Imaging a low viscosity zone beneath the Kutcharo caldera, eastern Hokkaido, Japan, using geodetic data

Abstract

Geodetically observed volcano deformation has been analysed in terms of either elastic or viscoelastic crustal rheology, for which the presence of magma has usually been considered as a deformation source. From a rheological viewpoint, however, a magma and the peripheral rocks it influences are presumably low viscosity materials, which allows detection of the presence of magma as a zone that has rheologically less strength. In this study, using a 3-dimensional finite element model that introduces a rectangular low viscosity zone (LVZ) into relatively high viscosity crust, the response of the linear Maxwell viscoelastic crust to the emplacement of a spheroidal sill is adopted for interferometric synthetic aperture radar (InSAR) data of the Kutcharo caldera, eastern Hokkaido, Japan, to image a spatial crustal viscosity variation. The observed temporal displacement at the surface point, where the line-of-sight (LOS) displacement is largest in the deformation field, requires the LVZ to have a spatially uniform viscosity of ∼4–5×1017 Pa s. The LVZ is also required to be present immediately below a 5-km elastic layer down to mid-crustal depths or deeper, with a horizontal extent of at least 10 km. Models with acceptable minimum sizes of LVZ are found to satisfy the overall temporal displacement field with root mean square misfits of less than ∼2 cm. The geodetic data reasonably accept the presence of an LVZ beneath the Kutcharo caldera that is consistent with the magnetotelluric image of the accumulated magma, providing the mechanical aspect of the low resistivity domain. The LVZ model behaviour suggests an important implication that the accumulation of magma in the middle to lower crust is detectable using geodetic data after magma emplacement into the upper crust, where the deformation rate represents the degree of magmatic influence on the crustal viscosity.