A novel approach to determine Coulomb stress changes in subduction megathrust earthquakes

Great subduction earthquakes cause Coulomb stress changes that can trigger intense aftershock seismicity, including strong earthquakes in the upper plate. The Coulomb stress changes are commonly computed assuming that the regional stress field does not change in the earthquake and that gravity can be neglected. However, geophysical observations from great subduction earthquakes indicate that the regional stress field changes in the earthquake and that aftershock seismicity in the upper plate may be driven by gravity.

Here, we use finite-element models of force balance to evaluate upper-plate stress changes in great subduction earthquakes. We show that gravitational stresses greatly influence the Coulomb stress change in a megathrust earthquake and cause a dependence of the stress change on the pre-earthquake stress conditions. While the dependence on pre-earthquake stress conditions complicates the assessment of the Coulomb stress change, accounting for it provides important information on the factors controlling the aftershock seismicity in the upper plate. We discuss these aspects at the examples of the 2011 M9 Tohoku-Oki (Japan) and the 2010 M8.8 Maule (Chile) earthquakes.