Topographic stresses affect stress changes caused by megathrust earthquakes and condition aftershock seismicity in forearcs: Insights from mechanical models and the Tohoku-Oki and Maule earthquakes

Abstract. Aftershocks of megathrust earthquakes at subduction zones may be driven by stresses arising from the topography of the forearc. However, the effect of topographic stresses on aftershock triggering is quantitatively not well understood and has been neglected in Coulomb failure stress models that assess whether the stress change caused by an earthquake promotes or inhibits failure on nearby faults. Here we use analytical and numerical models to examine the importance of topographic stresses on stress changes caused by megathrust earthquakes in the forearc. We show that the superposition of topographic and tectonic stresses leads to a dependence of the stress change on the stress state of the forearc. The dependence on the forearc stress state largely determines the coseismic stress change induced by a megathrust earthquake and must be considered when calculating Coulomb failure stress changes. We further show that increases in Coulomb failure stress promoting widespread failure in the forearc are only possible if topographic stresses dominate the regional stress field after the megathrust earthquake. Applying our modelling approach to the 2011 M_w 9.0 Tohoku-Oki and 2010 M_w 8.8 Maule megathrust earthquakes shows that the effect of topographic stresses caused Coulomb failure stress changes of up to ~40 MPa, which promoted the majority of aftershocks in the Japanese and Chilean forearcs. The model results further reveal that the spatial distribution of aftershocks was influenced by local differences in pre-earthquake stress states, fault strength and megathrust stress drop. Our analysis highlights the significance of topographic stresses in Coulomb failure stress calculations, enabling a better estimation of seismic hazard at subduction zones.