Landslide Surface Deformation Detected by Synthetic Aperture Radar (SAR) Interferometry in Shizu Area on the Southern Foot of Mt. Gassan, Japan

L-band (22.36 cm in wavelength) radars such as ALOS (Advanced Land Observing Satellite)/PALSAR (Phased Array type L-band Synthetic Aperture Radar) are suitable for observing landslide surface deformation in Japan. In this study, we produced InSAR (SAR interferometry) images using PALSAR data and observed the fringes produced; after subtracting orbital and topographic fringes, the resulted fringe indicated a 2 × 1 km landslide surface deformation on the southern foot of Mt. Gassan (stratovolcano, 1,994 m in elevation) in Japan; this landslide is actually a reactivated old landslide. According to a previous study, slide surfaces can be identified both in the deposits and in the underlying mudstone, which has a maximum depth of 160 m from the ground surface. The InSAR image indicated surface deformation at more than half of the L-band wavelength along the LoS (line of sight). Because of the good coherence (0.4–0.6) of the obtained InSAR image, we unwrapped the phase of the InSAR image and obtained a continuous phase. We found that the detected landslide deformation could be separated into two sections: a fluvial erosion-affected section and an upper slope section. In the former section, repeated surface deformation between 0 and 7 cm along the LoS implies the deformation by many sub-slide surfaces. In the latter section, surface deformation along LoS uniformly increased from 2.5 cm at the upper scarp to 13.5 cm at the foot of the slopes; this implies that one large rigid landslide block controls surface deformation. An on-site GPS survey of the fluvial erosion-affected section revealed the landslide’s surface deformation, and the measurement results from the GPS survey show good agreement with the InSAR results.