Coseismic and postseismic stress rotations due to great subduction zone earthquakes

The three largest recent great subduction zone earthquakes (2011 M9.0 Tohoku, Japan; 2010 M8.8 Maule, Chile; and 2004 M9.2 Sumatra‐Andaman) exhibit similar coseismic rotations of the principal stress axes. Prior to each mainshock, the maximum compressive stress axis was shallowly plunging, while immediately after the mainshock, both the maximum and minimum compressive stress axes plunge at ∼45°. Dipping faults can be oriented for either reverse or normal faulting in this post‐mainshock stress field, depending on their dip, explaining the observed normal‐faulting aftershocks without requiring a complete reversal of the stress field. The significant stress rotations imply near‐complete stress drop in the mainshocks, with >80% of the pre‐mainshock stress relieved in the Tohoku and Maule earthquakes and in the northern part of the Sumatra‐Andaman rupture. The southern part of the Sumatra‐Andaman rupture relieved ∼60% of the pre‐mainshock stress. The stress axes rotated back rapidly in the months following the Tohoku and Maule mainshocks, and similarly in the southern part of the Sumatra‐Andaman rupture. A rapid postseismic rotation is possible because the near‐complete stress drop leaves very little “background” stress at the beginning of the postseismic reloading. In contrast, there has been little or no postseismic rotation in the northern part of the Sumatra‐Andaman rupture over the 7 years since the mainshock. All M ≥8.0 subduction earthquakes since 1990 with an adequate number of pre‐ and post‐mainshock events were evaluated, and not all show similar coseismic stress rotations. Deeper earthquakes exhibit smaller coseismic stress rotations, likely due to increasing deviatoric stress with depth.