Alternative Titleシベリア北東部バタガイにおける山火事後の急激な永久凍土融解プロセス : InSARと現地観測による解明
Note (General)Permafrost thaw can lead to further positive feedback to temperature rising. Deepening the seasonally thawing layer induces microbial decomposition of organic carbon stored for tens of thousands of years. In particular, a localized but deep and drastic thawing at fire scars and deforested areas is called “abrupt thaw.” The abrupt thaw may release more greenhouse gases than the gradual thaw caused by temperature rising. For example, a wildfire burns a surficial organic layer, a heat insulator, increasing summer ground temperature. As a result, abrupt permafrost thaw is accelerated over several years to decades until vegetation recovers. However, the effects of abrupt thaw on carbon release are not fully revealed. Although the gradual thaw has begun to consider in the earth system models, the abrupt thaw is challenging to parameterize because of heterogeneity in the amplitude and distribution. Also, permafrost thaw causes topographic change, such as ground subsidence. The process and its characteristic landforms are called “thermokarst.” Thermokarst affects surrounding ecosystems, hydrological environments, and local infrastructures. On the other hand, it is difficult to observe permafrost broadly because it is a subsurface thermal structure. Optical satellites and aerial photographs can not see the thawing process directly.Therefore, it causes uncertainty in carbon emissions in the polar terrestrial region. For this important geophysical issue, remote-sensing observation using Interferometric Synthetic Aperture Radar (InSAR) has developed from the 2010s. InSAR is a geodetic method that can detect relative ground displacement from two SAR satellite images. The microwave can penetrate the cloud and observe dayand night. In addition, InSAR does not require ground-based observation points; it is also effective in polar regions where frequent field observations are difficult. Owing to the advantages, InSAR observation has spread from Alaska to other regions but has almost no progress in Siberia, the largest permafrost distributed area. The thesis observed multiple cases of abrupt thaw around Batagay, Sakha Republic, Northeastern Siberia, focusing on the world’s largest retrogressive thaw slump (Batagaika mega-slump).The outcrop of the slump exposes a massive ice layer about 20 meters thick. It indicates the distribution of an ice-rich permafrost layer (yedoma layer). InSAR images detected ground deformations at the fire scars around Batagay, and on-site observations verified the deformation signals. The thesis reports spatio-temporal process of ground deformation and abrupt thaw from three different perspectives: (1) seasonal and annual thaw subsidence at the 2014 fire scar for 2-5 years after the fire, (2) post-fire immediate deformation at the 2018-19 fire scars, (3) detection and verificationof spatial heterogeneity of post-fire deformation using ALOS-2 high-resolution images.The first topic is the 2014 fire scar located on the hills opposite the Yana River. InSAR detected the abrupt thaw process 2-5 years after the fire. Time-series analysis, called the small baseline subset method (SBAS), revealed the spatio-temporal changes of seasonal and annual grounddeformations, including a frost heave in early winter. In general, InSAR pairs in winter lose coherence due to changes in the backscatter process caused by snowpack. By comparing ALOS-2 andindependent Sentinel-1 interferograms, this thesis confirmed that consistent deformation signals could be detected even in winter. These results indicated that microwaves could penetrate dry snowpack around Batagay with a pronounced continental climate. The total secular subsidence 2-5 years after the fire suggests that 3.56×106m3 of massive ice melt. Furthermore, the seasonal uplift signal is interpreted by premelting dynamics of heave theory due to ice lens formation.The second topic is thawing immediately after the 2018-19 fires. The immediate process was unclear at the 2014 scar because the burned year was not included in the ALOS and ALOS-2 observation periods. In addition, the 2018-19 fires occurred on the same slope as the Batagaika megaslump, which may lead to the melting of massive ice and the second slump in the future. Therefore,we also focused on the difference in the deformation process from the 2014 fire. The Sentinel-1 interferograms detected an increase in the seasonal heave period from the burned year to the second year. The ALOS-2 interferograms detected that the heave signal was dominant in the burned year, and secular subsidence became dominant after the second year of the fire. We interpreted the spatiotemporal variation based on the local thaw depth data measured from 2019.The third topic is the spatial heterogeneity of ground deformation signals detected by the high-resolution ALOS-2 data. InSAR images detected spatially heterogeneous signals in both seasonal and annual deformation within the 2018-19 fire scars. The seasonal heave signals correlated with gully topography in the 2019 fire scar. On the other hand, within the 2018 fire scar, there are clear boundaries between well-deformed and non-deformed areas. In order to verify these spatial heterogeneities and clarify their causes, we conducted the on-site observation in September 2021 and measured the distribution of thaw depth and soil water content. This chapter reports the preliminary interpretation from InSAR and on-site data.To summarize the three cases of fire scars, the thesis found an increase in seasonal and annual deformation amplitude from the following year of the fires and an end of annual subsidence about ~5 years after the fire. These processes are qualitatively consistent with previous reports. The spatial pattern of deformation is not directly related to the elevation, the burn severity, and the vegetation index derived from the optical satellite images. Instead, the deformation is related to the slope direction and the presence of a gully. In particular, high-resolution InSAR images and on-site observations quantitatively revealed spatial heterogeneity in the deformation amount within the fire scars. Furthermore, no correlation between the thaw depth and the heterogeneity suggests that it is challenging to predict thaw depth directly from seasonal subsidence.
(主査) 教授 古屋 正人, 教授 日置 幸介, 准教授 高田 陽一郎, 教授 蟹江 俊仁 （工学研究院）, 准教授 石川 守 （地球環境科学研究院）
理学院（自然史科学専攻）
Collection (particular)国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
Date Accepted (W3CDTF)2023-06-02T22:06:10+09:00
Data Provider (Database)国立国会図書館 : 国立国会図書館デジタルコレクション