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Rotation of the principal stress directions due to earthquake faulting and its seismological implications

Pacific Geoscience Centre Geological Survey of Canada, Sidney, British Columbia Canada, V8L 4B2
School of Earth and Ocean Sciences University of Victoria, Victoria, British Columbia Canada

Abstract

Earthquake faulting results in stress drop over the rupture area. Because the stress drop is only in the shear stress and there is no or little stress drop in the normal stress on the fault, the principal stress directions must rotate to adapt such a change of the state of stress. Using two constraints, i.e., the normal stress on the fault and the vertical stress (the overburden pressure), which do not change before and after the earthquake, we derive simple expressions for the rotation angle in the ₁ axis. For a dip-slip earthquake, the rotation angle is only a function of the stress-drop ratio (defined as the ratio of the stress drop to the initial shear stress) and the angle between the ₁ axis and the fault plane, but for a strike-slip earthquake the rotation angle is also a function of the stress ratio. Depending on the faulting regimes, the ₁ axis can either rotate toward the direction of fault normal or rotate away from the direction of fault normal. The rotation of the stress field has several important seismological implications. It may play a significant role in the generation of heterogeneous stresses and in the occurrence and distribution of aftershocks. The rotation angle can be used to estimate the stress-drop ratio, which has been a long-lasting topic of debate in seismology.