We investigate the possibility that rock damage (Ashby and Sammis, 1990; Johnson and Sammis, 2001) can explain recent observations of Fisk (2006, 2007) that P- and S-wave corner frequencies from explosions differ by a factor approximately related to the ratio of near-source compressional and shear velocities (the Fisk conjecture). The observation of differing P- and S-wave corner frequencies from explosions forms the basis for the success of the high-frequency P/S discriminant. Using first-order considerations, we find that the damage mechanics model can indeed explain the Fisk conjecture (although this is certainly not a unique explanation). The key to our argument is that, although the radius of rock damage is less than that of the so-called elastic radius, the slow velocity of the outward propagating damage front can reduce shear-wave corner frequencies. This explanation differs slightly from that of Fisk (2006, 2007) who states that S waves from explosions occur at “a similar length scale, comparable to the elastic radius as for P waves.” Because of the significant reduction in VS caused by rock damage, we suggest that the shear waves generated within the damage region can have a radius significantly less than the elastic radius for a P wave. Additionally, because rock damage is controlled by the difference of the maximum and minimum principal stress, first-order considerations suggest that lithospheric overburden does not shut off rock damage until scaled depths of burial of up to forumla⁠. Thus, effects due to rock damage may be an important source of shear waves for nearly all tamped nuclear explosions for which we have experience.

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