Seismological evidence for a non-monotonic velocity gradient in the topmost outer core.

Vivian Tang,Li Zhao,Shu-Huei Hung

doi:10.1038/srep08613

Abstract

The Earth's core is mostly an Fe-Ni alloy with a fraction of light elements (~10 wt%, mainly O, S and Si). Accumulation of these light elements under the core-mantle boundary (CMB) may lead to chemical stratification. Seismic observations have been presented both for and against the stratification in the topmost region of the outer core. Here we investigate the structure under the CMB using differential travel times between SKKS and S3KS waves. We obtain 606 high-quality S3KS-SKKS differential travel times with global path coverage. Result from a Bayesian inversion of these differential times indicates that the seismic velocity in the top 800 km of the outer core is ~0.07% on average lower than that in model PREM. The depth-dependent velocity profile, in particular a low-velocity zone of up to ~0.25% lower than PREM at ~80 km below the CMB, strongly favors the existence of stratification at the top of the outer core.

Highlights

The Earth’s core is mostly an Fe-Ni alloy with a fraction of light elements (,10 wt%, mainly O, S and Si)
We investigate the structure under the core-mantle boundary (CMB) using differential travel times between SKKS and S3KS waves
Measuring S3KS-SKKS differential travel times In this study, we focus on the differential traveltimes between SKKS and S3KS waves in the distance range 120u– 140u and from deep earthquakes (400 km and deeper) to avoid the interference of near-source surface reflections

Summary

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Seismological evidence for a non-monotonic velocity gradient in the topmost outer core. Result from a Bayesian inversion of these differential times indicates that the seismic velocity in the top 800 km of the outer core is ,0.07% on average lower than that in model PREM. The latest seismic evidence supporting those observations has been from travel time inversions of SmKS waves along a few isolated paths under the Pacific Ocean, where the wave speed at the top of the core is 0.45% lower than PREM, and the anomaly gradually reduces to zero at 300 km below the CMB20,21. Notwithstanding the support for the existence of a stable compositional layering below the CMB, the mechanisms provided by dynamical modelings do not fully reconcile with our velocity profile obtained from S3KS-SKKS differential traveltimes. Our Bayesian inversion of the S3KS-SKKS differential travel times provides seismological constrains on the globally averaged wave speed profile in the topmost outer core. The alternating high and low velocity gradients may introduce more complexities in the interpretation of the seismic model, they offer more detailed seismic constrains that challenge geodynamicists to come up with new models to satisfy them

Methods

Author contributions

Findings

Additional information