Postseismic Deformation of Deep Earthquake Reflecting Viscous Structure of the Upper Mantle: A Case Study Examination of the 2012 Okhotsk Deep Earthquake

Abstract

 Activities are underway to estimate the overall viscosity structure of the upper mantle using geodetic observational data on postseismic deformation caused by deep earthquakes in subduction zones. Viscoelastic relaxation following a deep earthquake with a magnitude of Mw 7.7 that occurred on August 14, 2012, in the southern Sea of Okhotsk near Japan at a depth of 598.2 km is examined, as well as the extent to which this phenomenon is reflected in GNSS observation data in northern Hokkaido and Sakhalin, using a three-dimensional viscoelastic model. Multiple models with a stratified + slab structure are adopted, each differing in the depth of the low-viscosity layer within the upper mantle. It is found that efficient Maxwell viscoelastic relaxation can result in surface displacements exceeding 1 millimeter. This effect is observed when a low-viscosity layer, encompassing the depth range of the source fault, is present within the mantle transition zone—even when the viscosity of the upper mantle adheres to the conventional standard of 10²⁰ Pascal seconds. It is assumed that three-dimensional heterogeneity (low-viscosity region) in the asthenosphere or beneath the slab does not significantly alter these results, although three-dimensional heterogeneity caused by the presence of the slab plays an important role. However, at present, it is challenging to detect the postseismic deformation signal of the deep earthquake due to the dominance of surface displacement signals that are an order of magnitude larger, such as those from the postseismic deformation of the 2011 Tohoku megathrust earthquake, in observational data from the same period. It will become possible to overcome this challenge with future advances in observational and analytical technologies.