GPS Imaging of Vertical Land Motion and Earthquake Coseismic Displacements in the GPS Mega-Network

The Nevada Geodetic Laboratory’s (NGL) Global Positioning Systems (GPS) worldwide data holdings number nearly 21,000 GPS stations that comprise the GPS Mega-Network today. Advances in data processing software, final orbit and clock products, atmospheric modeling, and reference frames have improved the precision and accuracy of GPS positioning solutions to the sub-millimeter level. The rates of change in these GPS position time series can be calculated by the MIDAS robust trend estimator to identify the patterns and styles of crustal deformation. Additionally, the large number of global stations improves the spatial resolution of observable geophysical signals. Together, these improvements helped motivate the GPS Imaging technique, an analysis method that interpolates spatiotemporal GPS trends between stations to construct a crustal velocity field representative of coherent movement of the solid Earth. The research presented in this dissertation uses the GPS Imaging technique to identify and analyze a number of geophysical signals related to vertical land motion and earthquake deformation. Two studies examine vertical land motion trends in regions of the United States and try to pinpoint the underlying geological sources for their signals. In the first study, GPS Imaging is used to identify the scope and extent of a subsidence signal observed in the Pacific Northwest. This signal is subsiding at approximately –2 mm/year, a rate higher than surrounding subsidence, and is located at latitudes corresponding to the Cascadia subduction zone and approximate longitude of the Cascadia arc. Several methods tested the resolution of GPS Imaging and changes to the regional signal over time. GPS data was then compared to predictions of various hypothesized loading sources that might contribute to the subsidence feature. GPS Imaging and realistic regional geological properties constrained volcanic loading and end loading models. This revealed that both styles of loading matched the width of the subsidence feature. A postseismic relaxation model from the 1700 M9.1 Cascadia Earthquake was compared to the GPS Imaging result, and accounted for approximately half of the subsidence signal concentrated around the Cascadia arc. Glacial isostatic adjustment modeling of the region determined that lithospheric flexure contributes about –1 mm/year of subsidence to the region. By combining the postseismic relaxation and glacial isostatic adjustment models, the subsidence feature was removed, suggesting that these two processes are likely the dominant sources of the subsidence signal. However, climatic and hydrological data compared to vertical land motion trends indicate possible contributions from hydrological loading. This work demonstrates a way to analyze subsidence signals in geologically complex regions, and laid important groundwork for other vertical land motion research. The second vertical land motion study was located in the Great Plains, United States. Vertical velocity data indicated there was an enigmatic source of regional uplift of approximately ~2 mm/year centered around the Texas Panhandle, with uplift extending through to the surrounding ~670 km x 280 km area. This region is home to the High Plains aquifer, the largest aquifer in the country and a major source of groundwater for agriculture. Water levels for the southern part of the aquifer have declined over 45 m, with greatest declines centered near the Texas Panhandle. Hydrological unloading was investigated as the principal source of the uplift signal. Climatic and hydrological data indicate a correlation between periods of drought and an increased rate of uplift observed by GPS data in the region. A hydrological unloading model was constrained by GPS Imaging by locating the greatest water mass loss where the uplift signal was ≥1 mm/year. Results indicated that a water volume loss of –5.1 km3/ year was sufficient to create the uplift signal observed by GPS Imaging, and this unloading rate is substantiated by other estimated rates of High Plains aquifer depletion. Our results indicated that hydrological unloading from aquifer deletion from climatic and anthropogenic influences is causing vertical land motion in the southern High Plains aquifer. This challenges the common conception that aquifer depletion equates to a subsidence signal, and also proves that GPS Imaging can be used as a tool to monitor groundwater changes remotely. The last study shifts away from regional vertical land motion investigations to apply GPS Imaging to global earthquake research. Some of the ~21,000 GPS stations in GPS Mega-Network are situated in earthquake prone regions experiencing tectonic deformation from plate interactions and/or induced seismicity. Earthquakes captured by the GPS Mega-Network are recorded in GPS time series as immediate discontinuities that represent coseismic displacement. Several different strategies are first tested to estimate coseismic displacements for the NGL. Analysis of coseismic displacements, aided by GPS Imaging, suggests that estimations are improved by a hierarchical strategy and radius of influence used to approximate which stations may be potentially affected by an earthquake. Next, the coverage, completeness, and resolution of coseismic displacements in the GPS Mega-Network is examined using the GPS Global Earthquake Catalog built from the coseismic displacement data. Comparisons of the GPS Global Earthquake Catalog to the USGS National Earthquake Information Center Earthquake Catalog for events occurring between 1 Jan. 1994–20 Apr. 2022 reveal that the GPS Mega-Network’s ability to capture global earthquake activity has increased over time and that the availability of estimated GPS coseismic displacements is greatest for earthquakes M≥7. Of the 427 earthquakes M≥7 recorded by the USGS, 93% of earthquakes 7≤M<7.5 have estimated GPS displacements, and 100% of earthquakes 7.5≤M≤9.1 have coseismic displacement data available