Characteristics of Chemical Explosive Sources from Time-Dependent Moment Tensors

Abstract

Using a frequency-domain linear inversion technique and near-source broadband data, we inverted for the time-dependent source moment tensors of eight chemical explosions detonated in an open-pit coal mine during the Source Phenomenology Experiments (SPE) conducted by a consortium of U.S. research institutions to investigate a suite of explosive-source related problems. The moment tensors of the explosions from the inversion are dominated by their isotropic components regardless of variations between explosions in source size, confinement condition, and whether the explosion was on a bench and collapsed the vertical face of the bench. The percentage of isotropic moment-tensor component ranges from 96% to 98% for largest part of the source-time histories. Source-configuration variations result in differences that are most apparent in long-period moment-tensor spectra reflecting possible secondary source effects such as cylindrical source shape, spall, and compensated linear vector dipole (CLVD). Unconfined explosions show more oscillatory diagonal moment-tensor component time histories than confined and partially confined explosions possibly due to stronger free-surface effects such as material cast. Compared with pit explosions, deviatoric components of moment tensors of the two bench explosions are of higher amplitudes. There is a discernible long-period (<5 Hz) signal on one of the off-diagonal components, which could be related to the presence of the bench face in the source region and the horizontal material cast by the explosions. Although off-diagonal moment-tensor components comprise a small portion of the moment tensor, they are capable of generating a disproportionally large amount of shear waves.