Using SmKS Traveltimes to Investigate the Velocity Structure near the Top of the Earth’s Outer Core
Date Issued
2014
Date
2014
Author(s)
Tang, Vivian
Abstract
The solid inner core of the Earth consists of heavy minerals Fe and Ni with a fraction of light elements such as O, S and Si. These lighter elements are expelled from the inner core during its formation, rise up through the outer core as the result of buoyancy, and are trapped below the core-mantle boundary (CMB). Seismological evidence has been presented both for and against the existence of light materials at the top of the outer core. In this study, we use waveforms of recorded and modeled SmKS waves to investigate the effect of velocity perturbation under the CMB on the differential traveltimes between SKKS and S3KS waves. Due to the long propagation distance and interference with neighboring phases, the arrival times of SKKS and S3KS waves are difficult to define accurately in the records. Therefore, in our analysis we measure both the observed and model-predicted traveltimes by cross-correlating the waveform of the Hilbert-transformed S3KS with that of SKKS. We obtained 606 high-quality S3KS-SKKS differential traveltimes from 78 deep earthquakes (depth >= 400 km). We use synthetic seismograms calculated by the direct-solution method (DSM) in a suite of one-dimensional models with different structural profiles under the CMB to examine the existence of a zone of lowered velocity at the top of the outer core. Then we conduct a Bayesian inversion of the observed differential traveltimes for the velocity structure at the top of the outer core. The Metropolis-Hastings Monte Carlo algorithm is adopted for an efficient sampling of the model space. Inversion result indicates that the seismic velocity in the 550-km layer under the CMB is on average 0.11% lower than that in PREM. The clear depth-dependent velocity profile strongly favors the existence of light elements and chemical stratification at the top of the Earth''s outer core.
Subjects
外核
Direct-Solution Method (DSM)
SmKS波
Bayesian逆推
Type
thesis
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