Inner Core Anisotropy from Antipodal PKIKP Traveltimes

Abstract

The Earth's inner core (IC) and the innermost inner core (IMIC) are not merely subjects of academic curiosity, but they likely hold crucial information about the Earth's evolution and its geomagnetic field. In particular, the IMIC might contain a frozen anisotropic fabric different from the outer inner core (OIC). Seismological studies have probed IMIC to investigate its anisotropy – the directional dependence in seismic velocities – and reached different conclusions. Differing inferences are due to the scarcity of the seismological data sensitive to the deepest Earth's volumes and because outer layers of the Earth obscure the access to the IC by affecting the traveltime predictions and measurements. To contribute to the current efforts, we collect a global dataset of 1,150 antipodal PKIKP traveltimes for source-receiver epicentral distances between 165° and 180° sensitive to IMIC. The hand-picking of PKIKP absolute traveltimes at multiple band-pass filters is assisted by collecting P-wave records at the receivers with a similar azimuthal range. Along with relying on state-of-the-art seismological catalogs, tomographic mantle models, and OIC anisotropy models, this serves the purpose of eliminating the influences of outer layers on PKIKP traveltimes. Our new dataset is augmented by the Thai Seismic Array (TSAR) waveform data, antipodal to many South American events, and the Brazilian Seismographic Network (RSBR), antipodal to the events from Southeast Asia. The volumetric sampling with the bottoming depths between 75 and 560 km from the Earth's center provides new constraints on IMIC anisotropy and bulk IC. Both IC bulk anisotropy and the IC containing IMIC models, with the fast axis aligned with the Earth's rotation axis (ERA), the slow direction at ∼60° from the ERA, and an anisotropic strength in the range 3.2–5.0 ± 0.2% explain our new PKIKP dataset.