Generation of Seismic Waves by Explosions in Prestressed Media ~vl


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 os 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 explosiongenerated cavity alone could not account for the radiated transverse seismic energy. INTRODUCTION Seismic surface and body waves generated by underground explosions and recorded at distant stations quite often show characteristics differing from those of an ideal explosive point source in a homogeneous medium. Most notable among these differences are the presence of Love waves and the azimuthal asymmetry of the Rayleigh-wave radiation patterns. Through numerous studies, it has been shown that the Love waves are produced at the source region, although the exact mechanism of their generation still cannot be clearly defined. The purpose of this paper is to present and interpret data, obtained from underground nuclear explosions and controlled laboratory experiments, on seismic waves generated by explosive sources in stressed media. There have been a number of studies dealing with the radiation patterns and the generation of SH-type seismic waves by underground nuclear explosions (Press and Arehambeau, 1962; Brune and Pomeroy, 1963; Aki, 1964; ToksSz et al., 1964; Toks6z et al., 1965; Toks6z, 1967; Kehrer, 1969; Molnar et al., 1969; Tsai and Aki, 1971). In 1589 1590 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA addition, there are a number of previous theoretical studies on the problems of seismicwave generation by explosions (Zvolinskii, 1960; Bishop, 1963; Alverson, 1964; Cisternas, 1964; Butkovich, 1965; Archambeau, 1968). Difficulties in describing wave propagation from nuclear explosions arise because of very high stresses involved. In the immediate vicinity of the explosion, shock phenomena dominate, whereas in the far zone, elastic behavior prevails. The transition region is extremely important to the understanding of seismic-wave generation, yet this region is extremely difficult to describe mathematically. In the description of this complex problem, difficulties arise both in mathematical formulation and from inadequate knowledge of the behavior of geological materials under shock loading. In the zone immediately around tile explosion, relatively small volume, hydrodynamic compressible flow equations can be used without major assumptions or approximations. In the zones where the rocks are crushed and cracked, however, neither fluid nor elastic behavior can be assumed, and a semiempirical treatment based on approximations and available data must be followed. If the explosion is placed in a medium where tectonic stresses exist, other complications arise from the stress relaxation around the cavity, the crushed zone, and the zone of numerous shock-induced cracks. Controlled laboratory experiments can provide some of the information needed for the understanding of seismic-wave generation by explosions in prestressed media. Some experiments have been conducted in the field using explosive sources in soil (Kisslinger et al., 1961 ). Other model experiments have been carried out in the laboratory using explosive sources in prestressed plates. In these tests, the radiation patterns of seismic compressional and shear waves were observed with various transducers and oscilloscopes (Kim and Kisslinger, 1967; Kisslinger and Oupta, 1963). Most recently, photoelastic techniques have been used to observe wave generation and propagation in prestressed media (Thomson et al., 1969). In this paper, we will treat, in three steps, the problem of mechanisms of seismicwave generation by explosive sources in prestressed media. First, we describe the radiation pattern of surface waves from a typical underground explosion at the Nevada Test Site. We determine its source mechanism by using an amplitude equalization scheme. In part two, we describe the results of laboratory experiments in which explosive sources were detonated in plates under different prestress conditions. Here, the mechanisms of seismic-wave generation, crack formation, and source complications were observed using dynamic photoelasticity and high-speed photographic techniques. Finally, we interpret the field observations in light of laboratory findings. FIELD OBSERVATIONS Seismic waves from a large number of the U. S. underground explosions have been well-recorded by Long Range Seismic Measurements (LRSM), World Wide Standard Seismographic Network (WWSSN) and other seismograph stations in North America. LRSM stations provide the best recordings of surface waves in the period range of 10 to 40 sec. Most of the large explosions detonated in relatively hard geological media (granite, tuff) generate Love waves in addition to Rayleigh waves. Two examples of Love waves from explosions are shown in Figures 1 and 2. Explosions in loose alluvium and salt domes, and collapse events following explosions, do not generate a significant amount of Love waves. The main body of this study is devoted to the understanding of the generation of these transverse waves. Some sources that must be considered are: (1) relaxation of the prestressed medium around the explosion-generated cavity, (2) GENERATION OF SEIS]~IC WAVES BY EXPLOSIONS IN PRESTRESSED MEDIA 1591 triggering of an earthquake by the explosion, (3) radiation from explosion-induced cracks in the medium, and (4) a combination of all of these. In order to resolve this problem, it is useful to first determine the radiation patterns of Rayleigh and Love waves. A group of explosions have been studied using the amplitude equalization method (Toks6z, et al., 1964, 1965; Toks6z, 1967; Kehrer, 1969). l ~ ~ I / ....... 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@inproceedings{TOKSJZ2005GenerationOS, title={Generation of Seismic Waves by Explosions in Prestressed Media ~vl}, author={NAFI TOKSJZ and T. J. Ahrens}, year={2005} }