Inner Core Attenuation From Short-PeriodPkp(Bc)VersusPkp(Df)Waveforms

Abstract

S U M M A R YDifferential waveform analysis provides an excellent tool for studying the attenuation properties of the top of the inner core. W e analyse 108 P K P ( B C ) versus P K P ( D F ) waveforms from Global Digital Seismograph Network (GDSN) verticalcomponent seismograms to constrain the frequency and depth dependency of Qa in this region. W e use both frequency-and time-domain techniques. In the timedomain method, the BC phase is mapped onto the DF phase using an attenuation band operator. T h e mapping operator is parameterized by the upper and lower cut-off frequencies of the absorption band, the time shift required to align these two phases, and t*, the integrated effect of Q;' in the top of the inner core. In the frequency-domain analysis, multitaper spectral estimation is used to compute the complex spectrum of the two phases. T h e shape of the amplitude spectrum of the spectral ratio between these two phases gives an estimate of en.Similar results are obtained from frequency-and time-domain analysis but the Qa obtained from frequency-domain analysis is approximately 20 per cent greater than the value obtained from time-domain analysis. W e prefer the frequency-domain results since they are not affected by the presence of noise at higher frequencies.Apparent Qa values exhibit considerable scatter with no clear frequency or depth dependence. W e find that the average value of Qa in the t o p of the inner core is about 360 which is consistent with previous body wave studies but differs by a factor of two from values obtained from studies of the decay of free oscillations.

Strong seismic scatterers near the core–mantle boundary north of the Pacific Anomaly

“…In the calculation, all the attenuation and refraction coefficients are considered. The inner core attenuation is set to be 360 (Bhattacharyya et al, 1993). After convolving the energy kernels with the source time functions (in power), we calculate the square root of the energy power and normalize it to the theoretical amplitude of PKIKP.…”

Sharp and seismically transparent inner core boundary region revealed by an entire network observation of near-vertical PKiKP

“…Other than this phase, no other notable energy may be identified (there is a slight indication of pPKiKP ∼75 s). Figure 7(b) shows the waveform of a linear stack for the zero relative slowness; green lines indicate the amplitude of possible reflected phases from a sharp horizontal reflector with a 1% reflection (i.e., impedance contrast of 2%; ∼1% both in velocity and density jumps if they scale together) at frequencies of 1 and 2 Hz (Q = 360 is assumed; Bhattacharyya et al, 1993). As the linear stack results for other slownesses show a similar feature, we may conclude that there is no sharp (few kilometers thick) discontinuity of the 1% reflection in the top 600 km (corresponding ∼100 s after PKiKP) of the inner core.…”

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.…”