ROTATION OF THE PRINCIPAL STRESS DIRECTIONS DUE TO EARTHQUAKE FAULTING AND ITS SEISMOLOGICAL IMPLICATIONS

Earthquake faulting results in stress drop over the rupture area. Beca use the stress drop is only in the shear stress and there is no or lit tle stress drop in the normal stress on the fault, the principal stres s 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 rotatio n angle in the sigma(1) axis. For a dip-slip earthquake, the rotation angle is only a function of the stress-drop ratio (defined as the rati o of the stress drop to the initial shear stress) and the angle betwee n the sigma(1) axis and the fault plane, but for a strike-slip earthqu ake the rotation angle is also a function of the stress ratio. Dependi ng on the faulting regimes, the sigma(1) axis can either rotate toward the direction of fault normal or rotate away from the direction of fa ult normal. The rotation of the stress field has several important sei smological implications. It may play a significant role in the generat ion of heterogeneous stresses and in the occurrence and distribution o f aftershocks. The rotation angle can be used to estimate the stress-d rop ratio, which has been a long-lasting topic of debate in seismology .