Abstract.The

[1] It is crucial to obtain good spatial coverage of seismic data points for better understanding the Earth's core, but the core beneath the polar regions remains largely unexplored. We analyzed differential traveltimes and amplitude ratios of core phases whose raypaths run beneath Antarctica for determining the V p and Q p structure near the inner core boundary in the south polar region. The model we obtained (south polar region model, SPR) is described relative to the preliminary reference Earth model (PREM) as follows: a 0.05 km/s lower V p value at the top of the inner core, a 1.5 times steeper V p gradient in the upper 300 km of the inner core, a smaller Q p (300) in the upper 300 km of the inner core, and a 0.04 km/s lower V p at the bottom of the outer core. The V p values of SPR in the lowermost outer core lie between those of PREM and AK135, being closer to those of AK135. The lowermost outer core V p inside the tangent cylinder is thus close to the global average. In the upper inner core, SPR has lower V p than AK135 and PREM. The SPR V p profile is close to that of previous models for the Western Hemisphere, although most of our data sample the Eastern Hemisphere of the inner core. Our results indicate that the inner core does not have a simple hemispherical variation as usually supposed. Our data support an eyeball-shaped high-V p anomaly with compressional velocity higher than in 1-D reference Earth models, concentrated to a smaller region beneath eastern Asia.

Abstract.Broadband seismograms recorded by the seismic stations deployed on oceanic islands in the South Pacific for two deep earthquakes in 1998 are used to investigate the mantle transition zone structure beneath the South Pacific, where a large-scale hot plume might ascend from the core-mantle boundary (CMB).

[1] A family of core-reflected shear waves generated by the 28 June 2002 Vladivostok deep earthquake, including near vertical reflections from the 410-and 660-km discontinuities, were recorded by $500 tiltmeters of the Hi-net and $60 broadband seismometers of the F-net in Japan. The observed upper mantle reflections were crosscorrelated with synthetics calculated on the basis of the spectral element method for a fully three-dimensional Earth model using the Earth Simulator supercomputer to accurately determine the depths of the reflection points. The mapped upper mantle discontinuities were compared with a high-resolution P wave tomographic image. The 660-km discontinuity is depressed at a constant level of $15 km along the bottom of the horizontally extending aseismic slab under southwestern Japan. The transition from the normal to the depressed level occurs within a lateral distance of less than $200 km. Observations suggest that the reflections from the 410-km discontinuity interfere with those from slab-related structures on top of this discontinuity, leading to a spuriously large elevation of the 410-km discontinuity in and near the subducted slab. Records at stations relatively free from such interference effects, however, still imply elevation of this discontinuity within the slab.

No abstract