15:00 〜 16:00
[S17-P-01] Seismological evidence for heterogeneous lowermost outer core (F-layer) of the Earth
The asymmetric release of light-elements at the inner core boundary (ICB) may cause a regional variation of chemical components in the outer core, especially in the layer directly above the inner core (F-layer). To verify the variation, it is necessary to analyze detailed seismic structure in the layer of two or more laterally distinct regions. However, conventional observations are insufficient to resolve the detailed F-layer structure. We developed a new method to analyze dispersion in PKPbc and PKiKP-PKPbc travel times (dT), and investigated the velocity profile beneath the northeast Pacific (Ohtaki & Kaneshima, 2015). These two observations are particularly sensitive to F-layer and relatively insensitive to the mantle and inner core.
In this study, we analyze seismograms of South Sandwich events observed in Japan using the same method. The rays turn beneath Australia. The dispersion is sensitive to the velocity gradient just above the ICB but insensitive to velocity values in the F-layer. The dispersion observations require nearly constant velocity above the ICB, which is relatively close to AK135. The dT is sensitive to the velocity values between the PKPbc turning depth and the ICB but insensitive to the velocity gradient. The dT observations are close to those predicted by PREM. The velocity model that satisfies both the observations has nearly constant and smaller velocities than PREM on the ICB, and faster ones above.
Both the observations require disparate F-layer structures in between these two regions. Velocity in the liquid core has a little dependence on temperature (Ichikawa+, 2014). Thus the difference in velocities between the two regions is ascribed to the relative abundance of light elements. The reduced velocity gradient on the ICB beneath Australia signifies chemically unmixed materials there. The higher velocities than PREM and our NE Pacific model (FVW) indicate a localized higher abundance of light elements in the F-layer.
In this study, we analyze seismograms of South Sandwich events observed in Japan using the same method. The rays turn beneath Australia. The dispersion is sensitive to the velocity gradient just above the ICB but insensitive to velocity values in the F-layer. The dispersion observations require nearly constant velocity above the ICB, which is relatively close to AK135. The dT is sensitive to the velocity values between the PKPbc turning depth and the ICB but insensitive to the velocity gradient. The dT observations are close to those predicted by PREM. The velocity model that satisfies both the observations has nearly constant and smaller velocities than PREM on the ICB, and faster ones above.
Both the observations require disparate F-layer structures in between these two regions. Velocity in the liquid core has a little dependence on temperature (Ichikawa+, 2014). Thus the difference in velocities between the two regions is ascribed to the relative abundance of light elements. The reduced velocity gradient on the ICB beneath Australia signifies chemically unmixed materials there. The higher velocities than PREM and our NE Pacific model (FVW) indicate a localized higher abundance of light elements in the F-layer.