We studied seismic body‐wave generation from four fully contained explosions of approximately the same yields (68 kg of TNT equivalent, where TNT stands for trinitrotoluene) conducted in homogeneous granite in Barre, Vermont. The explosions were detonated using three types of explosives with different velocities of detonation: black powder (BP), ammonium nitrate fuel oil/emulsion (ANFO), and composition B (COMP B). The main objective of the experiment was to study differences in seismic‐wave generation among different types of explosives and to determine the mechanism responsible for these differences.
The explosives with slow burn rate (BP) produced lower P‐wave amplitude and corner frequency, which resulted in lower seismic efficiency (0.21%) in comparison with high burn rate explosives (1.3% for ANFO and 1.9% for COMP B). The seismic efficiency estimates for ANFO and COMP B agree with previous estimates for nuclear explosions. The body‐wave radiation pattern is consistent with an isotropic explosion with an added azimuthal component caused by vertical tensile fractures oriented along pre‐existing microfracturing in the granite, although the complexities in the P‐ and S‐wave radiation patterns suggest that more than one fracture orientation could be responsible for their generation. Analysis of the S/P amplitude ratios suggests that a significant fraction of the shear‐wave energy can be explained by opening of the tensile fractures and spall.