Seismic Q for the lithosphere of the Montenegro region (Yugoslavia): frequency,depth and time windowing effects

The availability of accelerometric data for the Montenegro earthquake of 15th April 1979 makes it possible to investigate seismic Q of the lithosphere in that region, in particular, its dependence on frequency, on the depth reached by seismic waves, and on the length of time windows in which signals are processed. Two different spectral methods, S phase energy ratio and coda envelope decay, are applied, respectively, to direct and scattered shear waves. Similar results are obtained using different portions of the recordings, i.e., coda waves for the envelope decay fit and the S wave train, with a significant duration of ～ 10 s, for the energy ratios. The same apparent Q (Q ～ 40 f, where f is the frequency expressed in Hz) that is found for other neighbouring central Mediterranean regions (e.g., Ancona, on the central Italian Adriatic coast; Valnerina, in the central Apennines; Irpinia, in the southern Apennines) is also found for the southern Yugoslavian coast, in the band 1–25 Hz up to a maximum range of ～ 120 km from the focus. This strong frequency dependence is probably connected with the type of small-scale heterogeneity and the same geological age and level of tectonic activity peculiar to all these seismotectonic areas.In order to compare the apparent Q of the whole S wave train, ～ 10 s long, with the (intrinsic) apparent Q of the single direct S wave (usually 1 s or less), the maximum entropy method is applied in the energy spectrum computation for shorter wave trains. The use of shorter time windows does not reveal any significant variation in the tendency of Q to increase linearly with frequency as the length of the time window containing the sample of the S waves decreases. This seems to indicate that scattering-dependent Q is generally inseparable from intrinsic Q in the lithosphere when estimates based on variations with distance of the seismic signal spectrum are used. While the type of linear growth with frequency does not seem to undergo any variations (it remains of the Q = qf type), the data show there are a considerable decrease in the coefficient of proportionality Q with decreasing duration of the window of S waves analysed, probably as a result of variations in seismic attenuation with depth.