Title: Azimuthal Sampling of Tectonic Release from Rayleigh Waves, Triggered by Shallow Explosions from the Source Physics Experiment: Theory

Anomalous surface waves including Love waves and phase reversed Rayleigh waves have been observed from explosions at NTS and from the eastern Kazakh areas of the Soviet Union. In this report we use a linear model for tectonic strain release to estimate the amount and type of prestress required to produce these anomalies. An important use of these results is to guide the input to fully nonlinear simulations of an explosion detonated in a prestressed environment.

Large samples of teleseismic P-wave amplitude and waveform data recorded from Shagan River underground explosions were collected and systematically analyzed in an attempt to identify any effects which may correlate with the amount of tectonic release accompanying these explosions. Results of these analyses indicate that these teleseismic P-wave data do not provide any unambiguous evidence of effects of tectonic release in the short-period range of interest in m/sub b/ determination. However, the results of a preliminary theoretical analysis indicate that such negative evidence is not definitive in that there are plausible models of tectonic release for which no detectable variationsmore » in the observed teleseismic P waveforms are theoretically expected to result from the superposition of these two sources. At the same time, such models of tectonic release predict significant positive bias in the network-averaged m/sub b/ values for explosions accompanied by the mode of tectonic release traditionally associated with the Shagan River test site. Thus, the currently available seismic data do not exclude the possibility that tectonic release may be introducing a positive bias of as much as several tenths of a magnitude unit into the network-averaged m/sub b/ values computed for some Shagan River explositons.« less

Tectonic strain release affects both the amplitude and phase of seismic waves from underground nuclear explosions. Surface wave magnitudes are strongly affected by the component of tectonic strain release in the explosion. Amplitudes and radiation patterns of surface waves from explosions with even small tectonic components change magnitudes significantly and show a strong dependence on receiver locations. A thrust-slip source superimposed on an isotropic explosion can explain observed reversals in waveform at different azimuths and phase delays between normal and reversed Rayleigh waves. The mechanism of this reversal is due to the phase relationship between reasonable explosion and tectonic releasemore » sources. Spallation or an unusual source time function are not required. The observations of Shagan River events imply thrust-slip motion along faults in a northwest-southeast direction, which is consistent with regional tectonics.« less

Detailed forward modeling of long-period shear waves for 2 large underground explosions at the Southern Novaya Zemlya test site indicates that the appropriate equivalent double-couple orientation for the tectonic release radiation is vertical strike-slip. Previous studies of observed teleseismic SH waveforms and SV amplitudes for the 27 Oct 1973 and 2 Nov 1974 events using geometric theory could not distinguish between vertical strike-slip and 45 deg-dipping thrust geometries. Either mechanism can match the observed four-lobed SH radiation pattern, and the two-lobed SV amplitude pattern can be produced by interference with an appropriate-size explosion pS signal. However, the complexity of themore » observed SV waveforms arising from Sp conversions near the receiver, diffracted Sp, and shear-coupled PL phases is not accounted for in the ray-theory synthetics. Incorporating more-realistic Green's functions using Baag and Langston's WKBJ spectral method allows more-complete modeling of the SV signals. Due to differences in frequency content between the explosion and double-couple SV waveforms, constructive interference occurs more efficiently than destructive interference when the two signals are linearly superimposed. As a result, using tectonic-release moments determined from the SH waves and the optimum F factors required to match the SV amplitude patterns, the waveforms produced by the strike-slip and thrust orientations differ substantially at some azimuths.« less

Comparing observed and synthetic seismograms, source amplitudes of NTS explosions are inferred from Rayleigh wave recordings from the WWSSN stations at Albuquerque, New Mexico (ALQ) and Tucson, Arizona (TUC). The potential influence of source complexities, particularly surface spallation and related phenomena, is studied in detail. As described in earlier work by Bache, Rodi and Harkrider, the earth model for the synthetic were converted from observations at ALQ and TUC. The agreement of observed and synthetic seismograms is quite good and is sensitive to important features of the source.