New Insights into Upper-Mantle Rheology from the Post-Seismic Deformation Following Deep Earthquakes

Abstract:

Upper-mantle rheology is one of the least constrained properties despite its importance in mantle dynamics. In this study, we introduce a new approach to understand the viscosity structure by examining the earth’s response to deep earthquakes occurring at 300+ km. We combine numerical modeling of viscoelastic deformation of upper mantle and the GPS observation of post-seismic deformation from deep earthquakes.

Our numerical predictions show that the viscoelastic deformation from deep earthquakes is more sensitive to viscosity structure at deeper depths than that from shallow earthquakes. This is because the spatio-temporal evolution of the post-seismic deformation is largely governed by the rheological structure between the earthquakes and the earth’s surface. Such distinctive sensitivity extending deeper beyond lithosphere allows independent constraints on upper-mantle viscosity structure including the thickness and viscosity of asthenosphere.

Major challenge in utilizing the post-seismic deformation from deep earthquakes, however, is on the observation side. So far, most GPS observations have been limited to shallow earthquakes since the amplitude of the surface deformation from deep events is considered small. Here, we take advantage of data processing techniques such as independent component analysis to extract post-seismic signals from deep earthquakes. We examine the GPS data of one of the largest deep earthquakes ever recorded, 2018 Mw 8.2 Fiji earthquake which occurred at ~600-km depth in an oceanic subduction environment. We detect a large scale post-seismic deformation that has been taking place for more than one and half years. The spatial extent of the surface deformation reaches farther than 1000 km from the epicenter, which is considerably large compared to what is often considered for shallow earthquakes. The overall directionality and amplitude of the deformation suggest strong presence of rheologically weak structures such as asthenosphere. Furthermore, the lateral variation in the deformation may indicate significant 3-D rheological structure, e.g., the contrast in asthenospheric structure on different sides of the oceanic convergent margin.