Abstract

We reexamine two canons of the seismological literature, that elastic displacements in the far field are proportional to slip velocities on the dynamical fault surface, and that dynamical in-plane slip on an earthquake fault has a double-couple body force equivalent. We show that if faulting takes place on a fault of finite thickness, and there is a strength-weakening zone near the advancing crack tip, there is an additional single-couple term in the body force equivalence and additional terms in the far-field displacement, which are proportional to the time rate of increase of stress drop in the advancing weakening zone. We also show that the single-couple equivalent does not violate principles of Newtonian mechanics because the torque imbalance in the single couple is counterbalanced by rotations within the fault zone; the crack therefore radiates torque waves and a rotational deformation field.

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