Seismic radiation from explosions in prestressed media and the measurement of tectonic stress in the Earth

Abstract

The radiation field from explosions in a prestressed medium can be theoretically predicted on the basis of a dynamical theory of stress relaxation in the vicinity of the shock-induced fracture zone created by an explosion. The field consists of the normal compressional wave field resulting from the conversion of the shock wave to an elastic wave plus an anomalous part due to the release of strain energy. In this study we consider the nature of the radiation field to be expected from such a source in an inhomogenous earth and determine the stress , field required to explain the observations from a large underground explosion. The field is described in terms of radiation patterns as functions of frequency or alternately as the amplitude and phase spectrums at particular distances. These theoretical predictions are compared to the Love-and Rayleigh-wave radiation patterns and spectra from the nuclear explosion Bilby. Using the known source parameters, we obtain agreement between the observed radiation patterns and the predicted patterns for a pure shear prestress field equivalent to a shear couple oriented approximately N10øW. Using the amplitude spectrum of the observed field a,djusted for propagational effects, we find that the prestress was 70 4-20 bars in the source area. We conclude that this approach can be utilized as a means of systematically measuring the stress field of the earth. The term 'stress relaxation' will be used to describe the change in the stress field in a prestressed medium due to the creation of a rupture zone within The dynamics of the process can be formulated as an initial value problem [Archambeau, 1964[Archambeau, , 1968. In this study we will compare the predicted properties of the radiated field associated with stress relaxation to the seismic observations from an hnderground nuclear explosion. The objectives will be determine source parameters and the prestress field existing before the explosion and to verify some aspects of the theory.Most of the basic ideas involved in the formulation of this problem have been discussed previously. Press and Archambea.u [1962] considered the energy release to be expected from the formation of a rupture zone in a prestressed medium. The calculations were based on the ides that the maximum energy to be released as seismic radiation would be just the difference in the total strain , energy for the medium with and without a rupture zone. The model chosen was appropriate to an explosive-induced rupture, and the calculations showed that x Contribution 1611 from the California Institute of Technology. 473 474 ARCHAMBEAU AND $AMMI$ a very significant energy release could result when the model parameters were chosen to be close to those expected in realistic situations. When the theory was applied to the Ranier nuclear explosion, it was found that the initial strain field required for approximate agreement with the 5bservations was somewhat higher than 10 -3 . Though this seemed very high at the time, it has since been recognized that this level is quite r...

The pulse shapes and spectra of elastic waves generated when a cavity expands in an initial shear field

“…This problem has been considered before by Archambeau [1972] and by Burridge and Alterman [1972] and was originally introduced as a model to explain the SH waves which are observed to be generated by nuclear explosions [ToksiOz et al, 1965;Press and Archambeau, 1962;Archambeau and Sammis, 1970]. (More recently, Aki and Tsai [ 1972] have shown that induced tectonic stress release is a more likely mechanism.)…”

Computation of complete waveforms in general anisotropic media-results from an explosion source in an anisotropic medium

“…In this article we also investigate the complexity of seismic radiation patterns and the generation of transversely polarized (SH) waves by an explosion in an azimuthally anisotropic medium. The causes of the azimuthal dependence of radiation patterns of P-, SV-, and Rayleigh waves and the generation of SH-and Love waves by underground nuclear explosions have been studied extensively (Kisslinger, Mateker & McEvilly 1961;Archambeau & Sammis 1970;Toksoz & Kehrer 1972;Mass6 1981;Wallace, Helmberger & Engen 1983;Gupta & Blandford 1983;Lynnes & Lay 1988;Johnson 1988). Various mechanisms, including tectonic strain energy release by relaxation or triggering of an earthquake, dislocation across cracks, spallation and 'slapdown', and scattering from heterogeneities, have been proposed and found to explain some of the data.…”

Transformation of multipolar source fields under a change of reference frame

“…Such a representation is very general, and can be obtained for a variety of source models, including relaxation sources (e.g. Archambeau 1964Archambeau , 1968Minster 1973;Burridge 1975), dislocation sources (Burridge 1969;Brune 1970;Savage 1966), as well as for underground nuclear explosions (Archambeau 1964;Archambeau & Sammis 1970;Haskell 1967). However, in most cases a multipolar representation is easily reached only in a particular coordinate system satisfying the conditions of symmetry of the source.…”