Date of Completion

7-28-2017

Embargo Period

7-28-2017

Keywords

Inner Core, Synthetic Seismograms, PKiKP

Major Advisor

Vernon Cormier

Associate Advisor

Michael Bergman

Associate Advisor

Winthrop Smith

Field of Study

Physics

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

The mechanism of inner core solidification drives the compositional and thermal convection in outer core, sustaining the Earth’s geodynamo. Hence any differences in the manner of crystal growth at the continuously growing inner core boundary(ICB) is essential in understanding secular variations of geomagnetic field.

By applying boundary element methods(BEM) to synthesize compressional waves, effects of ICB topography are predicted and compared with waveform observations in pre-critical, critical, post-critical, and diffraction ranges of the wave reflected from the ICB(PKiKP). In the pre-critical range, data require an upper bound of 2km at 1-20km wavelength for any ICB topography. Higher topography sharply reduces PKiKP amplitude and produces time-extended coda not observed in PKiKP waveforms. Topography of this scale smooths over minima and zeros in the pre-critical ICB reflection coefficient predicted from standard Earth models. In the diffracted range(>152°), topography as high as 5km leaves the PKPCdiff/PKIKP amplitude ratio unchanged from that predicted by a smooth ICB. The observed decay of PKPCdiff into the inner core shadow and the PKIKP-PKPCdiff differential travel time are consistent with a flattening of the outer core P-velocity gradient near the ICB and iron enrichment in the lowermost outer core.

A search for best fitting attenuation structure for the uppermost IC, shows that eastern hemisphere and the region under Pacific are more attenuating than the rest of the western hemisphere. The BEM study implies that this structure may be resulting from intrinsic attenuation and volumetric scattering. In conclusion, we discuss ways forward in refining the story of IC evolution.

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