Theses and Dissertations

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Open Access
Ambient and teleseismic elastic wavefields of the Ross Ice Shelf, Antarctica, and their application to crustal scale seismic imaging
(Colorado State University. Libraries, 2020) Baker, Michael G., author; Aster, Richard C., advisor; Schutt, Derek L., committee member; Krueger, David A., committee member; Dueker, Ken G., committee member
Laterally extensive floating tabular ice, such as the Ross Ice Shelf in Antarctica, is mechanically equivalent to a floating elastic plate and thus supports a variety of vibrational modes, including short-period (< 20 s) elastic waves, intermediate-to-long period (20–100 s) buoyancy-coupled elastic plate waves, and ultra long period (>100 s) gravity waves. Vibrational energy may be excited by near-field sources such as intra-shelf crevassing or the impingement of ocean gravity waves at the shelf ice front, and also by far-field sources such as teleseismic earthquake waves incident at the sub-shelf seafloor and the shelf grounded margins. Broadband seismometers deployed on an ice shelf readily observe these signals and facilitate large scale studies of ice shelf properties (via, e.g., travel-time tomography or velocity dispersion analysis) and near-field environment processes (via remote signal detection and analysis). Using two years of continuous data from a 34-station broadband seismic array deployed to the Ross Ice Shelf, Antarctica, I analyzed spatial and temporal variations in the short-to-intermediate period (0.4–25 s) ambient and teleseismic vibrational wavefields. I show that the ambient, ocean-wave-coupled wavefields are strongly modulated by sea ice concentrations in the adjacent Ross Sea, and identify three separate source processes operating in distinct period bands. Next, I show that body wave and surface wave arrivals from teleseismic earthquakes (>3000 km distant) are observed on the vertical components of ice shelf-sited seismometers with signal-to-noise ratios generally sufficient for crustal and mantle scale tomographic studies. I also show that teleseismic S-waves incident at the grounded margins routinely generate symmetric mode Lamb waves which propagate a minimum of 250 km into the ice shelf interior; this phenomenon occurs throughout the year, with broad azimuthal distribution, and may be exploited for travel-time tomography of the ice shelf. Finally, I present an algorithm for processing vertical channel autocorrelations of teleseismic P-wave coda, as recorded by floating-ice-sited seismometers, to illuminate crustal-scale features such as the Mohorovičić discontinuity. I present Markov Chain Monte Carlo inversions of these autocorrelations which yield crustal thickness estimates that are compatible with previous studies of crustal thicknesses for the Ross Sea Embayment and West Antarctica.
Open Access
Automated event detectors utilized for continental intraplate earthquakes: applications to tectonic, induced, and magmatic sequences
(Colorado State University. Libraries, 2018) McMahon, Nicole D., author; Aster, Richard C., advisor; Schutt, Derek L., committee member; Cheney, Margaret, committee member; Benz, Harley M., committee member
Event detection is a crucial part of the data-driven science of seismology. With decades of continuous seismic data recorded across thousands of networks and tens of thousands of stations, and an ever-accelerating rate of data acquisition, automated methods of event detection, as opposed to manual/visual inspection, allow scientists to rapidly sift through enormous data sets extracting event information from background noise for further analysis. Automation naturally increases the numbers of detected events and lowers the minimum magnitude of detectable events. Increasing numbers and decreasing magnitudes of detected events, particularly with respect to earthquakes, enables the construction of more complete event catalogs and more detailed analysis of spatiotemporal trends in earthquake sequences. These more complete catalogs allow for enhanced knowledge of Earth structure, earthquake processes, and have potential for informing hazard mitigation. This study utilizes automated event detection techniques, namely matched filter and subspace detection, and applies them to three different types of continental intraplate earthquake sequences: a tectonic aftershock sequences in Montana, an induced aftershock sequence in Oklahoma, and a magmatic swarm sequence in Antarctica. In Montana, the combination of matched filtering and multiple-event relocation techniques provided a more complete picture of the spatiotemporal evolution of the aftershock sequence of the large intraplate earthquake that occurred near Lincoln, Montana in 2017. The study reveals movement along an unmapped fault that is antithetical to the main fault system trend in the region and demonstrates the hazards associated with a highly faulted and seismically active region encompassing complex and hidden structures. In Oklahoma, subspace detection methodology is used in combination with multiple-event relocation techniques to reveal movement along three different faults associated with the 2011 Prague, Oklahoma induced earthquake sequence. The study identifies earthquakes located in both the sedimentary zone of wastewater injection as well as the underlying crystalline basement indicating that faults traverse the unconformity. Injecting fluid into the overlying sediment can easily penetrate to the basement where larger earthquakes nucleate. In Antarctica, subspace detection is again used in a very remote intraplate region with sparse station coverage to detail the sustained and ongoing magmatic deep, long-period earthquake swarm occurring beneath the West Antarctic Ice Sheet and Executive Committee Range in Marie Byrd Land, Antarctica. These earthquakes indicate the present-day location of magmatic activity, which appears appear to have increased in intensity over the last few years. This dissertation contributes to the growing bodies of literature around three distinctly interesting types of seismicity that are not associated to the first order with plate tectonic boundaries. Large tectonic intraplate earthquakes are relatively uncommon. Induced seismicity has only drastically increased in the central US during the last decade and created new insights into this process. Deep, long-period, magmatic earthquakes are still a poorly understood type of seismicity in volcanic settings.
Open Access
Event detection and analysis of a dense three-component near-summit seismic array deployed at Erebus volcano
(Colorado State University. Libraries, 2022) Jaski, Erika, author; Aster, Richard C., advisor; Schutt, Derek, committee member; Cheney, Margaret, committee member
Erebus volcano on Ross Island, Antarctica has maintained an erupting phonolitic lava lake for at least five decades. During active periods, the lava lake hosts large (up to ~10-m diameter) gas slugs rising through the conduit that create impulsive Strombolian eruptions and produce very long period (VLP) signals on broadband seismograms. We combine near-summit broadband observations and reanalyze data from a 100-station three-component short-period (4.5 Hz geophones) network deployed in an approximately 3 by 3 km region around the Main Crater during December 2008. Lava lake eruption template events are identified on broadband seismograms from their characteristic and repeating VLP spectral signature of nonharmonic modes between 0.033 and 0.2 Hz. Multi-channel and multi-station waveform matched filter correlations are performed across the short-period network using template events and correlation values that are three or more standard deviations are extracted into a working Inner Crater event catalogue, yielding 819 event detections over 19 days. While 94% of the signals in this catalogue are unique, 17 "families" of repeating lava lake events can also be identified through similar waveforms determined by Ward clustering on 5 stations, which are further interpreted for trends in location, size, and occurrence. We observe time-varying quasi-Poissonian interevent times and an approximately power-law size-frequency distribution with an excess of small events. Investigating the various event families that transpire in the Inner Crater region contributes to improved characterization and understanding of the seismogenic behavior of the lava lake degassing system and assists in the creation of a workflow that can be applied in volcanic and other circumstances that generate prolific low-level impulsive seismicity.
Open Access
Tidally induced seismicity at the grounded margins of the Ross Ice Shelf, Antarctica
(Colorado State University. Libraries, 2020) Cole, Hank M., author; Aster, Richard C., advisor; McGrath, Daniel, committee member; Cheney, Margaret, committee member; Benz, Harley, committee member
Repeating swarms of local icequakes were recorded by broadband seismograpghs deployed near the grounding line of the Ross Ice Shelf, Antarctica from late 2014 to early 2017. Swarms commonly persist for over six hours and contain thousands of events. Most swarms are induced or enhanced by tidal forcing. The number of events and event amplitudes in a swarm is most correlated with the modeled tide range. Some swarms only occur during cold periods of the austral winter. Icequakes are cataloged using a cross-correlation detector after building a template library from clustered STA/LTA picks and epicenters are estimated for high quality events. Events can be classified into four broad categories. The first event type is the most common (>95% of events) and occurs in diurnal swarms at all times of year. This type of event is interpreted to be sourced by propagation of near surface crevasses due to enhanced tensile stress from downward flexure of the ice shelf during falling tide. The second type of event has similar waveforms but occurs at the crest of large spring tides and appears to have an englacial or basal source. The third type of event is likely sourced from within the firn, possibly related to densification. It is also observed at stations in the ice shelf interior, but appears enhanced by tides at stations near the grounding line. The fourth type of event is only observed at a station on the Steershead Ice Rise. These are sweeping harmonic tremors lasting up to 8 s that start at low frequency and then tail upwards into an impulse like signal. This work characterizes these icequake types and their correlation to tidal and environmental forcing. It also details a single station event location scheme that is to used to further interpret events by finding their back azimuth with a polarization analysis and estimate their source-receiver distance with two methods. These observations provide insight into the deformation and brittle fracture at the grounded margins of the Ross Ice Shelf.

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