Publisher Correction: Measuring, modelling and projecting coastal land subsidence

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Unconstrained three-dimensional reaching in Rhesus monkeys

To better understand normative behavior for quantitative evaluation of motor recovery after injury, we studied arm movements by non-injured Rhesus monkeys during a food-retrieval task. While seated, monkeys reached, grasped, and retrieved food items. We recorded three-dimensional kinematics and muscle activity, and used inverse dynamics to calculate joint moments due to gravity, segmental interactions, and to the muscles and tissues of the arm. Endpoint paths showed curvature in three dimensions, suggesting that maintaining straight paths was not an important constraint. Joint moments were dominated by gravity. Generalized muscle and interaction moments were less than half of the gravitational moments. The relationships between shoulder and elbow resultant moments were linear during both reach and retrieval. Although both reach and retrieval required elbow flexor moments, an elbow extensor (triceps brachii) was active during both phases. Antagonistic muscles of both the elbow and hand were co-activated during reach and retrieval. Joint behavior could be described by lumped-parameter models analogous to torsional springs at the joints. Minor alterations to joint quasi-stiffness properties, aided by interaction moments, result in reciprocal movements that evolve under the influence of gravity. The strategies identified in monkeys to reach, grasp, and retrieve items will allow the quantification of prehension during recovery after a spinal cord injury and the effectiveness of therapeutic interventions

Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project

In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum-dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory

The application of satellite differential SAR interferometry-derived ground displacements in hydrogeology

The application of satellite differential synthetic aperture radar (SAR) interferometry, principally coherent (InSAR) and to a lesser extent, persistent-scatterer (PSI) techniques to hydrogeologic studies has improved capabilities to map, monitor, analyze, and simulate groundwater flow, aquifer-system compaction and land subsidence. A number of investigations over the previous decade show how the spatially detailed images of ground displacements measured with InSAR have advanced hydrogeologic understanding, especially when a time series of images is used in conjunction with histories of changes in water levels and management practices. Important advances include: (1) identifying structural or lithostratigraphic boundaries (e.g. faults or transitional facies) of groundwater flow and deformation; (2) defining the material and hydraulic heterogeneity of deforming aquifer-systems; (3) estimating system properties (e.g. storage coefficients and hydraulic conductivities); and (4) constraining numerical models of groundwater flow, aquifer-system compaction, and land subsidence. As a component of an integrated approach to hydrogeologic monitoring and characterization of unconsolidated alluvial groundwater basins differential SAR interferometry contributes unique information that can facilitate improved management of groundwater resources

Aquifer-system compaction and land subsidence : measurements, analyses, and simulations--the Holly Site, Edwards Air Force Base, Antelope Valley, California /

Includes bibliographical references (p. 61-65).Mode of access: Internet

Seasonal subsidence and rebound in Las Vegas Valley, Nevada, observed by Synthetic Aperture Radar Interferometry

Analyses of areal variations in the subsidence and rebound occurring over stressed aquifer systems, in conjunction with measurements of the hydraulic head fluctuations causing these displacements, can yield valuable information about the compressibility and storage properties of the aquifer system. Historically, stress‐strain relationships have been derived from paired extensometer/piezometer installations, which provide only point source data. Because of the general unavailability of spatially detailed deformation data, areal stress‐strain relations and their variability are not commonly considered in constraining conceptual and numerical models of aquifer systems. Interferometric synthetic aperture radar (InSAR) techniques can map ground displacements at a spatial scale of tens of meters over 100 km wide swaths. InSAR has been used previously to characterize larger magnitude, generally permanent aquifer system compaction and land subsidence at yearly and longer timescales, caused by sustained drawdown of groundwater levels that produces intergranular stresses consistently greater than the maximum historical stress. We present InSAR measurements of the typically small‐magnitude, generally recoverable deformations of the Las Vegas Valley aquifer system occurring at seasonal timescales. From these we derive estimates of the elastic storage coefficient for the aquifer system at several locations in Las Vegas Valley. These high‐resolution measurements offer great potential for future investigations into the mechanics of aquifer systems and the spatial heterogeneity of aquifer system structure and material properties as well as for monitoring ongoing aquifer system compaction and land subsidence

Detection and measurement of land subsidence using interferometric synthetic aperture radar and Global Positioning System, San Bernardino County, Mojave Desert, California

Land subsidence associated with ground-water-level declines has been recognized as a potential problem in parts of the Mojave Desert, California. Ground water has been the primary source of domestic, agricultural, and municipal water supplies in the desert since the early 1900s. Pumping of ground water from the Mojave River and Morongo ground-water basins in the southwestern Mojave Desert resulted in water-level declines of more than 30 meters (100 feet) between the 1950s and the 1990s. A Global Positioning System (GPS) survey of a geodetic network was used to determine the location, extent, and magnitude of vertical land-surface changes in Lucerne Valley in the Morongo ground-water basin. The GPS survey was conducted in 1998 to estimate historical elevation changes by comparing GPS-derived elevations with historical elevations (which were available for some of the monuments in the network as early as 1944) and to establish baseline values that can be used for comparisons with future GPS surveys. The GPS measurements indicated that about 600 millimeters (2 feet) [plus or minus 1,500 millimeters (5 feet)] of subsidence occurred at three of the monuments between 1969 and 1998 but that very little to no vertical change in position occurred at seven other monuments in the network. Water levels in the area of subsidence in Lucerne Valley declined about 15 meters (50 feet) during 1970-98. Interferometric synthetic aperture radar (InSAR) methods were used to characterize vertical land-surface changes in the Mojave River and Morongo ground-water basins during various intervals of time between 1992 and 1999. Interferograms, InSAR-generated displacement maps, show that subsidence ranging from 45 to 90 mm (0.15 to 0.3 ft) occurred in four areas of these two ground-water basins--the El Mirage, Lockhart-Harper Lake (dry), Newberry Springs, and Lucerne Valley areas. Some of the InSAR measurements were affected by the earthquakes at Landers and Hector Mine, California, and by atmospheric artifacts. Water-level data were examined for areas undergoing vertical land-surface changes to determine whether the vertical land-surface changes may be related to aquifer-system compaction caused by ground-water-level changes. Temporally relevant water-level data were sparse for some areas, particularly the El Mirage and Lockhart-Harper Lake (dry) areas. Water levels in wells proximate to the subsiding areas generally declined between 1992 and 1999; water levels in some wells proximate to the subsiding areas experienced seasonal periods of declines and recoveries

Measuring, modelling and projecting coastal land subsidence

Coastal subsidence contributes to relative sea-level rise and exacerbates flooding hazards, with the at-risk population expected to triple by 2070. Natural processes of vertical land motion, such as tectonics, glacial isostatic adjustment and sediment compaction, as well as anthropogenic processes, such as fluid extraction, lead to globally variable subsidence rates. In this Review, we discuss the key physical processes driving vertical land motion in coastal areas. Use of space-borne and land-based techniques and the associated uncertainties for monitoring subsidence are examined, as are physics-based models used to explain contemporary subsidence rates and to obtain future projections. Steady and comparatively low rates of subsidence and uplift owing to tectonic processes and glacial isostatic adjustment can be assumed for the twenty-first century. By contrast, much higher and variable subsidence rates occur owing to compaction associated with sediment loading and fluid extraction, as well as large earthquakes. These rates can be up to two orders of magnitude higher than the present-day rate of global sea-level rise. Multi-objective predictive models are required to account for the underlying physical processes and socio-economic factors that drive subsidence