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.
[1] At the New Hebrides (NH) subduction zone, ridges born by the subducting Australia plate enter the trench and collide with the overriding margin. Results from GPS surveys conducted on both sides of the trench and new bathymetry maps of the NH archipelago bring new light on the complex tectonics of this area. Convergence vectors present large variations that are not explained by Australia/Pacific (A/P) poles and that define four segments. Vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors. Slow convergence (i.e., 30-40 mm/yr) is found at the central segment facing the D'Entrecasteaux Ridge. The southern segment moves faster than A/P motion predicts (89 to 124 mm/yr). Relatively to a western North Fiji basin (WNFB) reference, the northern and southern segments rotate in opposite directions, consistently with the extension observed in the troughs east of both segments. Both rotations combine in Central Vanuatu into an eastward translation that ''bulldozes'' the central segment into the WNFB at $55 mm/yr. That model suggests that the motion of the central segment, forced by the subduction/collision of the D'Entrecasteaux ridge, influences the motion of the adjoining segments. The New Caledonia archipelago is motionless with respect to the rest of the Australia plate despite the incipient interaction between the Loyalty ridge and the NH margin. Southeast of the interaction area, convergence is partitioned into a $50 mm/yr trench-normal component accommodated at the trench and a $90 mm/yr trench-parallel component, close to the A/P convergence, and presumably accommodated by a transform boundary at the rear of the NH arc.
Isotopically dated corals from the central New Hebrides and New Georgia Island Group, Solomon Islands, indicate that both forearcs underwent rapid late Quaternary subsidence that was abruptly replaced by hundreds of meters of uplift at rates up to ∼8 mm/yr, while total plate convergence was only a few kilometers. Two mechanisms that might account for these rapid reversals in vertical motion include (1) a “displacement” mechanism in which the forearc is displaced upward by the volume of an object passing beneath on the subducting plate (as the object moves deeper and vacates the base of the forearc, the forearc subsides to near its original position) and (2) a “crustal shortening” mechanism in which the forearc thickens and uplifts because of horizontal shortening when a large object impinges on the forearc and abruptly increases interplate coupling on the shallow end of the main thrust zone. Rapid subsidence follows when the impinging object is broken or otherwise decoupled, shallow interplate coupling becomes weak, and the uplifted forearc extends and subsides. The displacement mechanism surely plays a role on timescales over which plates converge tens of kilometers, but it fails to explain the geographic pattern, short time frame, and abruptness of the change from subsidence to uplift that we observe. The crustal shortening mechanism is preferred because it allows the observed abrupt uplift when an object impinges on a forearc and causes locking of a shallow segment of the interplate thrust zone.
On March 11, 2011, a magnitude Mw 9.0 earthquake occurred off the coast of Japan's Tohoku region causing catastrophic damage and loss of life. The tsunami flow velocity analysis focused on two survivor videos recorded from building rooftops at Kesennuma Bay along Japan's Sanriku coast. A terrestrial laser scanner was deployed at the locations of the tsunami eyewitness video recordings. The tsunami current velocities through the Kesennuma Bay are determined in a four step process. The LiDAR point clouds are used to calibrate the camera fields of view in real world coordinates. The motion of the camera during recordings was determined. The video images were rectified with direct linear transformation. Finally a cross‐correlation based particle image velocimetry analysis was applied to the rectified video images to determine instantaneous tsunami flow velocity fields. The measured maximum tsunami height of 9 m in the Kesennuma Bay narrows were followed by maximum tsunami outflow currents of 11 m/s less than 10 minutes later.
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