Date of Award

Spring 5-15-2016

Author's School

Graduate School of Arts and Sciences

Author's Department

Earth & Planetary Sciences

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

The scope of this dissertation is broad, involving seismic array studies from Antarctica and Madagascar, and includes aspects of glaciology and oceanography as well as solid Earth geophysics. Chapter 2 focuses on the study of stickslip motion of the Whillans Ice Stream, West Antarctica. It includes methods combining seismic array and GPS time series, from ice stream based-sensors, to determine source dynamics in the framework of an earthquake source. The source characteristics are then analyzed to explain far-field seismic observations of ice stream- sourced surface waves detected throughout West Antarctica. Locations of asperities, or sticky- spots, that cause the Whillans Ice Stream to accelerate and generate seismic energy are found. Some of these asperities are in close proximity to the grounding line, where properties of the bed are altered through tidal flexure of the ice shelf and the influx of water into the subglacial till.

Chapter 3 explores ocean generated microseismic noise that is also detected on these ice stream seismometers, with the geometry of the array providing excellent azimuthal resolution. Stacked cross-correlations of seismograms enhance microseismic energy generated by the Southern Ocean in the form of both surface and body waves. The frequency spectra of these waves is analyzed as well as applying seismic array techniques, such as beamforming. Each frequency band provides different information on the source regions of that particular microseism suggesting multiple source mechanisms. Microseisms are modeled using ocean state hindcasts to compare with observations and identify microseism source regions and improve understanding of the effect of sea ice. It is shown that single-frequency microseisms are heavily damped by the presence of sea ice over the continental shelf. Long-period double frequency microseisms are observed and modeled to be sourced in the deep ocean. Short-period double frequency microseisms are also influenced by sea ice seasonality; however, this chapter provides evidence that shows that a component of this band may be sourced in the deep ocean.

The focus of Chapter 4 moves away from Antarctica, to Madagascar and the analysis of the first island-wide deployment of broadband seismometers. The priorities of this project are to better understand the crustal and upper mantle structure of Madagascar, and to assess the intraplate volcanism on the island from a seismological point of view for the first time. This chapter presents a surface wave tomography study producing the first shear velocity model of the crust and upper mantle of the island. A range of commonly employed surface wave methods is used to calculate phase velocities across the island. These are then amalgamated and inverted for shear velocity in the crust and the upper mantle. Low velocity regions are shown to extend to upper mantle depths beneath the center and north of the island above which lie intraplate volcanic provinces. This suggests that the mantle lithosphere has been significantly thinned, explaining the relatively high topography observed for a fragment of continental crust.

Language

English (en)

Chair and Committee

Douglas Michael A. Wiens Wysession

Committee Members

Slava Solomatov, Philip Skemer, Linda Warren,

Comments

Permanent URL: https://doi.org/10.7936/K7HM56QW

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