Journey to the Not-So-Center of the Earth: Possible Seismic Velocity Anomaly in the Outer-Core Tangent Cylinder

Abstract

Although the structure of Earth's liquid outer core is generally assumed not to vary laterally, seismic observations and geodynamical considerations have lead scientists to postulate that there are seismic velocity anomalies in the tangent cylinder, the portion of that outer core that is formed by a cylinder which circumscribes the inner core and is parallel to Earth's rotation axis. Most previous studies of the tangent cylinder have not found evidence to support the existence of such structure in the outer core; however, I have taken a different approach to the problem by making differential traveltime measurements of the PKP phases which do not travel in the inner core. I hypothesized that there would be no observable velocity anomaly in the tangent cylinder. However, I found that the traveltime duration in the tangent cylinder for PKPbc and PKPab is correlated with their residual compared to standard Earth models such as ak135, even when corrected for mantle structure. In order to be consistent with these findings, current models of the Earth's outer core (or mantle) need to be revisited in order to accommodate possible anisotropy or inhomogeneity in the outer-core tangent cylinder.