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 N10°W. Using the amplitude spectrum of the observed field adjusted for propagational effects, we find that the prestress was 70 ± 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.