INNER-CORE ATTENUATION FROM SHORT-PERIOD PKP(BC) VERSUS PKP(DF) WAVE-FORMS

Differential waveform analysis provides an excellent tool for studying the attenuation properties of the top of the inner core. We analyse 1 08 PKP(BC) versus PKP(DF) waveforms from Global Digital Seismograph Ne twork (GDSN) vertical-component seismograms to constrain the frequency and depth dependency of Q(alpha) in this region. We use both frequenc y- and time-domain techniques. In the time-domain method, the BC phase is mapped onto the DF phase using an attenuation band operator. The m apping operator is parameterized by the upper and lower cut-off freque ncies of the absorption band, the time shift required to align these t wo phases, and t, the integrated effect of Q(alpha)-1 in the top of t he inner core. In the frequency-domain analysis, multitaper spectral e stimation is used to compute the complex spectrum of the two phases. T he shape of the amplitude spectrum of the spectral ratio between these two phases gives an estimate of Q(alpha). Similar results are obtaine d from frequency- and time-domain analysis but the Q(alpha) obtained f rom frequency-domain analysis is approximately 20 per cent greater tha n the value obtained from time-domain analysis. We prefer the frequenc y-domain results since they are not affected by the presence of noise at higher frequencies. Apparent Q(alpha) values exhibit considerable s catter with no clear frequency or depth dependence. We find that the a verage value of Q(alpha) in the top of the inner core is about 360 whi ch is consistent with previous body wave studies but differs by a fact or of two from values obtained from studies of the decay of free oscil lations.