Tectonic Strain Release by Underground Nuclear Explosions and its Effect on Seismic Discrimination

The analysis of surface waves from a large number of underground nuclear explosions reinforces the hypothesis of tectonic strain release. Such strain release is dependent on rock type and ambient stress levels. In harder media, such as granite, strain release is considerably greater than in soft media such as loose alluvium and salt. In the case of three events for which the tectonic component exceeded that of the explosion itself, the effect on the Ms - mb discriminant due to the added surface wave energy was not noticeable. This was probably due to the averaging of magnitude over all azimuths. A decaying pulse-type source-time function is consistent with surface wave amplitude spectra and can explain the long-period/short-period spectral ratio.