Spectra of nuclear explosions, earthquakes, and noise from Wake Island bottom hydrophones

Spectral characteristics of P phases from 4 shallow focus earthquakes and 8 underground explosions, and of 52 samples of ocean bottom background noise, are examined by using tape recordings of ocean bottom hydrophones near Wake Island from July 1979 through March 1981. Significant differences are found between spectra of large shallow focus earthquakes and explosions (5.7 ≤ mb ≤ 6.3) observed at 61° to 77° epicentral distance. For similar magnitudes, explosions were found to have less energy at frequencies below 1.5 Hz and more energy at frequencies above 2.0 Hz. Earthquakes were found to have a spectral slope of −28 dB/octave (relative to pressure) over the band 1 to 6 Hz. Explosions were found to have the same spectral slope over the band 2.2 to 6 Hz, but a different slope of −12 dB/octave over the band 1.1 to 2.2 Hz. High frequencies (> 6 Hz) observed in the teleseismic P phases indicate high Q values for the deep mantle. Ambient noise levels on the ocean bottom near Wake are comparable to levels at the quietest continental sites for frequencies between 3 and 15 Hz. Also high levels of coherence (at least as high as 0.85) have been observed for P phases recorded on sensors with 40‐km separation.