Rapid postseismic uplift along the Pacific coast of northeastern Japan after the 2011 Tohoku-oki earthquake and forecasting pre-event relative sea-level recovery

The postseismic deformation of the 2011 Tohoku-oki earthquake has been explained by a range of postseismic models that includes viscoelastic relaxation (VR) in the lower crust-upper mantle and continuous afterslip (AS) on the plate boundary fault (e.g., Sun et al. 2014 Nature, Freed et al. 2017 EPSL). Most often, these postseismic models succeed in reproducing the observed horizontal motion, but reproducing the vertical deformation, in both in time and space, has proven more difficult. Hence, our understanding of the relative contribution of VR and AS across northeastern (NE) Japan remains elusive. Recent advancements in the stress-dependent model with high-resolution GNSS observations (Agata et al. 2019 Nat. Comm.; Muto et al. 2019 Sci. Adv.; Dhar et al. 2022 GJI) successfully decompose the postseismic crustal deformation into their individual VR and AS components during the transient period (~5 year) since the earthquake. Here, we deployed the 3-D rheological model incorporating laboratory-derived constitutive laws (Dhar et al. 2022 GJI) and simulate postseismic surface deformation across NE Japan over ten years after the mainshock.

Our model exhibits substantial agreement with the horizontal and vertical observations (both in time and space). Our results suggest that VR dominates in the early postseismic period (a few years) after the earthquake for the horizontal component at most GNSS stations. Although VR initially drives forearc uplift after the earthquake, predominantly due to the influence of the cold nose (Luo and Wang 2021 Nat. Geosci.), it eventually decays in the subsequent years. Over a decade of observations, the AS component dominates rapid postseismic uplift along the Pacific coast of NE Japan. This coastal uplift is due to the persistence of AS at the downdip of the main rupture area, which shows no sign of ceasing. One key take-home from our model, is that it can be used to forecast the crustal deformation over the next few years. We also estimated the recovery period (time to regain their pre-earthquake sea level) for coastal areas with significant coseismic subsidence. Hence, our results may benefit in rebuilding the port piers which are now way above the present sea level, hindering transshipment business (Iinuma 2018 JDR).