Heterogeneities of the Earth’s Inner Core Boundary from Differential Measurements of PKiKP and PcP Seismic Phases
The Earth’s crystalline inner core (IC) solidifies from the liquid Fe alloy of the outer core (OC), which releases latent heat and light elements sustaining the geodynamo. Variability in solidification regime at the inner core boundary (ICB) may result in compositional and thermal multi-scale mosaic of the IC surface and dissimilarity of its hemispheres. Both the mosaic and hemisphericity are poorly constrained, not least due to a lack of available sampling by short-period reflected waves. Measured amplitude ratio of seismic phases of PKiKP and PcP reflected, respectively, off the inner and outer boundary of the liquid core, yields direct estimate of the ICB density jump. This parameter is capable of constraining the inner–outer core compositional difference and latent energy release, but is not well known (0.2–1.2 g/cub. cm), and its distribution is obscure. Travel time measurements of PKiKP and PcP waveforms can be useful in terms of getting an insight into fine structure of ICB and its topography. We analyse a new representative sample of pre-critical PKiKP/PcP differential travel times and amplitude ratios that probe the core’s spots under Southeastern Asia and South America. We observe a statistically significant systematic bias between the Asian and American measurements, and carefully examine its origin. Separating the effects of core-mantle boundary and ICB on the measured differentials is particularly challenging and we note that a whole class of physically valid models involving D″ heterogeneities and lateral variation in lower mantle attenuation can be employed to account for the observed bias. However, we find that variance in PKiKP-PcP differential travel times measured above the epicentral distance of 16° is essentially due to mantle heterogeneities. Analysis of data below this distance indicates the ICB density jump under Southeastern Asia can be about 0.3 g/cub. cm, which is three times as small as under South America where also the thickness of the above liquid core can be by 1–3 km in excess of the one in the East. The findings preclude neither IC hemispherical asymmetry (whereby crystallization dominates in the West and melting in the East) nor patchy IC surface, but provide an improved and robust estimate of the ICB density jump in two probed locations.
KeywordsInner core boundary Reflected waves Density jump
This work is supported by the Russian Foundation for Basic Research grant #18-05-00619. The facilities of IRIS Data Services, and specifically the IRIS Data Management Center, were used for access to waveforms used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR-1261681. Seismic data were also obtained from Geophysical Survey of Russia (Federal research center of Russian Academy of Sciences), Data Management Center of the National Research Institute for Earth Science and Disaster Resilience (Japan) and Earthquake Research Institute, the University of Tokyo (Japan). Figure 1 was generated with Generic Mapping Tools .
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