A near-regional explosion source model for tuff

Abstract

Summary. A variety of near-regional (300 km) data, including spectral amplitudes of Pg, surface-wave forms, and close-in (5–10 km) accelerograms have been used to build an elastic seismic source model for a 1-Mton explosion in tuff at near-regional distances. The model consists of: (1) a pressure pulse which injects 3 × 1012 cm3 of volume into the medium, (2) a vertical, upward force impulse that imparts 1018 dyn-s of momentum to the medium, each source component having a time duration of 0.6 s and a depth of 1.3 km. The force impulse appears to be required by two considerations: (a) the striking similarity, apart from sign, of explosion surface waves with those of their cavity collapses, (b) the observation of considerable SV energy leaving the source of the 1-Mton explosions JORUM and HANDLEY. Scaling curves have been constructed which fit the proposed source model. These scaling curves employ: very slow decrease, as (yield)−0.10 of the primary corner frequency; decay as (frequency)4 or (frequency)3 to high frequency. While these scaling curves are unconventional, they appear to be the only ones which can satisfy the near-regional data. The slow scaling with yield of the spectral carner frequency suggests that it is caused by something other than the equivalent elastic radius, e.g. the time duration of motion at the source. The results, at odds with similar studies at teleseismic distances, suggest that significantly different equivalent elastic sources are required at near-regional (as compared with teleseismic) distances; therefore, the effect of the upward impulse might not be seen at teleseismic distances. Consequently, these results probably do not pertain to the seismic discrimination problem at teleseismic distances.