Aftershock activity following the 2011 Tohoku earthquake suggests near-stationnary afterslip rate at depth

We analyze the evolution of the interplate earthquake rate along the Japan Trench following the 2011 Tohoku earthquake. Nearly 14 years of aftershock activity allows to constrain with good accuracy how the rate relaxes after an initial jump, and how this relaxation depends on location, and most notably on depth. We find that specific intermediate depth areas display very little relaxation, i.e., that the rate of earthquake post-2011 stays constant at an elevated rate throughout the >10 years. This behaviour is specific to small, isolated areas, that tend to host repeating earthquakes, and that are located within the (large) GPS-inverted afterslip zone. The relaxation is found to be faster, tending to a classical Omori-like type, when averaging over larger areas. Our observations suggest that (1) afterslip kinematics following the 2011 megathrust is highly spatially dependent (showing significant variability at the kilometric scale), (2) that the usually accepted 1/t afterslip relaxation is only valid when averaged over large areas, (3) that the relaxation can be very slow in areas characterized by small, isolated asperities, in the transition zone between the locked updip fault and the deeper fault where no interplate activity is observed. This overall trend can be seen as caused by the stress-‘screening’ of rapidly healing asperities at shallow depth that cause the post-seismic deformation to quickly relax, while the slip rate / deformation remains nearly stationnary when moving away from these asperities.