Analysis of Near-Source Contributions to Early P-Wave Coda for Underground Explosions. II. Frequency Dependence

摘要

Analysis of dispersion in more than 1600 teleseismic short-period P waves from 46 underground explosions has established that near-source effects are responsible for systematic frequency-dependent variations observed in the first 15 sec of the P signals. Explosions from the Nevada, Amchitka, and Novaya Zemlya test sites exhibit a common magnitude dependence of the dispersive behavior, with smaller events having relatively enriched low-frequency (0.4 to 0.8 Hz) energy in the coda. For the Nevada and Amchitka sites, the larger events have relatively enhanced high-frequency (0.8 to 1.1 hz) energy in the coda as well, which may be a consequence of diminished high-frequency content of the direct arrivals. The dispersive behavior also correlates well with known source depths for the Nevada Test Site and Amchitka events, and with estimated pP delay times for the Novaya Zemlya events, indicating that burial depth and/or explosion size are important factors. Pahute Mesa tests show a secondary dependence on position in the site, with centrally located events having stronger dispersion, as well as more pronounced slowly varying azimuthal patterns in frequency dependence. Stations at azimuths NNE from the Mesa have particularly strong dispersion for centrally located events. Spatial and azimuthal variations for Pahute Mesa events do not appear to be the result of aftershock radiation but instead are associated with frequency-dependent defocusing and scattering from a high-velocity structure beneath the test site. Reprints. (EDC)