Inner core anisotropy measured using new ultra-polar PKIKP paths

Abstract

Summary We measure the seismic anisotropy of the inner core using PKPbc-PKPdf and PKPab-PKPdf differential travel times, 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 traveling 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 < 20o 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 20o − 5o. Our new data shows that the South Sandwich Island’s polar events are fast but still within the range of velocities measured from raypaths originating elsewhere. We further investigate the effect of mantle structure on our data set finding that the SSI data is particularly affected by fast velocities underneath the SSI originating from the subducted South Georgia slab which is currently located just above the CMB. 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 raypaths within the inner core and find that faster velocities significantly change the raypath 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 raypath changes in the inner core, producing a more reliable fractional travel time measurement. Combining the new ultra polar data with mantle and raypath corrections results in a more reliable inner core anisotropy measurement and an overall measured anisotropy of 1.9%-2.3% for the whole inner core. This is lower than previous body wave studies ( 3% anisotropy) and in better agreement with the value of inner core anisotropy measured by normal modes ( 2% anisotropy). We also identify regional variation of anisotropic structure in the top 500 km of the inner core which appears to be more complex than simple hemispherical variations. These regional variations are independent of the SSI data and are still present when these data are excluded. We also find a potential innermost inner core with a radius of 690 km and stronger anisotropy.

3D Transdimensional Seismic Tomography of the Inner Core

“…PKPdf is used either individually, or in combination with a reference phase (PKPcd, PKPbc, PKPab), which only traverses the mantle and outer core. We employ independent datasets from three previous studies consisting of 1603 PKPbc-PKPdf, 627 PKPab-PKPdf (Brett and Deuss, 2020), and 3102 PKPcd-PKPdf (Waszek and Deuss, 2011) differential travel time measurements, and 2344 absolute PKPdf arrival times (Lythgoe et al, 2014). The arrival times of Brett and Deuss (2020) and Lythgoe et al (2014) are visual picks, while Waszek and Deuss (2011) used a combination of both visual inspection and cross correlation between the inner core and outer core phases.…”

“…We employ independent datasets from three previous studies consisting of 1603 PKPbc-PKPdf, 627 PKPab-PKPdf (Brett and Deuss, 2020), and 3102 PKPcd-PKPdf (Waszek and Deuss, 2011) differential travel time measurements, and 2344 absolute PKPdf arrival times (Lythgoe et al, 2014). The arrival times of Brett and Deuss (2020) and Lythgoe et al (2014) are visual picks, while Waszek and Deuss (2011) used a combination of both visual inspection and cross correlation between the inner core and outer core phases. Combining these datasets provides good coverage of the inner core from its surface to 200 km radius (see Fig.…”

“…Multiple studies have highlighted the significant influence of mantle structure on inner core travel times (Tkalči ć 2010; Frost et al 2020;Brett and Deuss 2020). Mantle corrections will be especially large for absolute travel time measurements, but may also be important for differential travel time data.…”

“…A particular subset of raypaths, travelling from the South Sandwich Islands (SSI) to Alaska, have been noted to travel anomalously fast. It has been debated to what extent these travel times reflect inner core structure or mantle structure (Tkalči ć 2010; Frost et al 2020;Brett and Deuss 2020). To address the issue of mantle heterogeneity, we use multiple tomographic mantle models to assess unaccounted for mantle structure, and we develop inner core models which both include and exclude the South Sandwich Islands data.…”

Observing seismic attenuation in the Earth’s mantle and inner core using normal modes

“…Radial modes most strongly cross-couple to their closest inner core sensitive spheroidal mode (PKIKP equivalent modes), both through ellipticity and inner core anisotropy (Irving et al, 2008). Cylindrical anisotropy is a wellknown property of the inner core, with the fast axis roughly aligned to the axis of rotation (Woodhouse et al, 1986;Tromp, 1993;He and Tromp, 1996;Durek and Romanowicz, 1999;Beghein and Trampert, 2003;Deuss et al, 2010;Deuss et al, 2013;Mäkinen and Deuss, 2013;Brett and Deuss, 2020). Much less is known about radial anisotropy 1 in the inner core; only one recent study has shown that the centre frequencies of radial modes may be able to constrain radial inner core anisotropy (Lythgoe and Deuss, 2015).…”