Earth and Planetary Sciences ETDs

Publication Date

Summer 8-15-2023

Abstract

The field of observational seismology has made tremendous progress in the past two decades. This progress has been multi-faceted in form, but significant contributions emanated from 1) increases in both the quality and the quantity of seismic data 2) advances in computational power 3) advances in algorithmic capability, including machine learning. In this dissertation I report on three distinctly different seismic applications made possible by the aforementioned progress and discuss the insights these applications have provided in understanding volcanic and near surface processes of the Earth.

In the first chapter titled, “Shear Velocity Evidence of Upper Crustal Magma Storage Beneath Valles Caldera” I present the first local Vs tomographic images of the Valles Caldera magmatic system from ambient noise Rayleigh dispersion using a dense (∼750 m mean spacing) transect (∼71 km length) of nodal seismographs. An ∼6 km wide low-Vs anomaly (Vs < 2.1 km/s) is located at ∼3–10 km depth. Assuming magma in textural equilibrium, the new tomography suggests that melt fractions up to ∼17%–22% may be present within the upper crustal depth range where previously erupted rhyolites were stored. This work was peer reviewed and published in Geophysical Research Letters and is available for open access digital download here.

In the second chapter titled, “Seasonality and spatial variability in shallow velocity structure (dv/v) at the Albuquerque Seismic Laboratory using a Repeating Impulsive Source” I make use of a novel impulsive repeating source to measure shallow velocity structure (dv/v) over multiple years and investigate small-scale spatial variability of dv/v across a small-aperture (∼500 m) array of high quality posthole seismometers and two deep (90 and 188 m) borehole seismometers at the Albuquerque Seismic Laboratory (ASL). A consistent and clearly resolvable seasonal dv/v oscillation with an amplitude of ∼0.2% is broadly present on all ASL borehole and array stations. Through some simple logical deductions, a hydrological source mechanism is determined to be a likely candidate. However, the presence of small-scale spatial variability in dv/v indicates measurements are sensitive to structure on the order of 100’s of meters. I further demonstrate such sensitivity has the potential to obscure smaller amplitude shorter time scale signals, and highlight the importance of spatiotemporal analysis of dv/v. Work from this chapter has been submitted for peer review.

In the third and final chapter titled, “Background Seismic Noise Levels Among the Caribbean Network and the Role of Station Proximity to Coastline” I calculate power spectral density over > 15 years to examine seismic background noise within the 0.05-300 s period range from the nine-station Caribbean seismic network (CU) distributed throughout the Caribbean region. I document wide variability in noise levels among the stations and describe the most discernable first order signals observed. One of the most prominent signals occurs in the 0.75-3 s band where power levels are systematically elevated and decay as a function of proximity to coastline. A simple surface wave amplitude decay model fits the observed decay well with geometric spreading being the most important factor for stations near the coast (< ∼ 50 km). Furthermore, this relationship is absent in the 4-8 s range more typical of globally observed secondary microseism. This dichotomy suggests that 0.75–3 s power arises from nearshore wave action and locally overwhelms more distant and spatially distributed secondary microseism generation. Regardless of source, application of this basic model indicates that a power reduction of ∼25 dB can be achieved by simply installing the seismometer 25 km away from the coastline. Work from this chapter has also been submitted for peer review.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Brandon Schmandt

Second Committee Member

Adam Ringler

Third Committee Member

Karl Karlstrom

Fourth Committee Member

Lindsay Lowe Worthington

Fifth Committee Member

Julien Chaput

Keywords

Seismology, Seismic Tomography, Seismic ambient noise, Volcanology, Hydrology, Seismic monitoring, geology, Natural hazards

Document Type

Dissertation

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