Date of Award

2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

Meng (Matt) Wei

Abstract

Geodetic techniques make it possible to observe crustal deformation associated with many subsurface processes, both natural and anthropogenic. When a perturbation occurs beneath the ground surface, such as magma injection into a magma chamber, detonation of a clandestine nuclear weapon, slip on a tectonic fault, or withdrawal of groundwater from an aquifer, the surrounding crust undergoes a deformation response that can be measured on the surface. Those measurements, combined with our understanding of the Earth's expected response, can be inverted through deformation modeling to reveal details about subsurface events that we often cannot directly observe. Insights gleaned from geodetic measurements and models are valuable for understanding geologic hazards and improving our ability to mitigate their impacts. This dissertation explores three geodetic applications: an underground nuclear test conducted by the Democratic People's Republic of Korea (DPRK, formerly North Korea), the inflation of the submarine volcano Axial Seamount, and the removal of noise from seafloor geodetic data to improve measurements at seafloor volcanoes.

Satellite-based geodetic techniques such as Interferometric Synthetic Aperture Radar (InSAR) have revolutionized the field of geophysics by allowing for measurements to be made without needing to visit the location of interest. This is especially useful for monitoring clandestine nuclear testing, which is often conducted underground to avoid traditional detection methods. The first chapter of this dissertation utilizes InSAR data covering an underground DPRK nuclear test to develop a numerical model of cavity expansion, which was then used to estimate the nuclear yield of the explosion. The DPRK test site is within a stratified mountain and we are the first to investigate the mechanical effect of geological layering on yield estimate at this location.

While InSAR is a powerful tool for terrestrial applications, its electromagnetic energy cannot penetrate the oceans. Seafloor geodesy must rely on different techniques for measuring deformation of the oceanic crust. The second chapter uses data from seafloor pressure sensors and repeat bathymetric surveys to examine an inter-eruptive inflation period of Axial Seamount's long term geodetic time series. This data was jointly inverted using several analytical and numerical deformation models, constrained by the 3D volume of the magma reservoir from previous multichannel seismic studies. The results support the hypothesis that Axial's magma chamber is compartmentalized, with melt accumulating during inflation periods in isolated sills instead of uniform pressurization of the entire seismically-imaged body.

Axial Seamount is one of few geodetically monitored submarine volcanoes, and it is expected that more will be instrumented in the near future due to technological advancements and increased availability of sensors. A major challenge in using seafloor pressure to study crustal deformation is noise from the water column. The third chapter examines three methods to remove oceanic noise from seafloor pressure datasets at Axial Seamount and the Endeavour Segment on the Juan de Fuca Ridge (NE Pacific Ocean). The methods tested include 1) predicting seafloor pressure from the combination of conductivity-temperature-depth (CTD) data and sea surface height (SSH) anomalies, 2) predicting seafloor pressure from the Hybrid Coordinate Ocean Model (HYCOM) output, and 3) station differencing. Station differencing was most effective in isolating volcanic deformation at both sites. Notably, a likely deformation event of 2.5 cm was observed at the Endeavour segment between December 2022 and July 2023, marking the first documented deformation at this location to the best of our knowledge.

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