Numerical simulations of gravitational perturbations due to pre-seismic deep slab deformations before the 2011 Mw 9.1 Tohoku earthquake

The numerical simulation of gravity perturbations associated with deep slab deformations during the seismic cycle of great subduction earthquakes remains a significant challenge. This study presents a novel approach for simulating gravity anomalies induced by short-term slab deformations using the Spectral-Infinite-Element (SIE) method, implemented in the SPECFEM-X tool. Geodynamic models involving different fault settings are developed within a realistic 3D earth structure. The simulation includes a layer of infinite boundary elements surrounding the models in order to mimic a semi-infinite extent of the domain. Sensitivity analyses are carried out to assess the influence of the fault slip parameters (magnitude, mechanism, and location) as well as the density and velocity structure. The approach is first validated through synthetic benchmarks and then applied to a real-world scenario of the 2011 M_w 9.1 Tohoku earthquake. For this case, we design a 3D Earth model, incorporating a realistic Pacific slab in the region of the earthquake, and calculate the gravity anomalies induced by a sudden episode of slab extension, which is hypothesized to have occurred months before the rupture. The modelled gravity changes due to these pre-seismic deformations are compared with GRACE satellite gravity observations. This work highlights the importance of numerical simulations in satellite gravimetry and geodesy, offering new insights into the deformation processes that may result in gravity anomalies during the seismic cycle.