Simple physical model for spall from nuclear explosions based upon two-dimensional nonlinear numerical simulations. Technical report

Examine the effect of spall on the source function for explosion waveforms using two-dimensional nonlinear numerical simulations of explosions. A simple, physical model for spall is developed by comparing the complete two-dimensional waveform with the waveform generated by a one-dimensional explosion plus a shallow tension crack in the same layered medium. This is an extension to higher frequencies of the spall model. The tension crack is parameterized by its radius, depth and a distribution of takeoff velocities over its surface. P-waves generated by the two dimensional simulation are modeled very well by the simple tension crack plus explosion model, and the resulting parameters for the spall model are consistent with the limited set of near-field observations of spall. The model underestimates the shear waves generated in the two-dimensional calculations, indicating that the source of shear waves is more complex than a simple explosion plus tension crack model. P-waves generated by the tension are found to have a narrowband spectrum proportional to omega to the -5/2 power at high frequencies, with a peak typically in the frequency range between 0.5 and 5 Hz, which puts it in the middle of the frequency range where short period magnitudes are commonly measured.