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Browsing by Subject "Seismology"

Now showing 1 - 5 of 5
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    Apparent Attenuation Beneath the United States and its Correlation to Lithospheric Provinces
    (2016-09) Kowalke, Sara
    We map apparent attenuation beneath the continental United States through time-domain waveform analysis of 19 deep-focus teleseismic events recorded by the USArray Transportable Array. Results show good correlation with lithospheric boundaries. Low t* is common across the cratonic continent and high t* regions dominate in the western U.S. and east of the Appalachian front. Some geographic variations are not consistent with expectations, such as relatively low t* in the North Basin and Range. Comparisons with additional techniques, including heat flow, tomography, and seismicity, indicate regional influence of non-thermal attenuation factors. Different lithospheric provinces have distinct attenuation signatures that assist in understanding the behavior of the lithospheric continent.
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    Cross-correlation searches for persistent gravitational waves with Advanced LIGO and noise studies for current and future ground-based gravitational-wave detectors.
    (2018-05) Meyers, Patrick
    Over the last three years, the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) has detected signals from colliding black holes and a signal from colliding neutron stars. These detections ushered in a new era of gravitational-wave (GW) astrophysics and multimessenger astronomy that allows us to probe new regions of the universe. One of the next frontiers for gravitational-wave astronomy is the detection and characterization of the stochastic GW background (SGWB). A measurement of the SGWB from unresolved compact binary systems could come as Advanced LIGO reaches design sensitivity, and future detectors will be important for digging beyond that astrophysical background towards trying to measure signals from relic gravitational waves produced in the early universe. In this dissertation, I present cross-correlation-based searches for a SGWB and other persistent sources of GWs. I introduce and use a new method for setting limits on the strain amplitude of a potential source of GWs in the directions of Scorpius X-1, the galactic center, and Supernova 1987a in the frequency band from 20-1726 Hz. I also set limits on persistent, broadband point sources of GWs across the whole sky. Finally, I show how we can implement data analysis techniques to improve the Advanced LIGO detector sensitivity to persistent sources of GWs. Improving sensitivity of current detectors and planning for future detectors is vital to the effort to measure and understand the SGWB. This will requires a better understanding of the noise sources that limit sensitivity, especially at lower frequencies. To this end, I outline a method for estimating and modeling correlated magnetic noise between spatially separated GW detectors. I also present results from a 3D seismometer array deployed at the Homestake Mine, aimed at characterizing seismic and Newtonian noise for future GW detectors. I estimate the fundamental Rayleigh-wave eigenfunction, and then use it in a seismic radiometer algorithm to separate different components of the seismic field that contribute differently to the Newtonian noise. Finally, I present estimates of the Newtonian noise as a function of depth in the frequency band from 0.5-5 Hz based on results from the seismic radiometer.
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    Data for Lateral Variations in Teleseismic Attenuation of the Conterminous U.S. and New Insights Derived from its Relationship to Mantle Seismic Velocity
    (2023-10-26) Bezada, Maximiliano J; Byrnes, Joseph; Zhu, Zhao; Lee, Hwaju; mbezada@umn.edu; Bezada, Maximiliano J; University of Minnesota Seismology Lab
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    Knock Knock, Who's There?: Identifying the source of signals in the first DAS deployment in a fluvial setting
    (2023-05) Tate, Bill
    Over the past several decades, many studies have developed traditional seismic methods to monitor pressure fluctuations produced by turbulence and bedload sediment that impact riverbeds. A critical challenge in analyzing and interpreting fluvial processes with seismic signals is the overlap of numerous sources and processes. These signals are recorded by individual seismometers in heterogeneous environments adding further complexity. A new and exciting geoscience method called distributed acoustic sensing (DAS) has been developed that utilizes fiber optic cable as a pressure sensing device to record continuous ground motion, comparable to a large seismic array of single-channel geophones or accelerometers. This study aims to record and study the signal of processes occurring in the creek, but since the cable is not fixed, it is not a passive recorder. Changes in strain along the cable caused by interaction between the cable and the creek are also recorded. In this study, we examine the dataset of the first deployment of DAS in a fluvial setting. The focus is on spatial variations in the signal recorded by a cable submerged along the thalweg of Clear Creek in Golden, CO. Data analysis suggests that there are many different types of signals in the data, with various apparent velocities and frequency content. One such signal reminiscent of repeating “knock” sounds appears to propagate within the cable itself because the depth of the signal is near zero, and the propagation velocity is inconsistent with the propagation velocity of sound in water. We believe a point along the cable is hitting the bottom of the creek, creating these “knocks.” At the same time, we observe spectral gliding in a portion of the data we believe is associated with wave reflection, refraction, and interference phenomena. Another signal is investigated that is reminiscent of bubbles. The signal is localized near two waterfalls and may be present at other waterfalls in the creek but cannot be determined due to substantial amounts of noise at those points. A cross-correlation of each channel with every other channel reveals strong correlation between nearby channels and six regions of interest, associated with similar ambient sound. The cross-correlation matrix demonstrates one method of using DAS data to describe a general layout and other physical properties associated with a fluvial system. The methodology requires refinement to limit unknown variables and improve interpretation. We conclude that a more controlled study is required to utilize and understand DAS fully.
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    Mantle dynamics, composition, and state in regions associated with active and ancient subduction.
    (2008-05) Courtier, Anna Mahr
    Abstract not available.