Surface changes of soil liquefaction associated with the 2011 Tohoku earthquake derived from coherence in SAR interferometry

The 2011 Tohoku earthquake (Mw 9.0) occurred northeast of the Japan Trench on 11th March with a ruptured area as large as 500 km ~ 200 km. It was followed by numerous large aftershocks along the Japan Trench. In the Kanto region, including Tokyo about 350 km from the main shock, large areas of soil liquefaction occurred along the shore, causing extensive damage to residential buildings and infrastructure. Soil liquefaction is usually investigated by field reconnaissance and aerial photography. In this study, however, we successfully applied InSAR analysis, derived from satellite data, to identify and map the occurrence of soil liquefaction. When soil liquefaction is occurred, the ground surface might become more wetting and constructions might be inclined as compared with the before of liquefaction. Therefore we can detect these changes of surface properties of the ground in order to identify the soil liquefaction area from coherence reduction (decorrelation) of SAR data. Often, decorrelation is simply considered as damaged areas due to the disaster, however, we here proposed to use the coherence change threshold in order to discriminate significant decorrelation due to soil liquefaction from that due to ordinal surface covering changes. Moreover, local surface displacements caused by soil liquefaction were estimated using phase difference of the SAR datasets. Our results compare favorably with those from surveys of sand boils and aerial photography, showing that surface changes derived from SAR data are associated with soil liquefaction. Our results demonstrate that soil liquefaction occurred mainly near the waterfront along Tokyo Bay and the Tone River, and ground subsidence was widely distributed. Results of this study implied that there are liquefaction areas which were not found in previous studies.