High‐frequency P‐wave attenuation along five teleseismic paths from central Asia

Abstract

A model for the attenuation of high‐frequency (0.5 ‐8 Hz) P‐waves is developed for teleseismic paths from eastern Kazakhstan to five seismic arrays. Four are the UKAEA arrays in Scotland (EKA), Canada (YKA), India (GBA) and Australia (WRA) and the attenuation effects on these paths were studied in an earlier paper (Bache, Marshall & Bache). Here we add the path to NORSAR (southern Norway) and extend the analysis by introducing explicit corrections for the source spectrum. The events are underground explosions at the Soviet Semipalatinsk test site and the source corrections are based on the Mueller & Murphy model which is normalized to US experience. Averaging spectra from many array elements and many explosions leads to smooth spectra that essentially display the effects of attenuation on each path. There are three major features. First, from 0.5 to 3 Hz the attenuation effects are nearly the same for all five paths and indicate strong frequency dependence in Q. Second, from 3 to 8 Hz the four UKAEA paths are nearly the same, but the NORSAR path is quite different in that the source‐corrected spectrum is nearly flat. This means that the path‐average Q is nearly proportional to frequency in this band. Finally, above 7–8 Hz some additional attenuation effects are seen in the NORSAR data. Thus, for this teleseismic P‐wave path the maximum signal/noise occurs at 7 or 8 Hz. The attenuation effects from 0.5 to 8 Hz can be represented with absorption band models. For the paths to the UKAEA arrays the preferred model has t0* (travel time/Q at long periods) about 0.6 s and τ‐m about 0.5 s. Superimposed on this is a frequency‐independent t1* of about 0.1 s. This dominates the high‐frequency (f > 3 Hz) attenuation and appears to represent attenuation primarily by scattering. For the NORSAR path a single absorption band (t0*≈ 0.7, τm≈ 0.045) is adequate for the 0.5—7 Hz band. We have no explanation for why this path is so different, but it appears to be associated with some difference in the contribution of scattering to the attenuation of the initial P‐wave. Copyright © 1986, Wiley Blackwell. All rights reserved