Antipodal seismic reflections upon shear wave velocity structures within Earth's inner core

Abstract Seismic evidence is presented for a high shear wave velocity, apparent discontinuity near ~100 km depth within a portion of Earth's inner core. Antipodally (>179°) focused data are stacked for five source–receiver diametric ray paths traversing the inner core. Two antipodal paths follow ray surfaces which are aligned with diameters between Tamanrasset (TAM), Algeria and Tonga earthquakes and Pitinga (PTGA), Brazil and Sulawesi earthquakes, providing clear examples of precursors to PKIIKP (an underside reflection at the inner core boundary). Waveform and stacked data (T > 4 s) were engaged in testing more than 16 inner core model series, varying compressional and shear wave velocities in upper inner core structures. The precursory seismic phases are successfully modeled as reflecting beneath a core liquid/solid interface at 100 km depth below the inner core boundary. This interface is highly reflective, and sensitive to a shear wave velocity contrast ≥5 km/s. An earlier precursory phase is observed at TAM and PTGA which may be modeled as an apparent discontinuity near ~250 km depth. This intermediate region has high shear wave velocities more akin to hcp-Fe mineralogy than Preliminary Reference Earth Model (PREM) values. Three other antipodal observations (China–Chile) nearly orthogonal to TAM paths exhibit seismic waves whose waveforms are more consistent with the PREM velocities above 100 km depth, while offering modest evidence for a solid/solid discontinuity at 100-km-depth. This research focused on seismology of the inner core potentially has mineral physics and geodynamic implications too broad to be simply encapsulated herein. Acknowledging some of these implications, we have focused upon measuring and mapping the seismic anomalies.