Nur's (1972) explanation of the observed premonitory decrease in the travel-time ratio of shear and compressional waves, ξ, in terms of dilatancy is modified by considering the anisotropic characteristics of dilatant rock in the focal region. The expected change in ξ is influenced by the orientation and type of geologic fault as well as the geometry of source and observation points. The shear wave will in general split into two components traveling with somewhat different velocities.

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