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.

We estimate the velocity field in central and southern Calitbrnia using Global Positioning System (GPS) observations from 1986 to 1902 and very long baseline interferometry (VLB!) observations from 1984 to 1991. Our core network includes 12 GPS sites spaced approximately 50 km apart, mostly in the western Transverse Ranges and the coastal Borderlands. The precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 05% confidence ellipse with a semiminor axis of approximately 2 mm/yr oriented roughly north-south, and a semimajor axis of approximately 3 mm/yr oriented east-west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncertainty is larger. These uncertainties are calibrated by analyzing the scatter in three types of comparisons: (1) multiple measurements of relative position ("repeatability"), (2) independent velocity estimates from separate analyses of the GPS and VLBi data, and (3) rates of change in baseline length estimated t¾om the joint GPS+VLB! solution and from a comparison of GPS with trilateration. The dominant tectonic signature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model l¾om the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eastern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 _.+ I, 5 _.+ I, 5 _.+ 1, and 2 _.+ I mm/yr, respectively. North of Ihe Big Bend, some compression normal to the trace of the San Andreas fault can be resolved on both sides of the fault. The rates of rotation about vertical axes in the residual geodetic velocity field differ by up to a factor of 2 from those inferred from paleomagnctic declinations. Our estimates indicate that the "San Andreas discrepancy" can be resolved to within the 3 mm/yr uncertainties by accounting for deformation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of I-2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties. South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the ofl•horc islands and the mainland. INTRODU(q'!ONDetermining the velocity field in the vicinity of the Pacific-North America plate boundary in central and southern Calitbrnia (Figure 1) is a long-standing problem in tectonics. While most of the motion between these plates occurs on the San Andreas fault, the deformation extends for a substantial distance on either side of this structure. Such off-fault deformation is evident in geologic structures, seismicity, paleomagnetic declinations, and geodetic networks. Measuring that deformation with space geodesy is the primary objective of this study, w...

The Hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace. Dislocation models of the surface deformation adjacent to the Hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at approximately 7 millimeters per year to the bottom of the seismogenic zone along a approximately 20-kilometer-long northern fault segment. Microearthquakes with the same waveform repeatedly occur at 4- to 10-kilometer depths and indicate deep creep at 5 to 7 millimeters per year. The difference between current creep rates and the long-term slip rate of approximately 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern Hayward fault adjacent to the locked 1868 earthquake rupture, which broke the southern 40 to 50 kilometers of the fault. The potential for a major independent earthquake of the northern Hayward fault might be less than previously thought.

We use GPS measurements and block modeling to investigate the present‐day deformation of the Adriatic region, whose kinematics within the Nubia‐Eurasia plate boundary zone is not well constrained and remains controversial. Block modeling allows us to compute rigid‐plate angular velocities while accounting for elastic strain accumulation along block‐bounding faults. Results suggest that the Adriatic is a microplate (Adria) and that the southern boundary with the Nubia plate and the Aegean domain may be located along the Apulia Escarpment and the Kefallinia fault. Geodetic data alone cannot discriminate between a single block (AP) or a two blocks (GDAP) description of Adria, but the GDAP model predicts boundary slip rates that are in better agreement with observations from previous studies.