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

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 Qap 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 Qap 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.