Abstract

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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