Spallation and the generation of surface waves by an underground explosion

Underground nuclear explosions, even though they are thoroughly contained, invariably produce spallation. This mass of earth and rock, kicked up by the initial shock wave, falls back within a second or two. It is possible that the surface waves observed from explosions originate from the impact of this material. This hypothesis has been investigated, first, by estimating the spall impulse from accelerometer data, computing the surface wave amplitudes consistent with this impulse, and comparing the results with observed amplitudes and, second, by performing computer explosion simulation experiments with and without spallation and comparing the amplitudes of the resulting surface waves. The comparison of the amplitudes consistent with the spall momentum with the observed Rayleigh amplitudes at ranges of 50–500 km indicates that the spall has sufficient impulse to account for these waves. The computer simulation experiments show that the Rayleigh amplitudes corresponding to realistic inelastic spallation have several times the amplitude of Rayleigh waves computed from purely elastic behavior. The surface wave for the spallation case is also delayed relative to the wave from the elastic source.