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EARTH RESOURCES LABORATORY DEPARTMENT OF EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS 02139
Abstract
A comparison has been made of the relative scatter of classical mb and alternative measures of P-wave energy from underground nuclear explosions at test sites in East Kazakh, USSR. The scatter of the energy measures is observed in teleseismic arrays of short-period Global Digital Seismic Network (GDSN) stations and the local broadband array at Graefenburg, Federal Republic of Germany. Four measures of A in log(A/T), spectral magnitudes, peak velocity, rms coda, and integrated velocity-squared are compared. The measures are constructed to be in equivalent units of the flux rate of radiated elastic energy. All measures are assumed to have the same slope in a linear regression of log(yield) versus log(measure). Three independent tests were made of the stability of the yield estimators: the scatter of the measures using: (1) Graefenburg array data; (2) GDSN data normalized to a reference station; and (3) GDSN data normalized to a reference event. The differences among the standard deviations are small (
0.1 mb units), making it difficult using a small data base to conclude whether the performance of one estimator is significantly better than another. The relative order in the performance of the yield estimators, however, is preserved in each of the three tests. The coda measure is the most stable, followed by the spectral and time-domain A/T measures. The relations observed at Graefenburg between (1) the amplitude of direct P versus P coda, (2) the apparent azimuth of direct P, and (3) complexity, suggest that amplitude variations across an array are a product of scattering along the entire ray path as well as scattering, focusing, and defocusing localized in the lithosphere beneath the source and receiver sites.
Footnotes
The present address of J. T. Bullitt is: Terrametrics Research, 751 Main Street, Waltham, Massachusetts 02154.
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