A new Probe into the Innermost Inner Core Anisotropy via the Global Coda-correlation Wavefield

Costa de Lima, Thuany, Tkalcic, Hrvoje, and Waszek, Lauren (2022) A new Probe into the Innermost Inner Core Anisotropy via the Global Coda-correlation Wavefield. Journal of Geophysical Research: Solid Earth, 127 (4). e2021JB023540.

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Investigations of the Earth's inner core (IC) using seismic body waves are limited by their volumetric sampling due to uneven global distribution of large earthquakes and receivers. The sparse coverage of the IC leads to uncertainties in its anisotropy, the directional dependence of seismic velocity. Yet, detailed constraints on anisotropy, such as its magnitude, and spatial distribution, are required to understand the crystallographic structure of IC's iron and its solidification and deformation processes. Here, we present a new method to investigate the IC's anisotropic properties based on Earth's coda-correlation wavefield constructed from the late coda of large earthquakes. We perform a comprehensive travel time analysis of I2*, an IC-sensitive correlation feature identified as a counterpart of the direct seismic wavefield's PKIKPPKIKP waves, yet fundamentally different. Namely, I2* is a mathematical manifestation of similarity among specific seismic phases with the same slowness detected in global correlograms in the short inter-receiver distance range. Our new spatial sampling of the IC overcomes the shortage of direct seismic wavefield paths sensitive to the IC's central volume, also known as the innermost IC (IMIC). The observed I2*’s travel time variations relative to Earth's rotation axis (ERA) support a model of cylindrical anisotropy with 3.3% strength and a zonal pattern of slow axis oriented 55° from ERA. We thus find compelling evidence for a deep IC structure with distinct anisotropy, although we cannot resolve the depth at which the change occurs. This finding reinforces previous inference on the IMIC, with implications for Earth's evolution.

Item ID: 73230
Item Type: Article (Research - C1)
ISSN: 2169-9356
Copyright Information: © 2022. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
Funders: Australian Research Council (ARC)
Projects and Grants: ARC DE170100329
Date Deposited: 13 Oct 2022 00:44
FoR Codes: 37 EARTH SCIENCES > 3706 Geophysics > 370609 Seismology and seismic exploration @ 100%
SEO Codes: 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 100%
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