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Due to the blockage of seawater, seafloor displacement cannot be directly measured by space geodesy. The combination of Global Navigation Satellite Systems‐acoustic ranging (GNSS‐A) has been used to overcome the electromagnetic barrier, so that a GNSS‐determined sea surface vessel's coordinates can be transformed to seafloor benchmarks in a global reference frame. Due to the high cost and science priorities, previous GNSS‐A studies mainly targeted relatively deep water and a minimum of three transponders were used to form an array, equivalent to a precision geodetic station. With recent developments in unmanned autonomous surface vessels, low cost GNSS‐A surveys are poised to become practical. Here we demonstrate that with a carefully designed surveying trajectory, Wave Glider‐based GNSS‐A surveying of a single transponder in shallow water can provide centimeter‐level accuracy on horizontal seafloor positioning, even if the sound speed model deviates from the actual value by a few meters per second. Results from a nine‐month experiment conducted at ∼54 m water depth show that the repeatability of the seafloor horizontal positioning is better than 2 cm. When conditions allow, the acoustic observations should be collected symmetrically about the transponder and data redundancies are recommended to reduce the error associated with time‐dependent variations in sound speed. Plain Language Summary: Seafloor geodesy is important for monitoring offshore strain processes. Global Navigation Satellite Systems‐acoustic ranging (GNSS‐A) uses sound signals traveling in water as rulers to measure the distances between the sea surface vessel and the seafloor sensor (transponder), so that the GNSS‐determined sea surface vessel's coordinates can be transformed to the seafloor through triangulation. Multiple transponders deployed on the seafloor are often needed to form an array so that noise caused by irregular variations in the sound signals can be reduced. In shallow water, due to the relatively short distance, the range distortion caused by a sound signal's refraction is smaller, and the time it takes to conduct each complete cycle of survey is shorter. These conditions allow the seafloor displacements to be measured with centimeter‐level accuracy using only one transponder. Key Points: For single transponder Global Navigation Satellite Systems‐acoustic ranging in shallow water, seafloor vertical position estimation is sensitive to sound speed error and variationA geometrically symmetric surveying trajectory is preferable for error reduction in horizontal seafloor positioningSeafloor horizontal positions were measured with <2 cm repeatability at ∼54 m water depth over five repeat surveys with Wave Gliders [ABSTRACT FROM AUTHOR]

The boundary between the overriding and subducting plates is locked along some portions of the Cascadia subduction zone. The extent and location of locking affects the potential size and frequency of great earthquakes in the region. Because much of the boundary is offshore, measurements on land are incapable of completely defining a locked zone in the up‐dip region. Deformation models indicate that a record of seafloor height changes on the accretionary prism can reveal the extent of locking. To detect such changes, we have initiated a series of calibrated pressure measurements using an absolute self‐calibrating pressure recorder. A piston‐gauge calibrator under careful metrological considerations produces an absolutely known reference pressure to correct seafloor pressure observations to an absolute value. We report an accuracy of about 25 ppm of the water depth, or 0.02 kPa (0.2 cm equivalent) at 100 m to 0.8 kPa (8 cm equivalent) at 3,000 m. These campaign survey‐style absolute pressure measurements on seven offshore benchmarks in a line extending 100 km westward from Newport, Oregon from 2014 to 2017 establish a long‐term, sensor‐independent time series that can, over decades, reveal the extent of vertical deformation and thus the extent of plate locking and place initial limits on rates of subsidence or uplift. Continued surveys spanning years could serve as calibration values for co‐located or nearby continuous pressure records and provide useful information on possible crustal deformation rates, while epoch measurements spanning decades would provide further limits and additional insights on deformation. Plain Language Summary: The Cascadia subduction zone has produced large earthquakes and tsunamis whose potential size and interval is affected by the amount and distribution of locking between the tectonic plates. A large portion of the subduction zone is offshore, where typical land‐ and satellite‐based methods are ineffective at measuring crustal changes. Seafloor water pressure observations can be used to measure height changes, but pressure gauges inherently drift at rates typically greater than the expected vertical seafloor deformation rates. The absolute self‐calibrating pressure recorder (ASCPR) measures the true, absolute, sensor‐independent seafloor pressure by addressing and correcting sources of error caused by the internal piston gauge calibrator and recording pressure gauges. The accuracy of our measurements is about 25 ppm of the water depth, or about 2.5 cm of height per 1,000 m of water. Campaign survey‐style measurements using the ASCPR at seven benchmarks off the coast of Newport, Oregon from 2014 to 2017 establish a long‐term record of absolute measurements that can be referenced by studies decades or more in the future or can estimate and correct drift of nearby continuous pressure gauges. Continued measurements can provide insights on seafloor deformation and thus locking in Cascadia. Key Points: Campaign‐style surveys of absolute calibrated seafloor pressure measurements were made in the Cascadia subduction zone from 2014 to 2017These sensor‐independent measurements act as long‐term, absolute reference values that can be used for future vertical deformation studiesWe document and quantify the sources of error in the technique with a total uncertainty of ∼25 ppm, or ∼0.25 kPa per 1,000 m water depth [ABSTRACT FROM AUTHOR]

The southern Ryukyu subduction zone is one of the potential sources for tsunamigenic earthquakes. Despite a great seismic risk, the deformation pattern remains poorly known, primarily due to the absence of seafloor constraints. With GNSS‐acoustic measurements over years, we characterize the convergence rate across this margin growing from 92 mm/yr offshore eastern Taiwan to 123 mm/yr near the Gagua Ridge. The new data suggest the subduction interface is capable of hosting Mw 7.5–8.4 earthquakes. The orientations of seafloor movement and P‐axes in the Nanao Basin are both subnormal to the trench, notably deviate from the direction of plate convergence. By considering the combined effect of plate convergence and backarc rifting, different trends between the forearc convergence, P‐axes, and seafloor movement may indicate some degree of slip‐partitioning. The trench‐parallel component is likely accommodated in part by earthquakes near Taiwan, lower plate deformation, and strike‐slip faults within the accretionary wedge. Plain Language Summary: The southern Ryukyu margin near Taiwan accommodates the relative plate motion and backarc rifting. This region has suffered from tsunamis and large earthquakes with magnitudes of about eight in the history, yet insufficient offshore data still hamper our ability to fully characterize fault slip behaviors. We demonstrate how a seafloor geodetic technique, with the combination of the Global Navigation Satellite System and acoustic ranging, can help us constraining fault slip behaviors and assessing seismic risks. We infer a fraction of oblique plate motion is accommodated by the subduction interface capable of hosting earthquakes with moment magnitudes ranging from 7.5 to 8.4. Incorporating seismological observations, we find different trends of the compressive axes of earthquakes, seafloor movement, and the forearc convergence, which may indicate some degree of strain partitioning. The trench‐parallel motion is partially taken up by seismic slip near Taiwan, subducting plate deformation, and strike‐slip faults within the forearc. More near‐trench observations are crucial to assessing seismic hazard and evaluating strain partitioning along the southern Ryukyu Trench. Key Points: Rapid convergence across the southern Ryukyu margin increases eastward from 92 mm/yr offshore Taiwan to 123 mm/yr near the Gagua RidgeOblique convergence is accommodated in part by the plate interface fault capable of hosting earthquakes with Mw 7.5–8.4Different trends between the forearc convergence, P‐axes, and seafloor movement may indicate some degree of slip partitioning [ABSTRACT FROM AUTHOR]

Measuring seafloor motion in shallow coastal water is challenging due to strong and highly variable oceanographic effects. Such measurements are potentially useful for monitoring near‐shore coastal subsidence, subsidence due to petroleum withdrawal, strain accumulation/release processes in subduction zones and submerged volcanoes, and certain freshwater applications, such as volcano deformation in caldera‐hosted lakes. We have developed a seafloor geodesy system for this environment based on an anchored spar buoy topped by high‐precision GPS. Orientation of the buoy is measured using a digital compass that provides heading, pitch, and roll information. The combined orientation and GPS tracking data are used to recover the three‐dimensional position of the seafloor marker (anchor). A test system has been deployed in Tampa Bay, Florida, for over 1 year and has weathered several major storms without incident. Even in the presence of strong tidal currents which can deflect the top of the buoy several meters from vertical, daily repeatability in the corrected three‐component position estimates for the anchor is 1–2 cm or better. Plain Language Summary: To measure seafloor motion in shallow water, we built a spar buoy and put a GPS antenna and a digital compass (three‐dimensional orientation sensor) on top of it. The buoy rests on the sea bottom using a heavy concrete ballast. Rotation and other movements of the buoy are measured by the digital compass and GPS. Position of the ballast can be calculated based on these measurements. We tested the system in Tampa Bay, Florida, and found that it is able to measure motion of the anchor with an uncertainty of 1–2 cm or smaller. Key Points: A GPS‐buoy system has been designed, built, and tested to measure seafloor motion in shallow waterGPS and buoy orientation measured by a digital compass enable the anchor position and effects of water motion to be accurately determinedDaily repeatability of seafloor positioning is ~1–2 cm for horizontal components and better than 1 cm for the vertical component [ABSTRACT FROM AUTHOR]

One of the important issues on the GPS‐acoustic (GPS‐A) observation for sea bottom positioning is how to address the horizontal heterogeneity of the sound speed in oceans. This study presents an analysis method of GPS‐A data in the presence of a sloping sound speed structure. By applying this method and revising the analysis scheme to make full use of existing data, we reevaluated the horizontal postseismic deformations occurring ~1.5–5 years after the 2011 Tohoku earthquake. The revised horizontal movements have more uniform directions and rates between neighboring sites, suggesting enhancement of the array positioning accuracy. The revised displacement rate of the site on the incoming Pacific plate, located ~100 km northeast of the main rupture zone, was decreased significantly; it was only slightly, by 1.4 cm/year larger than the global motion of the Pacific plate, suggesting a relatively small effect of viscoelastic relaxation. The horizontal movements of the near‐trench sites above the main rupture zone were generally landward and were significantly faster than the Pacific plate motion, indicating a viscoelastic relaxation of 5–10 cm/year. The distribution of the fast landward movements peaked near 38°N at an updip of the mainshock hypocenter and extended significantly farther to the north than to the south. This implies the existence of a secondary coseismic slip patch in the northern area in addition to a primary slip patch at ~38°N. The occurrence of episodic slow slip in early 2015 to the north of the main rupture zone was also verified from the GPS‐A analyses. Key Points: We present a new analysis method of GPS‐acoustic observation data for seafloor positioning in the presence of sloping sound speed structureThe application to the 2011 Tohoku earthquake demonstrate enhanced position accuracy by the spatially coherent postseismic movementsThe occurrence of episodic slow slip in 2015, which has been inferred from repeating earthquakes, was also verified from our observations [ABSTRACT FROM AUTHOR]

Abstract: We performed observations of seafloor crustal deformation employing the Global Navigation Satellite System/Acoustic technique at two stations installed at about 45 and 70 km from the axis of the Nansei‐Shoto (Ryukyu) Trench, to the southeast off the Okinawa Main Island. The observations for 3‐ and 6‐year survey periods indicate that the two stations moved landward, in the opposite direction to the trench, by 63 and 21 mm/year relative to the Ryukyu Arc, suggesting interplate coupling around the stations. The observational results reveal the strong coupling state on the plate interface with coupling ratios of 0.9–1.0 at the slab depths of 10–13 km or 0.7–0.8 at slab depth ranging from 13 km up to the seafloor. The strongly coupled segment completely coincides with the source area of the 1791 tsunami event and does not overlap with the activity area of slow slip events. [ABSTRACT FROM AUTHOR]

We have developed a comprehensive inversion scheme to analyze traveltime data collected through GPS-acoustic observations on a campaign basis. Our method uses a quantity called the 'nadir total delay' (NTD), which is analogous to the zenith total delay in Global Navigation Satellite Systems analyses, to represent the time variation of the sound speed. The observation equation using the NTD is an approximated formulation. We examine its applicable scope using numerical experiments and demonstrate that the approximation holds well enough in practice. Traveltime data from all our observation campaigns were utilized together for inversion calculation to determine the following: (1) the positions of individual precision acoustic transponders at the time of a particular reference campaign, (2) the displacement from these positions of a rigid array at the time of each campaign, and (3) the time variation of the NTD during each campaign. We applied this method to actual data to fix the array geometry more precisely. Also, we have made a first step in detecting vertical motions as well as horizontal motions, though further enhancement of the accuracy will be required. In performing linear regression of array displacements obtained from the inversion, their error covariance has a significant effect on the resulting velocities. When the covariance is taken into account, nearly the same velocities are obtained no matter what campaign is taken as reference in the inversion. [ABSTRACT FROM AUTHOR]

The recently developed wave glider has the potential to be an effective unmanned platform for acoustic applications. We present the results of a variety of experiments that quantify this potential. The radiated self-noise of the autonomous platform is evaluated using an integrated passive acoustic recorder during a set of field trials off the coast of Hawaii. We present the radiated noise spectra from these trials to illustrate the dependence on hydrophone location and sea state. Using the same instrumentation, we demonstrate the ability of a modified wave glider to detect marine mammals using passive acoustic monitoring techniques. We also evaluate the performance of the wave glider operating as an active acoustic gateway, highlighting the potential of this platform to serve as a navigation reference and communications relay for scientific, industrial, and military subsea assets. To demonstrate the potential of the wave glider platform to support acoustic navigation, we assess the performance of time-of-flight range estimation and seafloor transponder localization. These tests were performed using commercial off-the-shelf acoustic positioning hardware integrated with the wave glider to illustrate that the low self-noise of the wave glider makes it possible to achieve acoustic positioning performance similar to previously reported results. Finally, we show that the glider can operate as a station-keeping surface communications gateway and provide recommendations for its use. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

We present calibration results from Jason-1 (2001-) and TOPEX/POSEIDON (1992-) overflights of a California offshore oil platform (Harvest). Data from Harvest indicate that current Jason-1 sea-surface height (SSH) measurements are high by 138 ± 18 mm. Excepting the bias, the high accuracy of the Jason-1 measurements is in evidence from the overflights. In orbit for over 10 years, the T/P measurement system is well calibrated, and the SSH bias is statistically indistinguishable from zero. Also reviewed are over 10 years of geodetic results from the Harvest experiment. [ABSTRACT FROM AUTHOR]

Geodesy, the oldest science, has become an important discipline in the geosciences, in large part by enhancing Global Positioning System (GPS) capabilities over the last 35 years well beyond the satellite constellation’s original design. The ability of GPS geodesy to estimate 3D positions with millimeter-level precision with respect to a global terrestrial reference frame has contributed to significant advances in geophysics, seismology, atmospheric science, hydrology, and natural hazard science. Monitoring the changes in the positions or trajectories of GPS instruments on the Earth’s land and water surfaces, in the atmosphere, or in space, is important for both theory and applications, from an improved understanding of tectonic and magmatic processes to developing systems for mitigating the impact of natural hazards on society and the environment. Besides accurate positioning, all disturbances in the propagation of the transmitted GPS radio signals from satellite to receiver are mined for information, from troposphere and ionosphere delays for weather, climate, and natural hazard applications, to disturbances in the signals due to multipath reflections from the solid ground, water, and ice for environmental applications. We review the relevant concepts of geodetic theory, data analysis, and physical modeling for a myriad of processes at multiple spatial and temporal scales, and discuss the extensive global infrastructure that has been built to support GPS geodesy consisting of thousands of continuously operating stations. We also discuss the integration of heterogeneous and complementary data sets from geodesy, seismology, and geology, focusing on crustal deformation applications and early warning systems for natural hazards. [ABSTRACT FROM AUTHOR]