Inner core anisotropy measured using new ultra-polar PKIKP paths

• Published in: Geophysical Journal International
• Main Authors: Brett, Henry, Deuss, Arwen
• Other Authors: European Research Council, Horizon 2020 Framework Programme, Nederlandse Organisatie voor Wetenschappelijk Onderzoek
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2020
• Subjects: Geochemistry and Petrology; Geophysics; Antarc*; Antarctic; South Sandwich Islands; Alaska
• Online Access: http://dx.doi.org/10.1093/gji/ggaa348; http://academic.oup.com/gji/advance-article-pdf/doi/10.1093/gji/ggaa348/33777468/ggaa348.pdf; http://academic.oup.com/gji/article-pdf/223/2/1230/34243555/ggaa348.pdf

SUMMARY We measure the seismic anisotropy of the inner core using PKPbc-PKPdf and PKPab-PKPdf differential traveltimes, as a function of the angle ζ between the Earth’s rotation axis and the ray path in the inner core. Previous research relied heavily on body waves originating in the South Sandwich Islands (SSI) and travelling to seismic stations in Alaska to sample inner core velocities with low ζ (polar paths). These SSI polar paths are problematic because they have anomalous travel time anomalies, there are no ultra-polar SSI paths with ζ < 20° and they only cover a small part of the inner core. Here we improve constraints on inner core anisotropy using recently installed seismic stations at high latitudes, especially in the Antarctic, allowing us to measure ultra-polar paths with ζ ranging from 20°–5°. Our new data show that the SSI’s polar events are fast but still within the range of velocities measured from ray paths originating elsewhere. We further investigate the effect of mantle structure on our data set finding that the SSI data are particularly affected by fast velocities underneath the SSI originating from the subducted South Georgia slab, which is currently located just above the core mantle boundary. This fast velocity region results in mantle structure being misinterpreted as inner core structure and we correct for this using a P-wave tomographic model. We also analyse the effect of velocity changes on the ray paths within the inner core and find that faster velocities significantly change the ray path resulting in the ray travelling deeper into the inner core and spending more time in the inner core. To remove this effect, we propose a simple but effective method to correct each event-station pair for the velocity-dependent ray path changes in the inner core, producing a more reliable fractional traveltime measurement. Combining the new ultra-polar data with mantle and ray path corrections results in a more reliable inner core anisotropy measurement and an overall measured anisotropy of ...