Isolation and Quantification of a Three-Component GPS-Signal Anomaly in the Western United States Occurring Between 2001 & 2005: A Geodetic Perspective
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Authors
Fuller, Robert Daniel
Issue Date
2025
Type
Thesis
Language
en_US
Keywords
Anomaly , Geodesy , GPS
Alternative Title
Abstract
A subtle, yet significant, anomalous signal is observed in the three components (east, north, up) of residual GPS station time-series across the western United States between the years 2001 and 2005. This anomaly is most prominent in the vertical and east components, with maximum amplitude occurring around mid-2003 roughly centered around Utah where a peak amplitude of 5-7 mm in the vertical component is measured. Localized variations are also observed in the Pacific Northwest. Here, we aim to isolate, quantify, and confirm the existence of this anomalous signal in the United States. We analyze a 10-year dataset (2000-2010) of daily solutions from 358 unique GPS stations across the United States and Canada. The signal persists after a robust time-series analysis is performed. The analysis removes long-term velocity trends, annual and semi-annual signals, and non-periodic surface loads of non-tidal atmospheric and oceanic loading (NTAOL). This time span is chosen to encapsulate and highlight the proposed anomalous period observed between 2002 and 2005. Data from before 2001 and after 2005 serve as a baseline for statistical comparison. A 365-day windowed average and median for each GPS station are computed and binned into various spatial extents, which are then plotted and mapped for regional analysis. Our analysis and subsequent results provide strong statistical evidence for the existence of a prominent anomaly across the United States between 2002 and 2005. Our comprehensive review of potential physical origins successfully explains and excludes various causes that include unaccounted hydrological loading/unloading and a mechanistic origin from a megathrust detachment under Nevada. While the precise underlying cause remains outside the scope of this study, leading theories include variations in the length of day (LOD) and deep Earth geophysics for which the hypothesized foundation originates from the interactions and dynamics at the core-mantle boundary.