Large variations in travel times of mantle-sensitive seismic waves from the South Sandwich Islands: Is the Earth's inner core a conglomerate of anisotropic domains?

Abstract

Cylindrical anisotropy in Earth's inner core has been invoked to account for travel times of PKP core-sensitive seismic waves, such as from the South Sandwich Islands (SSI) earthquakes observed in Alaska, which depart from predictions. Newly collected travel-time residuals from seismic waves from the SSI region that sample only Earth's mantle (PcP and P waves) have a comparable range to the PKP differential travel-time residuals, yet they are insensitive to core structure. This observation suggests that mantle structure affects PKP travel time residuals more than previously acknowledged and challenges the existing conceptual framework of a uniform inner core anisotropy. The inner core could be a conglomerate of anisotropic domains, and the PKP travel times are most likely influenced by the geometry of inner core sampling and inhomogeneous mantle structure. This concept reconciles observed complexities in travel times while preserving a net inner core anisotropy that is required by observations of Earth's free oscillations.