HR: 0830h
AN: S31D-0791    [PDF]
TI: Constraints on Density and Shear Velocity Contrasts at Inner Core Boundary
AU: * Cao, A
EM: acao@seismo.berkeley.edu
AF: Berkeley Seismological Lab, UC Berkeley, 215 McCone Hall, Berkeley, CA 94720 United States
AU: Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: Berkeley Seismological Lab, UC Berkeley, 215 McCone Hall, Berkeley, CA 94720 United States
AB: The density jump ($\Delta \rho_{ic}$) at the inner core boundary (ICB) is an important constraint on the dynamics and history of the earth's cores. Two types of seismological data sensitive to $\Delta \rho_{ic}$ have been studied since the 1970's: free oscillation eigenfrequencies and amplitudes of core reflected phases (PKiKP/PcP). The reference PREM model (Dziewonski and Anderson, 1981), based largely on normal mode data, has a relatively low value of $\Delta \rho_{ic}=0.65 g/cm^3$, whereas most studies based on PKiKP/PcP amplitude ratios find significantly larger values, sometimes in excess of $1.0 g/cm^3$. It has been argued that, because PKiKP is rarely observed in the distance range considered ($10-70^o$), the latter type of measurement provides only upper bounds on $\Delta \rho_{ic}$ (e.g. Shearer and Masters, 1990). We have analyzed 10 years of high quality global broadband data accumulated since the work of Shearer and Masters (1990). We systematically analyzed over 4500 seismograms from intermediate/deep events (depth $>$ 70 km) and nuclear explosions, in the distance range $10-70^o$. The data were filtered in the band-pass 0.7-3 Hz. We performed a rigorous data selection: 1) we only considered observations of clearly identified PKiKP and PcP (within 5s of theoretical arrival time) with signal to noise ratio $>4$; 2) we rejected data for which one or more of 7 other phases may be present within 15 s in front of PKiKP or PcP (P,pP,PP,PPP,S,SS,ScS). We identified 5 pairs of very clear (Quality A), and 16 possible (Quality A-) PKiKP and PcP arrivals. In addition, 62 records showed no PKiKP but a clear PcP. Together, we obtain a much less dispersed dataset than previously available, with the quality A data following the lower bound of the ensemble of amplitude ratios versus distance. We combine our high quality measurements with 2 measurements from the literature that fell within our rigorous selection criteria and obtain unbiased estimates of $\Delta \rho_{ic}$ in the range $0.65-0.91 gcm^{-3}$. We discuss trade-offs between $\Delta \rho_{ic}$ and the corresponding velocity jumps at the ICB.
DE: 1734 Seismology
SC: Seismology [S]
MN: 2003 Fall Meeting