Role of Slabs in Postseismic Deformation Following Deep Earthquakes

Abstract

The Earth's viscoelastic postseismic deformation reflects its rheological structure. Even though low-viscosity structures, such as the asthenosphere, are often thought to dominate postseismic deformation induced by shallow earthquakes, high-viscosity subducting slabs have also been found to considerably affect postseismic deformation following subduction zone earthquakes including deep-focus events. However, for deep earthquakes, the exact mechanism by which slab structures influence stress relaxation and the resulting deformation processes is poorly understood. Here, we conduct the first systematic study investigating the effect of a slab on the Earth's viscoelastic relaxation following a $\sim$600-km deep earthquake. We perform numerical modeling with and without a subducting slab for representative source mechanisms and slab geometries and compare the results. In general, we find that the slab structure significantly impedes stress relaxation. The high-viscosity slab sustains most of the coseismic stress, which leads to stress concentration within it; in the surrounding mantle, the relaxation of stress also becomes much slower compared to the case without the slab. Such differences in the spatiotemporal evolution of stress, which are further influenced by the geometries of the earthquake source and slab structure, result in the distinct patterns of postseismic deformation at the Earth's surface. Interestingly, we also find that even an extremely confined region of high viscosity surrounding the earthquake source can generate a significant slab effect. Our study provides a general framework for interpreting deep earthquake induced postseismic deformation and an improved understanding of the relationship between the Earth's 3D rheological structures and viscoelastic relaxation processes.