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Analysis of near-source contributions to early P-wave coda for underground explosions. II. Frequency dependence

DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF MICHIGAN, 1006 C. C. LITTLE BUILDING, ANN ARBOR, MICHIGAN 48109

Abstract

An 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 actually 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 the frequency dependence. Stations at azimuths to the north-northeast from the Mesa have particularly strong dispersion for centrally located events. The spatial and azimuthal variations for Pahute Mesa events do not appear to be the result of aftershock radiation, as suggested by Douglas (1984), but instead are associated with frequency-dependent defocusing and scattering from a high-velocity structure beneath the test site. Some of the dispersion from Novaya Zemlya events appears to result from deep path properties; however, the strong magnitude/depth dependence for all of the test sites indicates that very near-source conditions have the strongest influence on the frequency dependence. A likely explanation for the dispersive behavior is a combination of relatively enhanced surface wave excitation and scattering for shallower events, and frequency-dependent defocusing for sites overlying anomalous high-velocity structures, as is the case for both Pahute Mesa and Amchitka.

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