Abstract
Localized significant subsidence, associated with the 2011 megathrust Tohoku-Oki earthquake (MW 9.0), was revealed with Interferometric Synthetic Aperture Radar (InSAR) in five volcanic regions in northeastern Japan. Focusing on Mt. Zao, Sato (2017) attempted to quantitatively interpret the observed localized subsidence with 2-D finite element modeling of a vertical plane in the E–W direction. A horizontally elongated elliptic body representing hot-and-weak rock (magmatic complex including water) was assumed beneath the volcano. Sato (2017) compared calculated and observed vertical displacements approximately derived with the assumption that the direction of surface displacement almost coincides with that of line-of-sight (LOS) in InSAR observations, while this study compares calculated and observed surface displacements in the LOS direction. The observed surface displacements are interpreted with sufficient confidence. Hence, it seems possible to conclude that surface displacements, such as localized subsidence, in volcanic regions associated with large earthquakes are due to the existence of hot-and-weak rock bodies beneath the volcanoes. In the case of Mt. Zao, an appropriate combination of its size (length of the major axis) and Poisson's ratio is found to be 11 km and 0.49. Young's modulus is not absolutely determined; it would be in the range between approximately 1 and 8 GPa with a plausible value of ∼1 GPa (bulk modulus of ∼17 GPa) or slightly larger.
