Dissimilarity of the Earth's Inner Core Surface Under South America and Northeastern Asia Revealed by Core Reflected Phases

Abstract

Resolving topography of the inner core boundary (ICB) and the structure and composition of the nearby region is key to improving our understanding of solidification of the Earth's inner core. Observations of travel times and amplitudes of short-period seismic phases of PKiKP and PcP reflected, respectively, off the inner and outer boundary of the liquid core, provide essential constraints on the properties of this region. We revisit heterogeneities of ICB using a total of more than 1,300 new differential travel times and amplitude ratios of PKiKP and PcP measured at 3.2-35.2°and reflected off the core's boundaries under Northeastern Asia and South America. We observe a statistically significant systematic bias between the measurements collected in the two spots. We carefully examine its origin in terms of contributions by various Earth's shells and find that most of variance in PKiKP-PcP differential travel times measured above the epicentral distance of 16.5°in Northeastern Asia can be accounted for by mantle corrections. We find slight disparity of about 1-3 km between the outer core thickness under Asia and America; the ICB density jump under Northeastern Asia is about 0.3 g/cm 3 , which is three times as small as under South America. The findings are interpretable either as evidence for inner core hemispherical asymmetry, whereby crystallization dominates in the West and melting in the East (not vice versa), or in terms of two disconnected mosaic patches with contrasting properties.

Structure, Materials and Processes in the Earth’s Core and Mantle

“…The density jump at the present-day inner core boundary as provided by seismology (PREM model, Dziewonski and Anderson 1981) is as large as 600 kg m −3 , which requires enrichment in light element(s) in the outer core relative to the inner core. This value has been recently re-evaluated and the values are ranging from 420 to 820 kg m −3 , recognizing the possible influence of lateral heterogeneities in the inner core and seismic noise (e.g., Hirose et al 2013;Krasnoshchekov et al 2019;Wong et al 2021).…”