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[STT39-08] Deformation of reclaimed valleys in Sendai during the 2011 off the Pacific Coast of Tohoku Earthquake detected by InSAR and its background factors
Keywords:The 2011 off the Pacific Coast of Tohoku Earthquake, reclaimed valley, InSAR
1. Introduction
In Japan, beginning with the urbanization in the 1960s, a lot of valleys were filled with soil cut from original ground to use as residential estates in urban regions. Recent earthquakes, such as the 2011 off the Pacific coast of Tohoku Earthquake, the 2016 Kumamoto Earthquake, and the 2018 Hokkaido Eastern Iburi Earthquake, have triggered landslides in gentle slopes in residential area and resulted in severe damage to housing lots. Nakano et al. (2021) detected the deformed area of reclaimed valleys occurred during the 2016 Kumamoto and 2018 Hokkaido Eastern Iburi earthquakes by Synthetic Aperture Radar Interferometry (InSAR). InSAR enables us to extract not only the damaged area but also small deformation in wide areas. Therefore, it is important to examine the validity of InSAR at detecting reclaimed valley deformation. In this study, we detected reclaimed valley deformation in Sendai city, where a lot of reclaimed valleys distribute and suffered serious damage during the 2011 off the Pacific coast of Tohoku Earthquake, by InSAR and examined the background factors in reclaimed valley deformation.
2. Study methods
We processed ALOS ascending data which was acquired between March 3, 2011 and April 18, 2011 and ALOS descending data which was acquired between November 20, 2010 and April 7, 2011, which resulted in 2 interferograms. In order to compare those interferograms with the distribution of reclaimed valleys, we made distribution maps of reclaimed valleys in Sendai city subtracting the past altitude from the present altitude by GIS. Additionally, in order to examine the sites detected as phase change areas in the interferograms and the background factors in reclaimed valley deformation, we conducted a field survey.
3. Results & Discussion
The interferogram from ascending orbits showed surface deformation moving away from the satellite in some reclaimed valleys. Comparing with distribution maps of reclaimed valleys, the spatial patterns of surface deformation looked like branches, and those deformation areas coincided with reclaimed valleys. Similarly, the interferogram from descending orbits showed surface deformation corresponded with reclaimed valleys. However, the interferogram from ascending orbits showed more distinct phase changes and widespread deformation areas than that from descending orbits. These results suggest the possibility that some reclaimed valleys deformed again during the earthquake occurred on April 7, 2011 (M7.2) because the interferogram from ascending orbits is an image by a pair of March 3, 2011 and April 18, 2011.
In some reclaimed valleys, the thicker the valley fill was, the larger the displacement was. Strong earthquakes cause compaction of soils by vibration and subsidence in reclaimed valleys in Sendai city (Institute of Geology and Paleontology, Tohoku University, 1979). Therefore, there is a high possibility that one of the background factors in reclaimed valley deformation is compaction of soils by vibration and subsidence.
The sites where localized phase change areas or decorrelated areas were detected based on the interferograms were coincided with the area of the actual deformation. Additionally, based on the interferograms, we detected deformed areas where there was no report on damage to housing lots.
4. Conclusions
In this study, we detected surface deformation of reclaimed valleys based on interferograms, which indicates the validity of InSAR at detecting reclaimed valley deformation. In addition, there is a high possibility that one of the background factors in reclaimed valley deformation in Sendai city is compaction of soils by vibration and subsidence.
In Japan, beginning with the urbanization in the 1960s, a lot of valleys were filled with soil cut from original ground to use as residential estates in urban regions. Recent earthquakes, such as the 2011 off the Pacific coast of Tohoku Earthquake, the 2016 Kumamoto Earthquake, and the 2018 Hokkaido Eastern Iburi Earthquake, have triggered landslides in gentle slopes in residential area and resulted in severe damage to housing lots. Nakano et al. (2021) detected the deformed area of reclaimed valleys occurred during the 2016 Kumamoto and 2018 Hokkaido Eastern Iburi earthquakes by Synthetic Aperture Radar Interferometry (InSAR). InSAR enables us to extract not only the damaged area but also small deformation in wide areas. Therefore, it is important to examine the validity of InSAR at detecting reclaimed valley deformation. In this study, we detected reclaimed valley deformation in Sendai city, where a lot of reclaimed valleys distribute and suffered serious damage during the 2011 off the Pacific coast of Tohoku Earthquake, by InSAR and examined the background factors in reclaimed valley deformation.
2. Study methods
We processed ALOS ascending data which was acquired between March 3, 2011 and April 18, 2011 and ALOS descending data which was acquired between November 20, 2010 and April 7, 2011, which resulted in 2 interferograms. In order to compare those interferograms with the distribution of reclaimed valleys, we made distribution maps of reclaimed valleys in Sendai city subtracting the past altitude from the present altitude by GIS. Additionally, in order to examine the sites detected as phase change areas in the interferograms and the background factors in reclaimed valley deformation, we conducted a field survey.
3. Results & Discussion
The interferogram from ascending orbits showed surface deformation moving away from the satellite in some reclaimed valleys. Comparing with distribution maps of reclaimed valleys, the spatial patterns of surface deformation looked like branches, and those deformation areas coincided with reclaimed valleys. Similarly, the interferogram from descending orbits showed surface deformation corresponded with reclaimed valleys. However, the interferogram from ascending orbits showed more distinct phase changes and widespread deformation areas than that from descending orbits. These results suggest the possibility that some reclaimed valleys deformed again during the earthquake occurred on April 7, 2011 (M7.2) because the interferogram from ascending orbits is an image by a pair of March 3, 2011 and April 18, 2011.
In some reclaimed valleys, the thicker the valley fill was, the larger the displacement was. Strong earthquakes cause compaction of soils by vibration and subsidence in reclaimed valleys in Sendai city (Institute of Geology and Paleontology, Tohoku University, 1979). Therefore, there is a high possibility that one of the background factors in reclaimed valley deformation is compaction of soils by vibration and subsidence.
The sites where localized phase change areas or decorrelated areas were detected based on the interferograms were coincided with the area of the actual deformation. Additionally, based on the interferograms, we detected deformed areas where there was no report on damage to housing lots.
4. Conclusions
In this study, we detected surface deformation of reclaimed valleys based on interferograms, which indicates the validity of InSAR at detecting reclaimed valley deformation. In addition, there is a high possibility that one of the background factors in reclaimed valley deformation in Sendai city is compaction of soils by vibration and subsidence.