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Travel times and amplitudes of S waves from nuclear explosions in Nevada

DEPARTMENT OF GEOPHYSICS AND GEOPHYSICAL ENGINEERING INSTITUTE OF TECHNOLOGY SAINT LOUIS UNIVERSITY, ST. LOUIS, MISSOURI

Abstract

The underground Nevada explosions HALF-BEAK and GREELEY were unique in creating relatively large amplitude and long-period body S waves which could be detected at teleseismic distances. Observations of the travel times of these S waves provide a surface focus travel-time curve which in its major features is similar to a curve calculated from the upper mantle velocity model of Ibrahim and Nuttli (1967). This model includes a low-velocity channel at a depth of 150 to 200 km and regions of rapidly increasing velocity beginning at depths of 400 and 750 km. Observations of the S wave amplitudes suggest that a discontinuous increase in velocity occurs at 400 km, whereas at 750 km the velocity is continuous but the velocity gradient discontinuous.

Body wave magnitudes calculated from S amplitudes are 5.3 ± 0.2 for GREELEY and 4.9 ± 0.2 for HALF-BEAK. These are about one unit less than body wave magnitudes from P amplitudes as reported by others.

The shape and orientation of the radiation pattern of SH for both explosions are consistent with the Rayleigh and P-wave amplitude distribution of BILBY as given by Toksoz and Clermont (1967). This suggests that the regional stress field is the same at all three sites, and that the direction of cracking as well as the strain energy release in the elastic zone outside the cavity is determined by the regional stress field.

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