Overlapping post-seismic deformation processes: Afterslip and viscoelastic relaxation following the 2011Mw 9.0 tohoku (Japan) earthquake

Large tectonic earthquakes lead to significant deformations in the months and years thereafter.These so-called post-seismic deformations include contributions mainly from afterslip and viscoelasticrelaxation, quantification of their relative influence is of importance for understandingthe evolution of post-seismic crustal stress, strain and aftershocks. Here, we investigate thepost-seismic deformation processes following the 2011 M_w 9.0 Tohoku earthquake using surfacedisplacement data as observed by the onshore global positioning system network in thefirst ~1.5 yr following the main shock. We explore two different inversion modelling strategies:(i) we simulate pure afterslip and (ii) we simulate the combined effect of afterslip andviscoelastic relaxation. By assuming that the afterslip is solely responsible for the observedpost-seismic deformation, we find most afterslip activities to be located close to the downdiparea of the coseismic rupture at 20-80 km depth with a maximum cumulative slip of ~3.8mand a seismic moment of 2.3 × 10²² Nm, equivalent in moment to an M_w 8.84 earthquake.By assuming a combination of afterslip and viscoelastic components, the best data fit is foundfor an afterslip portion that is spatially consistent with the pure afterslip model, but reveals adecreased seismic moment of 2.1 × 10²² Nm, or M_w 8.82. In addition, the combined modelsuggests an effective thickness of the elastic crust of~50 km overlying an asthenosphere with aMaxwell viscosity of 2×10¹⁹ Pa s. Temporal analysis of our model inversions suggests that therate of afterslip rapidly decreases with time, consistent with the state- and rate-strengtheningfrictional law. The spatial pattern of afterslip coincides with the locations of aftershocks, andalso with the area of coseismically increased Coulomb failure stress (CFS). Only a small partof the coseismically increased CFS was released by the afterslip in 564 d after the event. Theeffect of the viscoelastic relaxation within this initial stage only plays a secondary role, butit shows an increasing tendency, that is, the contribution of viscoelastic relaxation increaseswith time. Further geodetic observations are needed for a robust quantification of the role ofthe viscoelastic relaxation in the post-seismic deformation.