Generation of seismic waves by explosions in prestressed media

Abstract

The mechanisms of generation of seismic waves by an explosion in prestressed media are studied using both field seismograms and controlled laboratory experiments. LRSM seismograms from the underground nuclear explosion BILBY are analyzed to determine the source parameters from the radiated Love and Rayleigh waves. From the normalized amplitudes of Rayleigh waves as well as the Love-Rayleigh amplitude ratios, a composite source consisting of an isotropic explosion and a double couple is synthesized for the explosion and the associated tectonic strain release. From BILBY and other explosions studied by similar techniques, it is found that the tectonic strain energy release strongly depends on the medium properties in the immediate vicinity of the explosion. For “harder” media (such as granite) the tectonic strain energy release and the relative amplitude of Love waves are significantly higher than for softer media such as alluvium. Source-time functions of Love waves associated with the explosions are closer to time functions of earthquakes than to those of explosions. The mechanisms of the pre-existing strain energy release by explosive sources are studied in two separate laboratory experiments. In a one-dimensional experiment where an explosive source is detonated in a rod stressed in torsion, the S-wave amplitudes are found to be linearly proportional to prestrain. In the second experiment, radiation of seismic waves and the near-source phenomena of explosive sources in prestressed plates are studied by photoelastic as well as strain gauge observations. The generation of S-waves is greatly enhanced by the prestress condition. It is found that extended cracking (faulting) occurs along directions determined by the prestress field. The transverse (SH) waves are generated primarily by the relaxation of the stress field along these cracks. The explosion-generated cavity alone could not account for the radiated transverse seismic energy.