PREM-like velocity structure in the outermost core from global SKS and ScS waveform modeling

Abstract

Seismic structure in the Earth's outermost core is important for our understanding of thermochemical stratification in the outer core, and is yet heavily debated. Here we study the compressional velocity structure in the Earth's outermost core based on waveform modeling of a unique untapped SKS and ScS dataset near bifurcation distances, collected from global seismic arrays for earthquakes occurring from 2000 to 2020. Using the SKS-ScS array dataset minimizes the effects of many uncertainties associated with earthquake source parameters and seismic heterogeneities in the mantle, and affords an opportunity to study and assess the seismic structure in the outermost core. We study outer core structure by testing two end-member models: 1) the D″ model that attributes any anomalous seismic observations to the effects of the lowermost mantle structure and is paired with a PREM (the Preliminary Reference Earth's Model) structure in the outermost core and 2) the outer core model that is paired with either a PREM or a tomographic structure in the lowermost mantle and attributes other unexplained seismic signals to an outer core structure. The results of the outer core models present unreasonable large lateral variations of >3.1% in the outermost core, while the inferred D″ models exhibit large-scale seismic anomalies that are consistent with the tomographic models and small-scale anomalies that are confirmed by further analysis of the seismic array data. Our analyses suggest a PREM-like seismic velocity structure and a lack of strong thermochemical anomalies in the topmost ∼200 km of the outer core, placing bounds on possible thermal and compositional conditions in the region of the Earth's outermost core. Our study also identifies the existence of small-scale seismic anomalies and sharp velocity variations in the lowermost mantle beneath the south coast of Alaska, northwestern Atlantic and the middle of Central America.