The Geodesy Advancing Geosciences and EarthScope (GAGE) Facility Global Positioning System (GPS) Data Analysis Centers produce position time series, velocities, and other parameters for approximately 2000 continuously operating GPS receivers spanning a quadrant of Earth's surface encompassing the high Arctic, North America, and Caribbean. The purpose of this review is to document the methodology for generating station positions and their evolution over time and to describe the requisite trade‐offs involved with combination of results. GAGE GPS analysis involves formal merging within a Kalman filter of two independent, loosely constrained solutions: one is based on precise point positioning produced with the GIPSY/OASIS software at Central Washington University and the other is a network solution based on phase and range double‐differencing produced with the GAMIT software at New Mexico Institute of Mining and Technology. The primary products generated are the position time series that show motions relative to a North America reference frame and secular motions of the stations represented in the velocity field. The position time series themselves contain a multitude of signals in addition to the secular motions. Coseismic and postseismic signals, seasonal signals from hydrology, and transient events, some understood and others not yet fully explained, are all evident in the time series and ready for further analysis and interpretation. We explore the impact of analysis assumptions on the reference frame realization and on the final solutions, and we compare within the GAGE solutions and with others.

Refinements to GPS analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 34 slow slip events located throughout the Cascadia subduction zone from 1997 through 2005. Timing of transient onset is determined with wavelet transformation of geodetic time series. Thirty continuous stations are included in this study, ranging from northern California to southwestern British Columbia. Our improvements in analysis better resolve the largest creep events and also identify many smaller events. At 48.5°N latitude, a 14‐month average recurrence interval has been observed over eight events since 1997. Farther north along Vancouver Island a host of smaller events with a distinct 14‐month periodicity also occurs. In southern Washington State, some of the largest transient displacements are observed but lack any obvious periodicity in their recurrence. Along central Oregon, an 18‐month recurrence is evident, while in northern California an 11‐month periodicity continues through 2005. We invert GPS offsets of the 12 best recorded events for thrust slip along the plate interface using a cross‐validation scheme to derive optimal smoothing parameters. These 12 events have equivalent moment magnitudes between 6.3 and 6.8 and have 2–3 cm of slip. Unlike other subduction zones, no long‐duration events are observed, and cumulative surface deformation is consistently less than 0.6 cm. The many newly resolved smaller transient events in Cascadia show that slow slip events occur frequently with GPS best capturing only the largest events. It is likely that slow slip events occur more frequently at levels not detectable with GPS.

A prototype earthquake early warning (EEW) system is currently in development in the Pacific Northwest. We have taken a two-stage approach to EEW: (1) detection and initial characterization using strong-motion data with the Earthquake Alarm Systems (ElarmS) seismic early warning package and (2) the triggering of geodetic modeling modules using Global Navigation Satellite Systems data that help provide robust estimates of large-magnitude earthquakes. In this article we demonstrate the performance of the latter, the Geodetic First Approximation of Size and Time (G-FAST) geodetic early warning system, using simulated displacements for the 2001 M w 6.8 Nisqually earthquake. We test the timing and performance of the two G-FAST source characterization modules, peak ground displacement scaling, and Centroid Moment Tensor-driven finite-fault-slip modeling under ideal, latent, noisy, and incomplete data conditions. We show good agreement between source parameters computed by G-FAST with previously published and postprocessed seismic and geodetic results for all test cases and modeling modules, and we discuss the challenges with integration into the U.S. Geological Survey's ShakeAlert EEW system.

Traditional real-time (RT) seismology has relied on inertial sensors to characterize ground motions and earthquake sources, particularly for hazards applications such as warning systems. In the past decade, a revolution in high-rate, RT Global Navigation Satellite Systems (GNSS) displacement has provided a new source of data to augment traditional measurement devices. The Ridgecrest, California, earthquake sequence in 2019 provided one of the most complete recordings of RT-GNSS displacements to date, helping aid in an initial source characterization over the first few days. In this article, we analyze and make available the archived RT displacement streams and compare their performance to postprocessed results, which we also provide. We find good agreement for all stations showing a noticeable signal. This demonstrates that simple modeling in RT, such as peak ground displacement scaling, would be practically identical to postprocessed results. Similarly, we find good agreement across the full spectral range, from the coseismic offsets (∼0  Hz) to the Nyquist frequency. We also find low latency between the measurement acquisition at the field site and the position calculation at the data center. In aggregate, the performance during the Ridgecrest earthquakes is strong evidence of the viability and usefulness of RT-GNSS as a monitoring tool.

Traditional real-time seismology has relied on inertial sensors to characterize ground motions and earthquake sources, particularly for hazards applications such as warning systems. In the past decade, a revolution in high-rate, real-time Global Navigation Satellite System (GNSS) displacement have provided a new source of data to augment traditional measurement devices. The Ridgecrest, California earthquake sequence in 2019 provided one of the most complete recordings of real-time GNSS displacements to date, helping to aid in an initial source characterization over the first few days. In this manuscript, we analyze and make available the archived real-time displacement streams and compare their performance to post-processed results, which we also provide. We find good agreement for all stations showing a noticeable signal. This demonstrates that simple modeling in real-time, such as peak ground displacement scaling, would be practically identical to post-processed results. Similarly, we find good agreement across the full spectral range, from the coseismic offsets (~0 Hz) to the Nyquist frequency. We also find low latency between the measurement acquisition at the field site and the position calculation at the datacenter. In aggregate, the performance during the Ridgecrest earthquakes is strong evidence of the viability and usefulness of real-time GNSS as a monitoring tool.