Analysis of the Effects of Spall on Regional and Teleseismic Waveforms Using Two-Dimensional Numerical Modeling of Underground Explosions

摘要

A simple, physical model for spall is developed by comparing the waveforms fromtwo-dimensional explosion simulations with the waveforms generated by a one-dimensional explosion plus a shallow tension crack in the same layered medium. The tension crack is parameterized by its radius, depth, and a distribution of takeoff velocities over its surface. This model accurately reproduces the P-waves from the two-dimensional simulations. The parameters for the spall model are consistent with the limited set of near-field observations of spall. P-waves generated by the tension crack have a narrow band spectrum proportional to omega-sq at low frequencies and omega to the -5/2 power. The spall model is used to generate complete regional seismograms for an explosion plus spall source. In high velocity structures, a spherically symmetric explosion source is a very poor generator of Lg while the tension crack is a good generator of Lg, so the Lg generated by spall completely dominates the regional Lg signal. In a structure with low velocities at the source, Lg generation by an explosion is comparable to the generation of Lg by spall. Regional seismic signals of the NTS explosions Duryea and Buteo, two explosions at the same location with very different scaled depths of burial, show no measurable difference other than that due to yield scaling. (jhd)