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Theses and Dissertations

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

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    Detection and relocation of earthquakes in the sparsely instrumented Mackenzie Mountains region, Yukon and Northwest Territories, Canada
    (Colorado State University. Libraries, 2020) Heath, David C., author; Schutt, Derek L., advisor; Aster, Richard C., committee member; Wald, David J., committee member; Cheney, Margaret, committee member
    The Mackenzie Mountains are an actively uplifting and seismogenic arcuate thrust belt lying within the Northwest Territories and Yukon, Canada. Seismic activity in the region is poorly constrained due to a historically sparse seismograph distribution. In this study, new data are analyzed from the 40-station, ~875 km-long Mackenzie Mountains temporary network (Baker et al., 2020) crossing the Cordillera-Craton region adjacent to and within the Mackenzie Mountains, in conjunction with Transportable Array and other sparsely distributed arrays in the region. Data from approximately August 2016 – August 2018 are processed and compared to the sparse-network earthquake catalog records maintained by the USGS and Natural Resources Canada. Using algorithms developed by Kushnir et al. (1990), Rawles and Thurber (2015), and Roecker et al. (2006), signals are identified and subsequently associated across the network to note potential events, estimate phase onsets, and resolve hypocenter locations. This study improves the regional earthquake catalog by detecting smaller-magnitude earthquakes and lowering the regional magnitude of completeness from Mc = 2.5 to 1.9. Within the Mackenzie Mountains and immediately surrounding areas we find 524 new events and additionally recommend an updated location for 185 previously cataloged events. Our b-value computation for the updated catalog (0.916 ± 0.08) likely indicates a relatively high level of regional differential stress. We identify the spatial distribution of earthquakes in the Mackenzie Mountains as diffuse, and offer far-field stress transfer as a mechanism for producing widespread reverse faulting observed in the region. Further, we associate regional seismicity with tectonic activity in the context of known faults and orogenic provinces such as the Richardson Mountains.
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    Distributed seasonal and annual mass balance measurements of Wolverine Glacier, Alaska, using geodetic surveys and emergence velocities
    (Colorado State University. Libraries, 2021) Zeller, Lucas R., author; McGrath, Daniel, advisor; Aster, Richard C., committee member; Leisz, Stephen J., committee member
    Glaciers are key components of human-environmental systems worldwide. They are a source of fresh water for human consumption, crop irrigation, and hydroelectric power even during times of drought. Glaciers promote environmental and ecological heterogeneity by modulating stream temperatures and providing key nutrient, geochemical, and sediment fluxes, are popular tourism destinations, and introduce risks from natural hazards such as glacier-lake outburst floods. Glaciers have undergone dramatic retreat and thinning over the past 50 years, and these trends are predicted to accelerate through the 21st century. Short term (seasonal to annual) measurements of glacier mass balance provide valuable insight on how glaciers respond to climatological forcings and the processes that drive those changes. However, in-situ measurements are prohibitively time consuming, logistically difficult, and prone to uncertainty, rendering them insufficient for global-scale analyses. The increasing availability of high-resolution geodetic products offers promising opportunities for measuring mass balance from a remote platform if the confounding effects of ice emergence velocities and firn compaction on surface elevation can be correctly constrained. In this study, I present spatially and temporally distributed measurements of emergence velocities on Wolverine Glacier, Alaska, derived from three methods: 1) repeat Global Navigation Satellite System (GNSS) measurements of mass balance stakes, 2) modelled from annual mass balance measurements and glacier thinning rates, and 3) a novel approach of differencing geodetic surveys and snow depths derived from ground penetrating radar surveys. These emergence velocities, in conjunction with estimates of firn compaction, were used to measure distributed mass balances of Wolverine Glacier over three winter seasons, one summer season, and two annual time periods via geodetic surveys. The three approaches to measuring emergence velocity showed overall agreement but had important spatiotemporal differences. Comparison of geodetic mass balances with in-situ point and glacier-wide average mass balances had root mean square errors of 0.42 and 0.46 meters water equivalent. These results indicate that if emergence velocities and firn compaction are carefully considered, geodetic methods can provide accurate measurements of distributed mass balances over seasonal and annual time frames, yielding an improved understanding of glacier response and trend over these time scales. Such an understanding will facilitate improvements in model physics and parameterizations, thus improving projections for the magnitude and timing of future glacier losses and their effects on downstream communities and ecosystems.
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    Evidence for a rotation in asthenospheric flow in northwest Canada: insights from shear wave splitting
    (Colorado State University. Libraries, 2021) Bolton, Andrew R., author; Schutt, Derek L., advisor; Aster, Richard C., committee member; Breidt, F. Jay, committee member
    The Mackenzie Mountains (MM) of northwest Canada are an actively uplifting, seismogenic salient of the northern Canadian Cordillera that lie 750 km NE of the nearest plate boundary. We present new shear wave splitting measurements for the region from a linear array which transects the region to characterize upper mantle anisotropy. A gradual rotation in anisotropy occurs across the Canadian Cordillera, with stations nearest to the craton yielding fast axis orientations that are subparallel to North America absolute plate motion (~230°). Moving SW from the craton, across the MM and towards the plate boundary, fast-axis orientations rotate to become aligned with major lithospheric fabrics (NW-SE). Previous work has shown that the Cordilleran lithosphere is thin (~50 km) in this region. We therefore interpret these results to primarily reflect sublithospheric flow. Three subduction-transpressional related hypotheses for flow are presented, where our preferred hypotheses invokes depth-dependent, subduction-induced flow.