A computer program has been developed to study the effect of subsurface geology on the propagation of seismic rays. Based on ray theory, and on numerical solution of Zoeppritz' equations for any angle of incidence at elastic boundaries, the program traces seismic rays and energy distribution through geological models of any complexity, assuming that the geological units comprising the model are isotropic elastic homogeneous media with no attenuation to seismic energy, and that the model is two-dimensional with interfaces between rock units normal to plane of section. Ray-path plots and amplitude calculation through two real twodimensional geological models of Basin and Range geology, more than 100 miles long and as deep as to the Moho, show clearly the strong influence of spatial distribution and elastic properties of rock units on ray-path distribution. Amplitude calculation for rays arriving at ground surface through an actual geological section permits the prediction of ground motion based on geology rather than on empirical relations obtained from observed displacements regardless of geologic structure between the source and the recording station. Comparison of calculated amplitudes and displacements at the earth's surface with observed displacements at a few recording stations along or near the models shows a reasonably good fit of data and suggests that seismic ray calculations, where sufficient subsurface data are available, would provide a valid means for ground motion prediction.
A computer program has been developed to study the effect of subsurface geology on the propagation of seismic body waves. The program, which is based on ray theory and on numerical solution of Zoeppritz’s equations for any angle of incidence at elastic boundaries, traces seismic rays and energy distribution through geological models of any complexity. The geological models are composed of rock units that are assumed to be isotropic, perfectly elastic, and homogeneous, and are two‐dimensional in distribution. The ray tracing program was applied to two geologic models of the crust, that are more than 100 miles long, starting at drill hole UE20F in the Nevada Test Site. The results, displayed as raypath plots and plots of the amplitude of vertical displacement at the earth’s surface, graphically illustrate the strong influence of the spatial distribution and elastic property variations of rock units in raypath distribution.
A method for accurate estimation of surface-wave magnitudes is presented as an alternative to Ms, the classical time-domain magnitude. The salient features of the method are the use of velocity and frequency windows to isolate seismic phases and the use of maximum spectral energy to characterize ground motion for magnitude estimation. Rayleigh-wave ground motion estimated by the new method is generally consistent, but not identical, to that measured in the time domain. The estimated ground motion is used to determine Rayleigh-wave spectral magnitudes for 27 presumed nuclear explosions, from several test sites. These magnitudes are studied for their accuracy and for their transportability between nuclear explosion test sites. For accuracy, we compare spectral magnitudes MR with their corresponding time-domain-measured magnitudes Ms. The comparison encompasses network magnitudes and their standard deviations and magnitude residuals. For transportability, we studied the variations in mb (body-wave magnitude), Ms (time-domain magnitude), and MR (spectral magnitude) for network-averaged mb-equivalent presumed nuclear explosions at the Shagan River and the Qingger test sites. These studies showed that this spectral magnitude has a smaller error than classical time-domain Ms and that it is transportable between the two test sites.
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