A New Probe Into the Innermost Inner Core Anisotropy via the Global Coda‐Correlation Wavefield

Abstract

AbstractInvestigations 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.