Journal Article

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

Harrington, R. M., Hampel, A., Dielforder, A., Bocchini, G. M., Kemna, K., Oncken, O.

2023

Overview

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. Plain Language Summary Earthquakes release stresses that build up in the Earth due to the motion of tectonic plates. The stress release can cause additional earthquakes called aftershocks. Several thousand onshore and offshore aftershocks followed the great Tohoku subduction earthquake in March 2011. Whether the stress release of the Tohoku earthquake triggered most of the aftershocks is not well understood, because it is largely unknown how the stress field changed following the earthquake. We therefore use a computer model to estimate the stress release and resulting stress change required to explain the aftershock distribution. We find that 78% of the aftershocks occurred in areas where the Tohoku earthquake caused a subsequent stress increase. Our model results are further consistent with the stress release of smaller earthquakes that occurred in Japan before and after the Tohoku earthquake. Our findings provide new insights into aftershock triggering and help to understand where aftershocks occur after great earthquakes at subduction zones. Key Points We show using force‐balance modeling that a megathrust earthquake stress drop can trigger forearc‐wide aftershock seismicity Model results explain the Tohoku earthquake aftershock distribution and reveal spatial variability in forearc stress and strength Most aftershocks occurred in areas that experienced an increase in deviatoric stress

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Aftershocks / Earthquakes / Failure analysis / force balance / Geological faults / megathrust earthquake / Offshore / Plate tectonics / Plates (tectonics) / Seismic activity / Seismicity / Stress / Stress distribution / stress drop / Subduction / Subduction (geology) / Subduction zones / Tohoku earthquake