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Theses and Dissertations

Permanent URI for this collectionhttps://hdl.handle.net/10217/100446

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Browsing Theses and Dissertations by Author "Aster, Richard, committee member"

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    A new method to test shear wave splitting: improving statistical assessment of splitting parameters
    (Colorado State University. Libraries, 2016) Corbalán Castejón, Ana, author; Schutt, Derek, advisor; Breidt, Jay, committee member; Aster, Richard, committee member; Egenhoff, Sven, committee member
    Shear wave splitting has proved to be a very useful technique to probe for seismic anisotropy in the earth’s interior, and measurements of seismic anisotropy are perhaps the best way to constrain the strain history of the lithosphere and asthenosphere. However, existent methods of shear wave splitting analysis do not estimate uncertainty correctly, and do not allow for careful statistical modeling of anisotropy and uncertainty in complex scenarios. Consequently, the interpretation of shear wave splitting measurements has an undesirable subjective component. This study illustrates a new method to characterize shear wave splitting and the associated uncertainty based on the cross-convolution method [Menke and Levin, 2003]. This new method has been tested on synthetic data and benchmarked with data from the Pasadena, California seismic station (PAS). Synthetic tests show that the method can successfully obtain the splitting parameters from observed split shear waves. PAS results are very reasonable and consistent with previous studies [Liu et al., 1995; Özalaybey and Savage, 1995; Polet and Kanamori, 2002]. As presented, the Menke and Levin [2003] method does not explicitly model the errors. Our method works on noisy data without any particular need for processing, it fully accounts for correlation structures on the noise, and it models the errors with a proper bootstrapping approach. Hence, the method presented here casts the analysis of shear wave splitting into a more formal statistical context, allowing for formal hypothesis testing and more nuanced interpretation of seismic anisotropy results.
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    Seismic anisotropy in northwestern Canada and eastern Alaska from shear wave splitting measurements
    (Colorado State University. Libraries, 2017) Witt, Derek Richard, author; Schutt, Derek, advisor; Aster, Richard, committee member; Breidt, Jay, committee member
    The Mackenzie Mountains are an actively uplifting and seismogenic mountain range that lies within the Yukon and Northwest Territories, Canada. The range is an eastward salient of the complexly deformed northern Canadian Cordillera, and lies ~500 kilometers away from and significantly off axis of the convergence direction of the Yakutat Indentor, a small oceanic-continental terrane that is subducting northward under North America in the Gulf of Alaska. To better assess the causes of the Mackenzie Mountains uplift and its broader relationship to deformation within the Northern Canadian Cordillera, shear wave splitting measurements have been performed on seismometers at over 150 locations within this region. Many of the measurements come from the Mackenzie Mountains Earthscope Project, a ~900 km NE-directed transect that spans from the complexly deformed coastal ranges near Skagway, Alaska, across the shortening axis of the Mackenzie Mountains, to the cratonic lithosphere at Great Bear Lake. This array is the first deployment of broadband seismometers within the Mackenzie Mountains and the current study is the first report from that array. Shear wave splitting provides a means to probe the seismic velocity anisotropy, and therefore the strain history, of the lithosphere and asthenosphere. Results indicate five distinct subregions of splitting behavior in our results: 1) chaotic, non-uniform splitting in the subduction zone complex; 2) fault-parallel fast axes along and between the Denali and Tintina dextral fault systems; 3) a short section of east-west fast axes near the British Columbia-Yukon border; 4) consistent fast axes aligned with North America absolute plate motion within the Canadian shield; and 5) the transitional inboard region between the Tintina fault and the Canadian shield, which includes the Mackenzie Mountains. Our findings support the hypothesis that shear from the Tintina and Denali faults penetrates the lithospheric mantle and has produced significant lithospheric anisotropy. The location of the strained mantle causing the observed anisotropy transitions from the lithosphere near the subduction zone and transpressional fault systems to the asthenosphere in the Canadian shield, where observations of asthenospheric flow are consistent with North America absolute plate motion.