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Revisit Smsks Differential Traveltime Data and the Inferred Stratification at Earth’s Outermost Outer Core

Description: 

Recent seismic studies using SmSKS waves showed that the top a few hundred kilometers of Earth’s core possess a P-wave velocity slightly lower than the PREM model, suggesting that stratification may exist within the outermost core. SmKS is a core phase being reflected m-1 times from the lower side of the core-mantle boundary (CMB) and can be observed at epicentral distances of 120°-180°. Differential arrival times between SmKS pairs, such as S3KS and SKKS, S4KS and S3KS are usually employed in determining the P-wave velocity structure in the top part of the outer core since these pairs have very similar traveling ray paths in the mantle. Since there is a π/2 phase shift between two consecutive SmKS arrivals due to the internal caustic surface for underside reflections, measuring the differential times between the two arrivals using waveform cross-correlation requires performing a Hilbert transform of the first arrival before the regular cross-correlation operation. Wang and Niu (2019) found that postcritical refractions and reflections at the CMB of SmKS could lead to phase shifts of the arrivals, causing anomalous differential traveltimes that are not associated with velocity structure of the outermost outer core. In this study, we analyzed SmKS waves recorded by a large dense broadband array in southern China from deep earthquakes occurring in southern America. We found after correcting the postcritical reflection/refraction induced phase shifts, the differential SmSKS data do not support for the presence of a low velocity layer at the top of the outer core.

Session: Earth’s Structure From the Crust to the Core

Type: Oral

Date: 4/18/2023

Presentation Time: 10:45 AM (local time)

Presenting Author: Fenglin Niu

Student Presenter: No

Invited Presentation: 

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