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Transmission and attenuation of the primary seismic wave, 100 to 600 km

LELAND TIMOTHY LONG^* and JOSEPH W. BERG, JR.

DEPARTMENT OF OCEANOGRAPHY OREGON STATE UNIVERSITY, CORVALLIS, OREGON

Abstract

The observed frequency dependence of attenuation can be used to calculate Q values which are independent of losses due to geometrical spreading or to reflections at boundaries. The observed apparent rates of attenuation in terms of Q_ap along two refraction lines were computed to be 169 ± 42 at 5 cps in eastern New Mexico and 116 ± 38 at 4 cps in Nevada. The geometrical spreading factor which satisfied the observed amplitudes and both of the observed Q_ap values is r^–0.5. Thus, the primary seismic wave at epicentral distances of 100 to 600 km would appear to be a continuous refraction (direct wave). Attenuation is the dominant factor in determining the waveform, spectral content, and amplitude of the first arrival. A Q of 100 implies a functional relation between frequency and distance which shows that amplitudes at frequencies above 4 cps at 150 km and above 3 cps at 400 km will be less than one tenth the peak spectral amplitude and cannot be reliably related to the source.

Footnotes

^* Now with Georgia Institute of Technology.

Now with National Academy of Sciences, Committee of Seismology, 2101 Constitution Avenue, Washington, D. C. 20418.

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