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Geodetic observations after large subduction earthquakes reflect multiple postseismic processes, including megathrust relocking. The timing of relocking and the observational constraints on it are unclear. Relocking was inferred to explain some observed landward motion that occurs within months. It was also considered unable to explain other, greater landward motion, including that off the coast of Japan beginning weeks after the 2011 Tohoku earthquake, attributed to postseismic relaxation. We use generic, 3D numerical models to show that relocking, particularly of the shallow interface, is needed for postseismic relaxation to produce landward motion on the tip of the overriding plate. We argue that this finding is consistent with previous simulations that implicitly relock the megathrust where afterslip is not included. We conclude that the Tohoku megathrust relocked within less than 2 months of the earthquake. This suggests that the shallow megathrust probably behaves as a true, unstably sliding asperity. Plain Language Summary: In the largest earthquakes, a tectonic plate suddenly slides under another, where previously the interface between them was largely locked. The deformation of the Earth's surface reflects multiple deep processes. One process is the restoration of interface locking. This means that elastic energy starts to accumulate again, building up toward the next earthquake. How long after an earthquake does the interface relock? Relocking is suggested to have caused some deformation observed months after some earthquakes, and so to have occurred by then. It is also thought to not explain the rapid landward motion of the seafloor on the upper plate, observed beginning less than 2 months after the 2011 Japan earthquake. We use numerical simulations to show that locking the shallow portion of the plate interface is needed for landward motion to transmit from the lower plate to the upper plate, as happened in Japan. Landward motion is understood to occur below and in the lower plate because of mantle flow and deep sliding taking place after the earthquake and caused by it. We conclude that this result agrees with simulations in previous studies, even though they were not interpreted in terms of locking by their authors. Key Points: Landward motion of the overriding plate, attributed to postseismic relaxation, was observed offshore 2 months after the Tohoku earthquakeModels show that landward motion only transmits from the slab to the overriding plate, reaching the surface, if the megathrust is lockedObservations imply shallow megathrust relocking within 2 months of the Tohoku earthquake, consistent with unstably sliding behavior [ABSTRACT FROM AUTHOR]

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]

After the 2011 Mw 9.0 Tohoku earthquake ruptured the subduction megathrust offshore Japan, many normal faulting earthquakes were observed in regions that previously hosted dominantly thrust faulting events. The rate of normal faulting seismicity has decreased since 2011, but is still significantly higher than before the Tohoku earthquake. Similar normal faulting earthquake sequences have been observed after other Mw 8.5+ megathrust events and in some cases appear to last for decades. In this study, we investigate whether the spatial distribution and temporal evolution of normal faulting seismicity in subduction zones can be explained by earthquake cycle deformation processes, particularly co-seismic and post-seismic stress changes. We develop a three-dimensional finite element model of the subduction zone offshore Honshu with an appropriate plate interface geometry and a rheology consisting of an elastic upper plate above a linear visco-elastic mantle wedge. We spin up the model using internally consistent earthquake cycles to generate pre-stresses; stresses build across the locked plate interface during the inter-seismic stage, driving co-seismic slip. Afterwards, the interface is re-locked as afterslip and bulk viscous relaxation occur, leading back into the subsequent inter-seismic stage. We incorporate the co-seismic slip from known megathrust earthquakes occurring before 2011 to better represent the slip deficit and state of stress at the time of the Tohoku earthquake. The model results show that the immediate co-seismic effect of a great megathrust earthquake is to cause approximately trench-normal extension of the upper plate (relative to the pre-earthquake stress field). This extension occurs above and landward of the rupture zone in the regions that currently host significant normal faulting earthquakes. Early afterslip occurring on the subduction interface down-dip of the rupture results in further extension of both the upper and subducting plates, promoting normal faulting earthquakes in these regions. Although the upper plate continues to move trenchward as a consequence of the bulk viscous relaxation of the deeper visco-elastic regions, this does not have a significant effect on the stress field in the shallower, elastic regions. The dominant process responsible for reestablishing the pre-earthquake state of stress is the plate convergence after the re-locking of the subduction megathrust. We find good agreement between the predicted spatial and temporal pattern of tensile stresses and the post-seismic normal faulting earthquakes in Japan. [ABSTRACT FROM AUTHOR]