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.

Automatically detected and located tremor epicenters from episodic tremor and slip (ETS) episodes in northern Cascadia provide a high‐resolution map of Washington's slow slip region. Thousands of epicenters from the past four ETS events from 2004 to 2008 provide detailed map‐view constraints that correlate with geodetic estimates of the simultaneous slow slip. Each of these ETS events exhibits remarkable similarity in the timing and geographic distribution of tremor density and geodetically inferred slip. Analysis of the latest 15‐month inter‐ETS period also reveals ageodetic tremor activity similar both in duration and extent to ETS tremor. Epicenters from both ETS and inter‐ETS tremor are bounded between the 30‐ and 45‐km plate interface depth contours and locate approximately 75 km east of previous estimates of the locked portion of the subducting Juan de Fuca plate. Inter‐ETS tremor overlaps but is generally downdip of ETS tremor and does not yet correlate with geodetically observed slip, but this is likely because the slip is below current GPS detection levels. Based on the tremor and slip correlation and the tremor‐duration and slip magnitude relationship, we suggest that the well‐resolved, sharp updip edge of tremor epicenters reflects a change in plate interface coupling properties. The region updip of this boundary may accumulate stress with the potential for coseismic shear failure during a megathrust earthquake. Alternatively, plate convergence in this region could be accommodated by continuous slow slip with no detectable tremor or by slow slip events with sufficiently long recurrence intervals that none have been detected during the past 10 years of GPS observations.

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.

[1] A comparison of GPS and seismic analyses of 23 distinct episodic tremor and slip events, located throughout the Cascadia subduction zone over an 11-year period, yields a highly linear relationship between moment release, as estimated from GPS, and total duration of nonvolcanic tremor, as summed from regional seismic arrays. The events last 1-5 weeks, typically produce $5 mm of static forearc deformation, and show cumulative totals of tremor that range from 40 to 280 h. Moment released by each event is estimated by inverting GPS-measured deformation, which is sensitive to all rates of tremor-synchronous faulting, including aseismic creep, for total slip along the North American-Juan de Fuca plate interface. Tremor, which is shown to be largely invariant in amplitude and frequency content both between events and with respect to its duration, is quantified using several different parameterizations that agree to within 10%. All known Cascadia events detected since 1997, which collectively span the Cascadia arc from northern California to Vancouver Island, Canada, release moment during tremor at a rate of 5.2 ± 0.4 Â 10 16 N m per hour of recorded tremor. This relationship enables estimation of moment dissipation, via seismic monitoring of tremor, along the deeper Cascadia subduction zone that poses the greatest threat to its major metropolitan centers.

We describe the detection of teleseismic surface waves from the 3 November 2002 Mw 7.9 Denali fault earthquake in Alaska with a dense network of 1 Hz GPS stations in southern California, about 3900 km from the event. Relative horizontal displacements with amplitudes in excess of 15 mm and duration of 700 seconds agree with integrated velocities recorded by nearby broadband seismometers with an rms difference of 2–3 mm. The displacements are derived from independent 1 Hz instantaneous positions demonstrating that a GPS network can provide direct measurements of arbitrarily large dynamic and static ground horizontal displacements at periods longer than 1 s and amplitudes above 2 mm, with an inherent precision (signal to noise) that improves indefinitely with amplitude without clipping and in real time. High‐rate, real‐time GPS networks can enhance earthquake detection and seismic risk mitigation and support other applications such as intelligent transportation and civil infrastructure monitoring.