Rupture Dynamics Along Subduction Zones: Structural and Geometrical Complexities and the Case of Tohoku-Oki Earthquake

We aim to characterize the rupture dynamics along the subduction zones. We investigated the shear/normal stress coupling when geometrical discontinuities and/or realistic velocity fields induce normal traction perturbations. The Spectral Element Method (SEM) is shown to be a powerful numerical tool to perform dynamic simulations for subduction earthquakes due to its geometrical flexibility and to the easy implementation of classical seismological boundary conditions. Sharp variations of normal stress are induced when a rupture propagates between dissimilar materials. Performing dynamic simulations along bimaterial interfaces, we show how the Coulomb friction law leads to unstable solutions due to the missing time/length scale of shear/normal coupling. We also show how the shear stress response has to be properly delayed to provide stable physical reliable solutions and how this delay can allow to define a length, comparable with the dissipation zone, which can be interpreted as the length of coupling. Free surface interaction is shown to generate a break of symmetry in the shallow part of dipping faults. In particular larger ground motion on the hanging wall and thus larger coseismic slip is evidenced. Due to the fault/free surface interaction this slip is shown to be generally associated with low-frequency radiation. Finally, exploiting these results some dynamic models of Tohoku earthquake are presented. The main source features of that event can be addressed in terms of influence of geometry and structure and thus of shear/normal coupling. Along dip we find a bilateral rupture faster trenchward where the largest coseismic slip is measured. Strong rupture accelerations due to geometrical and velocity discontinuities imply high-frequency sub-sources in the deep part of the subduction. We finally show how taking into account these dynamic features the tsunami scenarios for the Tohoku earthquake lead to higher estimates for tsunami hazard.