Regionally heterogeneous uppermost inner core observed with Hi‐net array

Abstract

Studies of velocity structure in the Earth's inner core and its volumetric variations illuminate our understanding of inner core dynamics and composition. Here we use an extensive number of seismograms recorded by the Hi-net array to construct complete empirical curves of PKP BC -PKP DF differential traveltimes. The nature of these curves implies that significant variations in traveltimes are accumulated during the passage of PKP DF waves through the uppermost inner core and rules out outer core structure effects. Uniform cylindrical anisotropy of a plausible strength in the uppermost inner core can also be ruled out as a cause of the observed traveltime variations because the range of sampled ray angles is too narrow. The configuration and strength of inhomogeneities from a recent tomographic model of the lowermost mantle cannot account for the observed traveltime variations. Therefore, we infer that either variations of P wave isotropic velocity on the scale of about hundred km and less are present in the uppermost inner core or the material may be organized in distinctive anisotropic domains, and both of these features may be superimposed on long-wavelength hemispherical structure. If the former holds true, the absolute magnitude of required P wave velocity perturbations from referent values is 0.60 ± 0.10% in the quasi-eastern and 1.55 ± 0.15% in the quasi-western hemisphere (0.85 ± 0.05% and 1.10 ± 0.10%, respectively, with the lowermost mantle correction). The existence of these variations is a plausible physical outcome given that vigorous compositional convection in the outer core and variations in heat exchange across the inner core boundary may control the process of solidification.

Measuring the seismic velocity in the top 15 km of Earth’s inner core

“…The hemisphere boundaries have been observed as sharp , and may vary with depth in the inner core . Further irregularities and variation in the location of the Pacific boundary have been observed (Miller et al, 2013;Yee et al, 2014;Irving and Deuss, 2015;Yu et al, 2017). Small scale features on the inner core boundary have been detected in this area (Waszek and Deuss, 2015;Tian and Wen, 2017;Shen et al, 2016), which may be linked to regional solidification of the inner core (Cormier, 2015).…”

Partial melting of a Pb‐Sn mushy layer due to heating from above, and implications for regional melting of Earth's directionally solidified inner core

“…In addition, seismology has also inferred east-west variations in IC elastic anisotropy [Tanaka and Hamaguchi 1997], attenuation anisotropy [e.g., Cao and Romanowicz 2004], and isotropic velocity [Tanaka and Hamaguchi 1997;Niu and Wen 2001]. More recently, Cormier and Attanayake [2013], Iritani et al [2014], and Yee et al [2014] argue for smaller-scale variations in seismic properties.…”

Polymorphic Nature of Iron and Degree of Lattice Preferred Orientation Beneath the Earth's Inner Core Boundary

“…If the heat flow across the ICB is governed by the distribution of heterogeneities in the lowermost mantle through quasistationary flows in the outer core (Gubbins et al, 2011), it is possible that Geochemistry, Geophysics, Geosystems 10.1002/2017GC007285 seismologically detected multiscale heterogeneities in the lowermost mantle can complicate the iron crystallization pattern at ICB. This complex pattern has not yet been fully revealed by the seismological probes, but the recently observed regional variation in compressional velocity (Huang et al, 2015;Wu & Irving, 2017;Yee et al, 2014) and attenuation (Attanayake et al, 2014;Iritani et al, 2014;Pejić et al, 2017) suggest more wide-spread complexity that yet has to be fully mapped and understood. According to the scenario envisaged by the constrained geodynamic simulations, the regional melting and freezing mechanism of the core material provides itself a cause for seismic anomalies in the inner core.…”