An analysis of shear waves generated by the Sterling explosion

Abstract

We show that the near‐field shear waves from the decoupled explosion Sterling were likely caused by the cavity shape, which was approximately spherical except for a flat floor due to melted and recrystallized salt. We model the impact of the explosion shock wave on the cavity walls using a two‐dimensional (2‐D) Eulerian finite difference code that simulates the evolution of the air shock in the cavity coupled with 2‐D Lagrangian finite difference code that simulates the nonlinear region outside the cavity. Calculated shear waves generated by the asymmetric impact of the shock wave on the cavity walls match the observed initial shear wave amplitudes and radiation pattern. The observations also have substantial P and S coda, which are not reproduced by the calculations. Similarity of coda waveforms with distance indicates that their source is at or very near the cavity. Longer time modeling of the air shock evolution appears to produce a more realistic source function and provide a better match to the coda and indicates that the coda are caused by air reverberations between the top and bottom of the cavity. We assess the likelihood that fractures created by the tamped explosion that formed the cavity Salmon were reopened by Sterling. Modeling of hydrofracture propagation driven by the Sterling explosion, by coupling stress wave dynamics in rock with fluid mechanics in the fractures, shows that the cavity pressure is insufficient to overcome the overburden to propagate fractures into rock, except for the area immediately below the explosion on the cavity floor.