Insights on the postseismic surface displacement field of the 2011 Tohoku earthquake via 3D viscoelastic seismic-cycle FEM models.

The 2011 Tohoku-oki earthquake occurred in the well-studied Japan convergent margin andproduced coseismic and postseismic surface displacements well observed at several GPSsites, including a dense onshore network and offshore GPS-A stations. Such geodeticdata, particularly large landward postseismic displacements in the primary rupturearea, suggest that relocking occurred shortly after the earthquake. This has clearimplications for the slip deficit accummulation and thus the seismic hazard in theregion. However, the landward motion recorded at the sites closest to the trench issignificantly faster than the plate convergence rate, which suggests that it cannot beattributed solely to relocking. Instead, it has been explained through numericalmodelling as due to viscoelastic relaxations of the stresses induced by coseismicdisplacement. Numerical modelling is valuable as it can quantitatively connect the surface displacementfield to the relocking history of the megathrust interface as well as to the postseismicrelaxation history. We thus construct a finite-element model of the Japan subduction zonewith a simplified, spherical-shell 3D geometry. The model is driven to realistic preseismicstresses and strains by imposing the earthquake history of the last several decades. We thenuse it to investigate the geodetic signature of relocking history in the context of specificsubduction zone geometry and physical features. The hypothesis is that a combination ofrapid relocking and significant viscous relaxation is needed to explain the observedpostseismic surface velocities. The lithospheres in the model have linear elastic rheology, neglecting the minor anelasticcomponent of deformation, while the asthenospheres have Maxwell viscoelastic rheologies.The areas of the megathrusts modelled as non-seismogenic are also viscoelastic and thusallow for afterslip in response to stresses induced during coseismic slip. The lithosphericstructure is imported from the Slab2 dataset, derived from seismological data. The effect ofdifferent lithospheric thicknesses of both the overlying and downgoing plates is thenexplored. This accounts for uncertainties, heterogenity and the discrepancy between differentdefinitions of plate thickness. The spatial extent of the coseismically slipping patch in themegathrust is varied in different models, especially its updip limit. This is crucial inattempting to reproduce the surface displacement close to the trench. Furthermore, it reflectsuncertainties and possible variations in locking behaviour of peripheral areas of theinterface. Preliminary results show that rapid relocking is not sufficient to explain the landward anddownward displacements observed offshore on the upper plate. Work is ongoing to fullycharacterize the primary features of the subduction zone responsible for the observed surfacedisplacements. [ABSTRACT FROM AUTHOR]