Numerical Modeling of Localized Subsidence Observed in Volcanic Regions Associated with the 2011 off the Pacific Coast of Tohoku Earthquake

Abstract

 Interferometric Synthetic Aperture Radar (InSAR) revealed unprecedentedly significant localized subsidence in some volcanic regions, namely Mt. Akitakoma, Mt. Kurikoma, Mt. Zao, Mt. Azuma, and Mt. Nasu, in the northeastern part of Japan associated with the 2011 megathrust earthquake (M_W 9.0), which occurred off the Pacific coast of Tohoku (Takada and Fukushima, 2013, 2014). Maximum subsidence ranges from approximately 5 cm (Mt. Nasu) to 15 cm (Mt. Azuma). Subsided regions are roughly elliptically elongated with major axes of approximately 15-20 km in almost the N-S direction, i.e., nearly perpendicular to the axis of coseismic horizontal extension due to the earthquake. To quantitatively investigate volcanic deformation triggered by the earthquake, we performed numerical modeling with the 2D finite element method (FEM) focusing on Mt. Zao. We used two types of FE model for the E-W cross section through the Mt. Zao volcanic region: one with an elliptic body of hot-and-weak rock (including magma reservoir and water within it) elongated horizontally beneath the volcano, and another without it. To impose the reverse coseismic slip of the earthquake, nonuniform tangential displacement is assigned based on the estimated slip distribution along the model boundary corresponding to the interface between the Pacific and North American plates. To clarify the dependence of maximum subsidence and spatial dimensions of the subsided region upon the characteristics of the hot-and-weak rock body, we consider many cases with different size and elastic parameters, such as Young's modulus and Poisson's ratio of the hot-and-weak rock body. It is found that the existence of the hot-and-weak rock body can cause localized subsidence above it, and that maximum subsidence increases with size and Poisson's ratio, but decreases with Young's modulus, of the hot-and-weak rock body, and spatial dimensions of subsided region also increase with size and Poisson's ratio of the hot-and-weak rock body, but do not depend significantly on Young's modulus. The most appropriate combination of size (length of the horizontal major axis), Young's modulus, and Poisson's ratio of the hot-and-weak rock body that can best fit the calculated subsidence pattern to the observed one is found to be that with values of 15 km, 20 GPa, and 0.35.