Inferring 3‐D Rheology of Low‐Viscosity Zone Around Quaternary Volcanoes of NE Japan From Postseismic Deformation of the 2011 Tohoku‐Oki Earthquake

Abstract Several geological and geophysical studies suggest the small‐scale presence of low viscosity zone (i.e., LVZ) beneath the Quaternary volcanoes of northeastern Japan. Before the 2011 M w 9.0 Tohoku‐oki earthquake, scientists hypothesized that these LVZs cause localized crustal deformations around the Quaternary volcanoes. However, the deformation‐signals related to these LVZs were too weak to properly understand the LVZ rheology. After the 2011 Tohoku‐oki earthquake, InSAR and Global Navigation Satellite System (GNSS) observations reported significant ground movements around five Quaternary volcanoes including the Mt. Akitakoma, Mt. Kurikoma, Mt. Zao, Mt. Azuma, and Mt. Nasu. Using the early years of postseismic GNSS displacements, we extracted the short‐wavelength components of strain‐rate distribution, which clearly show the localized crustal contraction near the five volcanoes. To explain such spatial pattern of localized contraction, we propose a 3‐D rheological model of LVZs near the five volcanoes, using power‐law Burgers rheology. Most of our modeled LVZs have narrow tops (width of 20–40 km), wide roots (width of 80–100 km), limited arc‐parallel dimensions (≤80 km), and are located at the depth range of 15–55 km in the lower crust‐upper mantle. Based on the localization of postseismic strain rate, newly proposed 3‐D LVZ models highlight an arc‐parallel heterogeneity of subsurface rheology along the volcanic front of northeastern Japan, which is consistent with previously reported clusterization pattern of late Cenozoic calderas and high geothermal gradient near the five Quaternary volcanoes.