Physics
Scientific paper
Aug 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991pepi...68..183s&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 68, Issue 1-2, p. 183-199.
Physics
36
Scientific paper
Differential travel times of SKS, SKKS and S3KS seismic phases are analysed to specify the structure of the outermost liquid core, which is poorly sampled by the PKP waves currently used for core tomography. The data are mainly broadband records at epicentral distances of 83-140°. They have been selected to obtain a good global Earth coverage. SKKS-SKS and S3KS SKKS travel-time residuals have been computed with respect to the mean Earth model Preliminary Reference Earth Model (PREM), and corrected for ellipticity. The SKKS-SKS residuals plotted as a function of epicentral distance exhibit a general positive slope for distances greater than 105°, with mean positive values of approximately 2 s at 120-130°C. These features may be accounted for by slight perturbations in PREM, involving a 2-5% S velocity increase in the D'' layer at the base of the mantle, and a small decrease in the P velocity in the upper 800 km of the liquid core. The structure in the uppermost 100 km of the core is mainly constrained by the S3KS data. A high-velocity gradient followed by a low gradient is obtained immediately beneath the core-mantle boundary; this may suggest the existence of a thin low-velocity layer at the top of the liquid core, as a result of a possible accumulation of light elements beneath the core-mantle boundary.
The SKKS-SKS residuals exhibit strong coherent regional variations, whereas the S3KS-SKKS residual variations remain small. This indicates that the lateral heterogeneitics at the core side of the core-mantle boundary, if any, are very small. The simultaneous analysis of S3KS-SKS and SKKS-SKS residuals suggests a prevailing contribution of long-wavelength heterogeneities in D''.
Poupinet Georges
Souriau Annie
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