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MODELING TELESEISMIC S V WAVES FROM UNDERGROUND EXPLOSIONS WITH TECTONIC RELEASE: RESULTS FOR SOUTHERN NOVAYA ZEMLYA BY BRIAN P. COHEE AND THORNE LAY

Detailed forward modeling of long-period shear waves for two 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 SVamplitudes for the 27 October 1973 and 2 November 1974 events using geometric ray theory could not distinguish between vertical strike-slip and 45°-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 the 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 (1985b) WKBJ spectral method allows more complete modeling of the SVsignals. Due to differences in frequency content between the explosion and double-couple SVwaveforms, 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 SVamplitude patterns, the waveforms produced by the strike-slip and thrust orientations differ substantially at some azimuths. The strike-slip solution yields a consistently superior match to the data. Using the EU2 model of Lerner-Lam and Jordan (1987) for the source region and either EU2 or TNA (Grand and Helmberger, 1984) for the receiver structure, together with an attenuation model similar to SL8, we obtain a double-couple moment, Mo = 3 .2 x 1024 dyne-cm and explosion source strength, ~ = 3.8 ± 0.5 x 1011 cm 3 for the 27 October 1973 event, and Mo = 1.7 × 1024 dynecm and ~ = 2.0 _+ 0.3 x 101~ cm 3 for the 2 November 1974 event. Complete waveform modeling of SV signals can thus provide improved constraints on tectonic release radiation and explosion source strength. INTRODUCTION Numerous studies have addressed the existence of nonisotropic seismic radiation accompanying large underground nuclear explosions (e.g. Press and Archambeau, 1962; ToksSz et al., 1965; Hirasawa, 1971; Wallace et al., 1983, 1985). Long-period horizontal shear wave motions, including Love waves and teleseismic S H body waves, result directly from this nonisotropic component and can be used to constrain the transverse source contribution. Although many uncertainties remain concerning the precise physical source of this transverse energy, it is generally attributed to the release of preexisting tectonic strain accumulation in the vicinity of the explosion (tectonic release). Two general theories for tectonic release have been suggested: the triggering of displacement on a nearby fault (Brune and Pomeroy, 1963; Aki et al., 1969; Aki and Tsai, 1972), or the release of in si tu stress from vaporized and fractured rock during formation of the ex-