An analysis of the RULISON/GASBUGGY mb anomaly is presented which encompasses comparative studies of near-regional, regional, and teleseismic data recorded from these two explosions, as well as theoretical simulation analyses of selected near-regional and teleseismic data sets. Previously proposed explanations for this mb anomaly, such as differences in explosive source coupling and variations in upper mantle attenuation beneath the two test sites, are critically examined and shown to be inconsistent with various aspects of the observed data. However, both long-period and short-period evidence is presented, which indicates that significant tectonic release accompanied the RULISON explosion. It is demonstrated that the short-period, near-regional SH observations from RULISON can be theoretically explained by an explosion-induced tectonic release equivalent to a 250 to 300 bar stress drop at the elastic radius of the explosion. The inferred prestress field is homogeneous to first order and oriented as a pure shear on planes dipping at about 45°, with normal, or dip-slip displacements occurring as a consequence of stress relaxation. Such a tectonic release mechanism would be expected to decrease the associated explosion mb value by more than 0.3 units and, therefore, can provide a quantitative explanation of the observed RULISON/GASBUGGY mb anomaly. We conclude that tectonic release can significantly affect the short-period teleseismic P waves from explosions, at least in cases where the tectonic stress is large and favorably oriented.

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