Attenuation in the uppermost inner core from broad-band GEOSCOPEPKPdata

Abstract

S U M M A R YThe attenuation in the uppermost 500 km of the inner core is investigated at the global scale from the comparison of PKP(DF) and PKP(BC) phases recorded at broad-band GEOSCOPE stations. From the amplitude ratio DFIBC, we show that for most paths the attenuation is not frequency-dependent in the range 0.2-2 Hz. This result follows from the linearity of ln(DF/BC) with frequency. A notable exception occurs for paths from the south-west Pacific to Europe, as analysed in previous studies which infer a frequency dependence of inner core attenuation.The DF/BC amplitude ratios are determined at 3 s periods for the different paths. The data include the diffracted branch of BC at long distances, providing a deeper sampling of the inner core by the DF phase. The variations of the DF/BC amplitude ratio with epicentral distance exhibit a rather large scatter around a mean curve. This mean curve is compared to the theoretical ratios obtained for several earth models using synthetic seismograms. A trade-off exists between the velocity at the base of the liquid core and the attenuation in the uppermost inner core. Robust features include a velocity decrease in the lowermost liquid core, inferred from previous studies of the PKP(BC) propagation times, and a Q decrease at the top of the inner core, required by the DF/BC amplitude ratios. The preferred model exhibits a constant velocity gradient in the lowermost 150 km of the liquid core, and Q=200 in the uppermost 100 km of the inner core, Q = 440 below. With the adopted velocity model, an increase of Q with depth is required by the data.The amplitude ratio DF/BC exhibits regional variations, which cannot be attributed with certainty to regional variations of Q inside the inner core, due to the large Fresnel zone at the inner core and the possibility of wave focusing and defocusing in the mantle. On the other hand, with the available data, we failed to detect an anisotropy in attenuation inside the inner core.

Error bars for the global seismic Q profile

“…Further, it must be recalled that this is an average over the entire inner core and, if Q a increases with depth [31], more compressional attenuation is permitted at the top of the IC. When we test this by splitting the IC into two layers, we observe that an increase with depth is slightly favored and Q a =450 is allowed at the 1-j model error level in the upper layer.…”

Intricate heterogeneous structures of the top 300 km of the Earth's inner core inferred from global array data: II. Frequency dependence of inner core attenuation and its implication

“…Cormier et al (1998) and Li and Cormier (2002), who analyzed global core phase data for a distance range of 150-180 • , also demonstrated the frequency dependent attenuation over a broader frequency range (0.02-2 Hz), although they did not confirm the hemispherical variation of the attenuation. In contrast, Bhattacharyya et al (1993) and Souriau and Roudil (1995) analyzed spectral amplitude ratios between PKP(DF) and PKP(BC) for globally observed data filtered with corner frequencies of 0.2-1.5 Hz and 0.2-2 Hz, respectively, and concluded that the attenuation of the inner core is not frequency dependent. Recently, the existence of anisotropy of the frequency dependent attenuation has been suggested.…”

Inner-Core Anisotropy and Rotation

“…Bhattacharyya et al (1993) and Souriau & Roudil (1995) find a comparable P-wave quality factor of roughly 360 for the top of the core. This translates into a shear quality factor of 50, which is distinctly lower than the value of 110 ± 25 per cent inferred from normal modes (Widmer et al 1991, Durek & Ekström 1996.…”