato Furuya2
1Department of Natural History Sciences, Graduate School of Science, Hokkaido University.
2Department of Earth and Planetary Dynamics, Faculty of Science, Hokkaido University.
Corresponding author: Kazuki Yanagiya (k.yanagiya@frontier.hokudai.ac.jp) and Masato Furuya (furuya@sci.hokudai.ac.jp)
Key Points:
Abstract
Thawing of ice-rich permafrost and subsequent ground subsidence can form characteristic landforms, and the resulting topography they create are collectively called “thermokarst”. The impact of wildfire on thermokarst development remains uncertain. Here we report on the post-wildfire ground deformation associated with the 2014 wildfire near Batagay, Eastern Siberia. We used Interferometric Synthetic Aperture Radar (InSAR) to generate both long-term (1-4 years) and short-term (sub-seasonal to seasonal) deformation maps. Based on two independent satellite-based microwave sensors, we could validate the dominance of vertical displacements and their heterogeneous distributions without relying on in-situ data. The inferred time-series based on L-band ALOS2 InSAR data indicated that the cumulative subsidence at the area of greatest magnitude was greater than 30 cm from October 2015 to June 2019, and that the rate of subsidence slowed in 2018. The burn severity was rather homogeneous, but the cumulative subsidence magnitude was larger on the east-facing slopes where the gullies were also predominantly developed. The correlation suggests that the active layer on the east-facing slopes might have been thinner before the fire. Meanwhile, C-band Sentinel-1 InSAR data with higher temporal resolution showed that the temporal evolution included episodic changes in terms of deformation rate. Moreover, we could unambiguously detect frost heave signals that were enhanced within the burned area during the early freezing season but were absent in the mid-winter. We could reasonably interpret the frost heave signals within a framework of premelting theory instead of assuming a simple freezing and subsequent volume expansion of pre-existing pore water.
Plain Language Summary
Wildfires in arctic regions not only show an immediate impact on nearby residents but also long-lasting effects on both regional ecosystems and landforms of the burned area via permafrost degradation and subsequent surface deformation. However, the observations of post-wildfire ground deformations have been limited. Using satellite-based imaging technique called Interferometric Synthetic Aperture Radar (InSAR), we detected the detailed spatial-temporal evolution of post-wildfire surface deformation in Eastern Siberia, which helps in understanding permafrost degradation processes over remote areas. Post-wildfire areas are likely to be focal points of permafrost degradation in the Arctic that can last many years.
1 Introduction
Wildfires in boreal and arctic regions are known to have increased over recent decades in terms of both frequency and areal coverage (e.g., Kasischke & Turetsky, 2006; Hu et al., 2010), and have had significant impacts on permafrost degradation (e.g., Jafarov et al., 2013; Zhang et al., 2015; Gibson et al., 2018). Although fires do not directly heat up the subsurface space deeper than 15 cm (Yoshikawa et al., 2003), severe burning decreases surface albedo, and removes vegetation and the surface organic soil layer that previously acted as insulators buffering from changes in air temperature. Subsequent increases in both soil temperature and thickness of the active layer, a near-surface layer that undergoes a seasonal freeze-thaw cycle, have been documented for up to several years after the fire (e.g., Yoshikawa et al., 2003). Meanwhile, in ice-rich permafrost regions, the thawing of permafrost and the melting of massive ice can lead to formation of characteristic landforms such as thaw pits and ponds, and retrogressive thaw slumps. While there are a variety of classifications in terms of morphological and hydrological characteristics (Jorgenson, 2013), those thaw-related landforms and the topography they create are collectively termed as “thermokarst”. However, the role of wildfires in developing thermokarst terrain remains quantitatively uncertain. Moreover, in comparison to the controlled warming experiments in Alaska (Hinkel and Hurd Jr, 2006; Wagner et al., 2018), wildfires in arctic regions may also be viewed as uncontrolled disturbance experiments that aid in understanding the permafrost degradation processes.
Ice-rich permafrost deposits, known as the yedoma ice complex (yedoma), are widely distributed in the lowland of Alaska and Eastern Siberia (Kanevskiy et al., 2011; Schirrmeister et al., 2013). The greatest subsidence within the 2007 Anaktuvuk River tundra fire scar was identified in the yedoma upland by LiDAR (Jones et al., 2015). Yedoma is a unique permafrost deposit in terms of its extraordinarily high volume of ice (50-90 %) and organic-rich sediments. While the organic carbon trapped in permafrost regions is estimated to be twice that in the current atmosphere, permafrost thawing and related thermokarst processes may release the carbon as greenhouse gasses (CO2 and CH4) via microbial breakdown, which may further promote global warming (Mack et al., 2011; Schuur et al., 2015). Thus, in order to estimate the volume of greenhouse gasses released, it is important to evaluate the volume of thawed ice associated with thermokarst processes in yedoma-rich areas.
Near the village of Batagay, Sakha Republic, Eastern Siberia (Figure 1), there exists the Batagaika megaslump, known as the world’s largest retrogressive thaw slump, exposing roughly 50-90 m thick yedoma deposits on the north-east facing slope (e.g., Kunitsky et al., 2013; Murton et al., 2017). Thaw slumps are characterized by a steep headwall surrounding a slump floor and develop as a result of rapid permafrost thawing. The Batagaika megaslump was initiated at the end of the 1970s by deforestation but still appears to be growing (Günther et al., 2016). Considering this feature, it is worth considering whether new disturbances in the proximity will result in the formation of similar landforms. A wildfire incident occurred in July 2014 near Batagay, which, like deforestation, will change the ground thermal regime. Therefore, it is important to examine whether future catastrophic thermokarst development could be similarly initiated at the fire scar, whose area is much larger than the Batagaika megaslump (Fig 1b).
The first objective of this study was to assess the effectiveness of satellite Interferometric Synthetic Aperture Radar (InSAR) in detecting surface deformation signals due to wildfire-induced thermokarst over different temporal scales. InSAR has been used to detect long-term and seasonal displacements over several thaw-related landforms in permafrost areas (e.g., Liu et al., 2010, 2014, 2015; Short et al., 2011; Iwahana et al., 2016; Molan et al., 2018; Antonova et al., 2018; Strozzi et al., 2018; Chen et al., 2018). Although subsidence signals as a result of thermokarst associated with Alaskan wildfires have been detected using InSAR (Liu et al., 2014; Iwahana et al., 2016a, 2016b; Molan et al., 2018; Michaelides et al., 2019), no such studies have been conducted on Siberian fires, to our knowledge. Also, all previous InSAR-based post-wildfire deformation mapping has been performed over relatively flat terrains, but no reports over hillslopes have been shown. Moreover, in contrast to previous studies, we employed two independent SAR imageries with distinct carrier frequencies and polarizations, L-band (1.2 GHz) HH- and C-band (5.4 GHz) VV-polarized microwave. Because the imaging geometries were different and had different sensitivities to the 3D displacement vector, we could not only take advantage of the performance of each sensor in mapping deformation signals but could also cross-validate the measurements by two InSAR data sets.