Seismic Resolution of the Earth's Outermost Core in Relation to Lower Mantle Heterogeneity

Abstract

Issues concerning resolution of seismically determined outermost core properties are presented. Particular attention is given to effects of both large and small scale lower mantle heterogeneity on seismic phases most commonly used for determining outermost core properties. The family of SmKS waves, which travel as S in the mantle, P in the core, reflecting (m-1) times at the underside of the core-mantle boundary (CMB), are the outer core's equivalent to upper mantle multiple S waves (S, SS, SSS, ... ), and are well-suited for studying outermost core structure. The higher multiples of SmKS have outer core wave paths restricted to the outermost few hundred km of the core (see figure). Travel time and waveform behavior of SmKS waves are analyzed over a large distance range (125° - 165°) and correlated to overlying mantle structure. Long-period (LP) World Wide Seismographic Station Network data are utilized due to the presently unsurpassed ≈20 year time span of operation for global station coverage. This data set is augmented by available broadband data. In regions where lower mantle heterogeneity is predicted small, SmKS observations are well predicted by the PREM reference model, with the addition of a slight reduction in Vp in the top 50 km of the core (1.5%). Such a reduction implies chemical stratification in this 50 km zone, though this model feature is not uniquely resolved. Data having wave paths through areas of known D" heterogeneity (± 2%) exhibit systematic anomalies in SmKS differential times. 2-D wave propagation experiments using a modified WKBJ method demonstrate how large scale lower mantle velocity perturbations can explain long wavelength behavior of such anomalous SmKS times, though heterogeneity on smaller scales may be responsible for the observed scatter about these trends. A 2-D model having anomalously slow lower mantle velocities beneath the Indonesia region produces SmKS differential time residuals that agree with observations of Fiji-Tonga events recorded in Eurasia and Africa. In general, information from previously published 3-D maps of mantle heterogeneity can be used to construct starting models of 2-D cross sections appropriate for source-receiver geometries of interest.

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