Deformation of Reclaimed Valleys in Sendai City during the 2011 Tohoku-Oki Earthquake Detected by InSAR and Its Background Factors

In Japan, beginning with the urbanization starting in the 1960s, a lot of valleys were reclaimed to use as residential estates. Recent earthquakes, such as the 2011 Mw 9.0 Tohoku-Oki Earthquake, the 2016 Mw 7.0 Kumamoto Earthquake, and the 2018 Mw 6.6 Hokkaido Eastern Iburi Earthquake, have triggered landslides in gentle slopes in reclaimed valleys and resulted in severe damage to housing lots. In this study, we detected reclaimed valley deformation in Sendai city occurred during the 11 March 2011 Tohoku-Oki Earthquake by InSAR and examined its background factors.

We made 2 interferograms using ALOS/PALSAR data acquired from both ascending and descending orbits. The ascending data was acquired between March 3, 2011 and April 18, 2011, and the descending data was acquired between November 20, 2010 and April 7, 2011. Additionally, we made a distribution map of reclaimed valleys in Sendai city by subtracting the past altitude from present altitude using ArcGIS. The interferograms showed phase change corresponding to subsidence or decorrelation in the reclaimed valleys distributed like branches. The phase change was observed where actual damages have been reported. Moreover, we detected up to ~10 cm of displacements away from the satellite on some reclaimed valleys where no damage to housing lots has been reported. The interferogram from the ascending orbit showed more distinct phase change in the reclaimed valleys than that from the descending orbit, suggesting the possibility that some reclaimed valleys deformed not only in response to the Tohoku-Oki Earthquake but also during the earthquake occurred on April 7, 2011 (M7.2) whose epicenter was approximately 95 km from Sendai. In Sendai city, the fill material of the deep part of reclaimed valleys was composed of loose soils cut from the shallow part of surrounding original ground and contained a lot of water (Institute of Geology and Paleontology, Tohoku University, 1979). On many of the subsided areas, we found a proportional relation between the thickness of the reclaimed soil and the amount of displacement, suggesting that the dominant mechanism of the deformation is the compaction of soil due to the strong seismic motion. This study indicates the validity and usability of InSAR on detecting reclaimed valley deformation.