Megathrust Stress Drop as Trigger of Aftershock Seismicity: Insights From the 2011 Tohoku Earthquake, Japan

Numerous normal-faulting aftershocks in subduction forearcs commonly follow large megathrust earthquakes. Postseismic normal faulting has been explained by stress changes induced by the stress drop along the megathrust. However, details of forearc stress changes and aftershock triggering mechanisms remain poorly understood. Here, we use numerical force-balance models combined with Coulomb failure analysis to show that the megathrust stress drop supports normal faulting, but that forearc-wide aftershock triggering is feasible within a narrow range of megathrust stress drop values and preseismic stress states only. We determine this range for the 2011 Tohoku earthquake (Japan) and show that the associated stress changes explain the aftershock seismicity in unprecedented detail and are consistent with the stress released by forearc seismicity before and after the earthquake.