Comparison of seismic body wave and coda wave measures of Q

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Pure and Applied Geophysics
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Measurements of seismic attenuation (Q(-1)) can vary considerably when made from different parts of seismograms or using different techniques, particularly at high frequencies. These discrepancies may be methodological, or may reflect earth processes. To investigate this problem, we compare body wave with coda Q(-1) results utilizing three common techniques: i) parametric fit to spectral decay, ii) coda normalization of S waves, and iii) coda amplitude decay with lapse time, Q(-1) is measured from both body and coda waves beneath two mountain ranges and one platform, from recordings made at seismic arrays in the Caucasus and Kopet Dagh over paths less than or equal to 4 degrees long. If Q is assumed frequency independent, spectral decay fits show Q(s) and Q(coda) near 700-800 for both mountain paths and near 2100-2200 for platform paths. Similar values are determined with the coda normalization technique. However, frequency-dependent parameterizations fit the data significantly better, with Q(s) (1 Hz) and Q(coda) (1 Hz) near 200-300 for mountain paths and near 500-600 for platform paths. Lapse decay measurements are close to the frequency-dependent values, showing that both spectral and lapse decay methods can give similar results when Q has comparable parameterizations. Above 6 Hz, coda measurements suggest some enrichment relative to body waves, perhaps due to scattering, but intrinsic absorption appears to dominate at lower frequencies. All approaches show sharp path differences between the Eurasian platform and adjacent mountains, and all are capable of resolving spatial variations in Q.


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