University of Connecticut

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PhD Dissertation Defense

Monday, July 24, 2017
10:00am – 12:00pm

Storrs Campus
Physics Building, P121

Susini de Silva, Department of Physics, University of Connecticut

Exploring Heterogeneities near Earth's Inner Core Boundary using Seismic Body Waves

The mechanism of inner core solidification drives the compositional and thermal convection in fluid outer core, sustaining the geodynamo that gives rise to Earth’s magnetic field. Hence any differences in the manner of crystal growth and heat flux at the continuously growing inner core boundary (ICB) is essential in understanding secular variations of geomagnetic field around the Earth. Many previous geodynamic modeling and seismic investigations provide evidence of such differences in solidification rates. Properties of inner core needs to be well constrained in order to understand the growth and deformation mechanisms. We thoroughly investigate the possibility and dimensions of topography on ICB as well as use a cross correlation method to invert for the best fitting attenuation structure above and below this growing discontinuity.

Applying boundary element methods (BEM) to synthesize compressional waves interacting with the ICB, effects of 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 2 km at 1 to 20 km 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 5 km 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 100 km of the outer core.

A search for best fitting attenuation structure for the uppermost IC, shows that eastern hemisphere and the region under Pacific is more attenuating than the rest of the western hemisphere. The BEM study implies that this structure may solely be resulting from intrinsic attenuation and volumetric scattering. In conclusion, we discuss avenues of extending our results/methods in refining the story of inner core evolution.


Prof. V. Cormier

Physics Department (primary), UConn Master Calendar

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