Seismological evidence for heterogeneous lowermost outer core of the Earth from PKiKP-PKPbc differential traveltime and dispersion in PKPbc
Abstract
We present seismic evidence for heterogeneous structure in the lowermost outer core (F-layer) based on the method developed by Ohtaki and Kaneshima (JGR, 2015). The method uses two observations that are particularly sensitive to the layer structure and are relatively insensitive to the structure of the other parts of the Earth: the frequency dispersion in P-waves that graze or are diffracted at the inner core boundary (PKPbc), and differential traveltimes between the P-waves reflected from the inner core boundary (PKiKP) and those that turn above the boundary (PKPbc). Ohtaki and Kaneshima (2015) have obtained the FVW model for the velocity structure of the F-layer on the "western" hemisphere of the inner core. In this study, we examined the structure for the "eastern" hemisphere using the core phases on vertical-component seismograms of Hi-net in Japan for earthquakes near the South Sandwich Islands. The PKPbc dispersion analysis requires nearly constant velocity on the inner core boundary in this region. The CD-BC analysis necessitates that cumulative velocity in the F-layer is close to that of PREM. The velocity model which satisfies both has nearly constant and smaller velocity than PREM on the boundary and faster ones above. This feature is contrast to the FVW model, which has a somewhat gentler velocity gradient and smaller velocities than PREM in the whole F-layer. The reduced velocity gradient on the inner core boundary beneath Australia signifies chemically unmixed materials there. Its higher velocity than FVW and also PREM indicates that the unmixed materials stem from a higher concentration of light element above the boundary. Thus the light element cloud in the F-layer suggests solidification in the "eastern" hemisphere.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.V41B3120O
- Keywords:
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- 1015 Composition of the core;
- GEOCHEMISTRYDE: 3919 Equations of state;
- MINERAL PHYSICSDE: 3672 Planetary mineralogy and petrology;
- MINERALOGY AND PETROLOGYDE: 7207 Core;
- SEISMOLOGY