Frequency dependence of Q for seismic body waves in the Earth's mantle

Abstract

In this paper an attempt is made to determine the frequency dependence of Q in the Earth's mantle in the frequency range 0.03–1.5 Hz from the spectral ratio of teleseismic S- and P-waves. Digital broad-band data of 17 earthquakes at 40° < ∆ < 90° recorded at the Central Seismological Observatory of the Federal Republic of Germany at Erlangen were analysed. The method implies the following assumptions: frequency independence of the crustal transfer function, proportionality of Q_p (f) and Q_s (f), and proportionality of P- and S-source spectra. This last and most critical assumption was carefully investigated by kinematic and dynamic source models. The calculated Q-spectra for the individual events vary considerably but all have in common a general increasing trend with frequency which can best be represented by a power law Q ≈ f ^α with 0.25 < α < 0.6. A further increase in slope near 1 Hz suggests an absorption band corner with an upper cut-off relaxation time τ_m = 0.33 ± 0.18 s. The significance of the Q-spectra and their variability is estimated by manipulating semi-synthetic seismograms with different error-producing processes such as length and shape of the time window, superposition of noise, digital filter process and source spectra. It is concluded that none of these processes is able to destroy or to imitate the observed increasing trend of Q with frequency. The results are compared with those from other seismological investigations and from laboratory experiments on mantle rocks at high temperature and in the seismic frequency band.

          ARK: https://n2t.net/ark:/88439/y081538

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