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At least 325 records · Page 18Linked to original sources

USGS2010

Satellite observations of a surtseyan eruption: Hunga Ha'apai, Tonga

On March 17, 2009, a surtseyan eruption occurred around Hunga Ha'apai Island, Tonga. A series of observations from the high-spatial resolution Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and the high-temporal resolution Moderate Resolution Imaging Spectroradiometer (MODIS), was used to estimate the magnitude, location, start time, and duration of the eruption, and measure the evolving characteristics of the new Hunga Ha'apai Island. The eruption start time was estimated to be between 01:50 and 11:10 local time, on March 17, 2009 (i.e., between 12:50 and 22:10 UTC, March 16). The initial explosive phase lasted 3–5 days and consisted of multiple steam and tephra explosions from two distinct vent sources, one on the northwest side, and another about 100 m off the south shore of the pre-existing island. The eruption plume reached 4.0 to 7.6 km altitude above sea level, and tephra added new land around each of the vents, initially tripling the area of the pre-existing island. The next phase of steaming from newly formed crater lakes around the vents lasted a few days. Three warm crater lakes formed initially, but disappeared with time as the shoreline eroded. After ~ 2 months, vegetation that was initially buried by tephra was recovering; after ~ 10 months, the size of the island had eroded down to ~ twice that of the pre-existing island, and the one remaining crater lake had a temperature of ~ 68 ± 17 °C. The volume of erupted material was estimated to be ~ 0.0176 km 3 and the volcanic explosivity index (VEI) was estimated to be VEI = 2.

Hunga Ha'apai Island↗

USGS2019

Three-dimensional basin and fault structure from a detailed seismic velocity model of Coachella Valley, Southern California

The Coachella Valley in the northern Salton Trough is known to produce destructive earthquakes, making it a high seismic hazard area. Knowledge of the seismic velocity structure and geometry of the sedimentary basins and fault zones is required to improve earthquake hazard estimates in this region. We simultaneously inverted first P wave travel times from the Southern California Seismic Network (39,998 local earthquakes) and explosions (251 land/sea shots) from the 2011 Salton Seismic Imaging Project to obtain a 3-D seismic velocity model. Earthquakes with focal depths ≤10 km were selected to focus on the upper crustal structure. Strong lateral velocity contrasts in the top ~3 km correlate well with the surface geology, including the low-velocity (<5 km/s) sedimentary basin and the high-velocity crystalline basement rocks outside the valley. Sediment thickness is ~4 km in the southeastern valley near the Salton Sea and decreases to <2 km at the northwestern end of the valley. Eastward thickening of sediments toward the San Andreas fault within the valley defines Coachella Valley basin asymmetry. In the Peninsular Ranges, zones of relatively high seismic velocities (~6.4 km/s) between 2 to 4 km depth may be related to Late Cretaceous mylonite rocks or older inherited basement structures. Other high-velocity domains exist in the model down to 9 km depth and help define crustal heterogeneity. We identify a potential fault zone in Lost Horse Valley unassociated with mapped faults in Southern California from the combined interpretation of surface geology, seismicity, and lateral velocity changes in the model.

California↗

USGS2013

Seasonal circulation over the Catalan inner-shelf (northwest Mediterranean Sea)

This study characterizes the seasonal cycle of the Catalan inner-shelf circulation using observations and complementary numerical results. The relation between seasonal circulation and forcing mechanisms is explored through the depth-averaged momentum balance, for the period between May 2010 and April 2011, when velocity observations were partially available. The monthly-mean along-shelf flow is mainly controlled by the along-shelf pressure gradient and by surface and bottom stresses. During summer, fall, and winter, the along-shelf momentum balance is dominated by the barotropic pressure gradient and local winds. During spring, both wind stress and pressure gradient act in the same direction and are compensated by bottom stress. In the cross-shelf direction the dominant forces are in geostrophic balance, consistent with dynamic altimetry data.

Journal of Geophysical Research C: Oceans↗

USGS1997

Estimation of directional surface wave spectra from a towed research catamaran

During the High-Resolution Remote Sensing Main Experiment (1993), wave height was estimated from a moving catamaran using pitch-rate and roll-rate sensors, a three-axis accelerometer, and a capacitive wave wire. The wave spectrum in the frequency band ranging roughly from 0.08 to 0.3 Hz was verified by independent buoy measurements. To estimate the directional frequency spectrum from a wave-wire array, the Data-Adaptive Spectral Estimator is extended to include the Doppler shifting effects of a moving platform. The method is applied to data obtained from a fixed platform during the Risø Air–Sea Experiment (1994) and to data obtained from a moving platform during the Coastal Ocean Processes Experiment (1995). Both results show that the propagation direction of the peak wind waves compares well with the measured wind direction. When swells and local wind waves are not aligned, the method can resolve the difference of propagation directions. Using the fixed platform data a numerical test is conducted that shows that the method is able to distinguish two wave systems propagating at the same frequency but in two different directions.

Journal of Atmospheric and Oceanic Technology↗

USGS2013

Storm surges and climate change implications for tidal marshes: Insight from the San Francisco Bay Estuary, California, USA

Tidal marshes are dynamic ecosystems, which are influenced by oceanic and freshwater processes and daily changes in sea level. Projected sea-level rise and changes in storm frequency and intensity will affect tidal marshes by altering suspended sediment supply, plant communities, and the inundation duration and depth of the marsh platform. The objective of this research was to evaluate if regional weather conditions resulting in low-pressure storms changed tidal conditions locally within three tidal marshes. We hypothesized that regional storms will increase sea level heights locally, resulting in increased inundation of the tidal marsh platform and plant communities. Using site-level measurements of elevation, plant communities, and water levels, we present results from two storm events in 2010 and 2011 from the San Francisco Bay Estuary (SFBE), California, USA. The January 2010 storm had the lowest recorded sea level pressure in the last 30 years for this region. During the storm episodes, the duration of tidal marsh inundation was 1.8 and 3.1 times greater than average for that time of year, respectively. At peak storm surges, over 65% in 2010 and 93% in 2011 of the plant community was under water. We also discuss the implications of these types of storms and projected sea-level rise on the structure and function of the tidal marshes and how that will impact the hydro-geomorphic processes and marsh biotic communities.

California↗

USGS2009

Northern Monterey Bay upwelling shadow front: Observations of a coastally and surface-trapped buoyant plume

During the upwelling season in central California, northwesterly winds along the coast produce a strong upwelling jet that originates at Point A??o Nuevo and flows southward across the mouth of Monterey Bay. A convergent front with a mean temperature change of 3.77 ?? 0.29??C develops between the warm interior waters and the cold offshore upwelling jet. To examine the forcing mechanisms driving the location and movement of the upwelling shadow front and its effects on biological communities in northern Monterey Bay, oceanographic conditions were monitored using cross-shelf mooring arrays, drifters, and hydrographic surveys along a 20 km stretch of coast extending northwestward from Santa Cruz, California, during the upwelling season of 2007 (May-September). The alongshore location of the upwelling shadow front at the northern edge of the bay was driven by: regional wind forcing, through an alongshore pressure gradient; buoyancy forces due to the temperature change across the front; and local wind forcing (the diurnal sea breeze). The upwelling shadow front behaved as a surface-trapped buoyant current, which is superimposed on a poleward barotropic current, moving up and down the coast up to several kilometers each day. We surmise that the front is advected poleward by a preexisting northward barotropic current of 0.10 m s -1 that arises due to an alongshore pressure gradient caused by focused upwelling at Point A??o Nuevo. The frontal circulation (onshore surface currents) breaks the typical two-dimensional wind-driven, cross-shelf circulation (offshore surface currents) and introduces another way for water, and the material it contains (e.g., pollutants, larvae), to go across the shelf toward shore.Copyright 2009 by the American Geophysical Union.

Journal of Geophysical Research C: Oceans↗

USGS1974

Detailed near-bottom geophysical profile across the continental slope off northern California

A geophysical profile was run across the continental slope off the California-Oregon border by use of a deeply submerged instrument package, approximately 50 m above the sea floor, containing a proton magnetometer and a 3.5-kHz transducer for shallow seismic penetration. Surface-towed seismic reflection equipment was operated concurrently. The deep-tow data show that the slope is relatively smooth; this agrees with interpretation of surface seismic profiles. Features smaller than 1 km in length can be resolved with the deep profiles but not with the surface profiles. The deep profiles show smallscale undulations less than 0.5 km long that are especially well formed on the steeper parts of the slope. These undulations may be caused by minor downslope movement of surface sediment or by depositional processes. The deep-tow and surface seismic profiles can be combined to give a more accurate structural picture of the continental slope than can be obtained with surface data alone.

California↗

USGS1991

Paleoshorelines, reefs, and a rising sea: South Florida, USA

The porous limestone bedrock, thin sediment cover, and tectonic stability of the Florida Platform during the past 15 ka BP provide an exceptionally suitable setting for reconstruction of paleoshorelines and onshore projection of future shorelines in a rising-sea scenario. Paleoshorelines for 8, 6, 4 and 2 ka BP show that 1) a series of limestone islands formed, then drowned, along the outer platform; 2) a distinct trough, called Hawk Channel, separated the outer islands from shore; 3) the lower Keys flooded earlier and more rapidly than the rest of the Keys; and 4) Florida Bay and tidal passes through the middle Keys into the bay developed within the past 4 ka BP. Projection of future shorelines onto land shows that most land forming the Florida Keys would flood in a rise of 1 to 2 m and that a rise of little more than 5 m would submerge all land. Offshore reefs would die, while nearshore reefs would shift landward as the mainland shoreline migrated northward. -from Authors

Florida↗

USGS2001

Estimation of population size using open capture-recapture models

One of the most important needs for wildlife managers is an accurate estimate of population size. Yet, for many species, including most marine species and large mammals, accurate and precise estimation of numbers is one of the most difficult of all research challenges. Open-population capture-recapture models have proven useful in many situations to estimate survival probabilities but typically have not been used to estimate population size. We show that open-population models can be used to estimate population size by developing a Horvitz-Thompson-type estimate of population size and an estimator of its variance. Our population size estimate keys on the probability of capture at each trap occasion and therefore is quite general and can be made a function of external covariates measured during the study. Here we define the estimator and investigate its bias, variance, and variance estimator via computer simulation. Computer simulations make extensive use of real data taken from a study of polar bears (Ursus maritimus) in the Beaufort Sea. The population size estimator is shown to be useful because it was negligibly biased in all situations studied. The variance estimator is shown to be useful in all situations, but caution is warranted in cases of extreme capture heterogeneity.

Journal of Agricultural, Biological, and Environme↗

USGS1976

Rise of a variable-viscosity fluid in a steadily spreading wedge-shaped conduit with accreting walls

Relatively rigid plates making up the outer 50 to 100 km of the Earth are steadily separating from one another along narrow globe-circling zones of submarine volcanism, the oceanic spreading centers. Continuity requires that the viscous underlying material rise beneath spreading centers and accrete onto the steadily diverging plates. It is likely that during the rise the viscosity changes systematically and that the viscous tractions exerted on the plates contribute to the unique pattern of submarine mountains and earthquake faults observed at spreading centers. The process is modeled by viscous creep in a wedge-shaped conduit (with apex at the sea floor) in which the viscosity varies as r m where r is distance from the apex and m is a parameter. For these conditions, the governing differential equations take a simple form. The solution for the velocity is independent of r and of the sign of m . As viscous stresses vary as r m-1 , the pattern of stress on the conduit wall is sensitive to viscosity variation. For negative m , the viscous pressure along the base of the conduit is quite uniform; for positive m , it falls toward zero in the axial region as the conduit base widens. For small opening angles, viscous forces push the plates apart, and for large ones, they oppose plate separation. Though highly idealized, the solution provides a tool for investigating tectonic processes at spreading centers.

Journal of Research of the U.S. Geological Survey↗

USGS2003

Agreement among observers classifying larval sea lamprey ( Petromyzon marinus ) habitat

Estimates of larval sea lamprey ( Petromyzon marinus ) abundance are used to rank Great Lakes tributaries for lampricide treatment. Observers subjectively stratify habitat into three categories: type I = preferred, type II = acceptable, type III = unacceptable. Agreement was evaluated among eight observers classifying habitats in small discrete plots in two Lake Superior tributaries, the Rock and Chocolay rivers, and among four observers classifying and measuring the amount of each habitat type along random transects in the Rock River. Agreement among the eight observers classifying habitat plots was high (Chocolay, k = 0.742 and Rock, k = 0.785). The amounts of types I, II, and III habitat estimated were statistically different among observers. However, the amount of variability found in the classification and measurement of habitat by observers had little effect on the ranking of 51 streams considered for lampricide treatment.

Journal of Great Lakes Research↗

USGS2007

Seismic detection and analysis of icequakes at Columbia Glacier, Alaska

Contributions to sea level rise from rapidly retreating marine-terminating glaciers are large and increasing. Strong increases in iceberg calving occur during retreat, which allows mass transfer to the ocean at a much higher rate than possible through surface melt alone. To study this process, we deployed an 11-sensor passive seismic network at Columbia Glacier, Alaska, during 2004–2005. We show that calving events generate narrow-band seismic signals, allowing frequency domain detections. Detection parameters were determined using direct observations of calving and validated using three statistical methods and hypocenter locations. The 1–3 Hz detections provide a good measure of the temporal distribution and size of calving events. Possible source mechanisms for the unique waveforms are discussed, and we analyze potential forcings for the observed seismicity.

Journal of Geophysical Research↗

USGS2022

Microgravity change during the 2008-2018 Kı̄lauea summit eruption: Nearly a decade of subsurface mass accumulation

Results from nine microgravity campaigns from Kı̄lauea, Hawaiʻi, spanning most of the volcano's 2008–2018 summit eruption, indicate persistent mass accumulation at shallow levels. A weighted least squares approach is used to recover microgravity results from a network of benchmarks around Kı̄lauea's summit, eliminate instrumental drift, and restore suspected data tares. A total mass of 1.9 × 10 11 kg was determined from these microgravity campaigns to have accumulated below Kı̄lauea Caldera during 2009–2015 at an estimated depth of 1.3 km below sea level. Only a fraction of this mass is reflected in surface deformation, and this is consistent with previously reported discrepancies between subsurface mass accumulation and observed surface deformation. The discrepancy, amongst other independent evidence from gas emissions, seismicity, and continuous gravimetry, indicate densification of magma in the reservoirs below the volcano summit. This densification may have been driven by degassing through the summit vent. It is hypothesized that during the final years of the summit eruption, magma densification resulted in a buildup of pressure in the reservoirs that may have contributed to the lower East Rift Zone outbreak of 2018. The observed mass accumulation beneath Kı̄lauea could not have been detected through other techniques and illustrates the importance of microgravity measurements in volcano monitoring.

Hawai'i↗

USGS2016

Detecting sea-level hazards: Simple regression-based methods for calculating the acceleration of sea level

This report documents the development of statistical tools used to quantify the hazard presented by the response of sea-level elevation to natural or anthropogenic changes in climate and ocean circulation. A hazard is a physical process (or processes) that, when combined with vulnerability (or susceptibility to the hazard), results in risk. This study presents the development and comparison of new and existing sea-level analysis methods, exploration of the strengths and weaknesses of the methods using synthetic time series, and when appropriate, synthesis of the application of the method to observed sea-level time series. These reports are intended to enhance material presented in peer-reviewed journal articles where it is not always possible to provide the level of detail that might be necessary to fully support or recreate published results. The purpose of this report is to document and compare three simple methodologies that have previously been used to provide estimates with associated errors of the acceleration of sea-level elevation. These techniques have been used by coastal scientists and planners in assessing coastal risk over a wide range of spatial and temporal scales. Because relative sea-level (SL) elevation time series contain energetic fluctuations at many time scales, extracting what can be relatively small rate and acceleration signals (along with estimates of the error) from much larger “noise” has proven to be both difficult and controversial. Acceleration is a preferred measure of SL response to recent changes in the Earth’s climate because over time scales of 100 years or less slow vertical land motions (such as glacial isostatic adjustment) contribute only to the linear signal and not to acceleration, thus reducing the complexity of the analysis. Hence acceleration is useful if the goal of a study is to characterize and quantify the hazard associated with the changing relative elevation of water with respect to land on decadal time scales. Although in some cases it may be necessary to determine the cause of relative sea level rise, as a first step, it is important to accurately estimate the magnitude of the threat. Most researchers agree that global sea level (GSL) rose persistently through much of the 20th century at about 1.5–2.0 millimeters per year (mm/yr). There is far less agreement about whether the rate of sea-level rise (SLR) is increasing (that is, an acceleration in SL). Recent studies, and most of their predecessors, use tide gage data to quantify SL acceleration, A SL (t). In the current study, three techniques were used to calculate acceleration from tide gage data, and of those examined, it was determined that the two techniques based on sliding a regression window through the time series are more robust compared to the technique that fits a single quadratic form to the entire time series, particularly if there is temporal variation in the magnitude of the acceleration. The single-fit quadratic regression method has been the most commonly used technique in determining acceleration in tide gage data. The inability of the single-fit method to account for time-varying acceleration may explain some of the inconsistent findings between investigators. Properly quantifying A SL (t) from field measurements is of particular importance in evaluating numerical models of past, present, and future SLR resulting from anticipated climate change.

Open-File Report↗

USGS2008

Historical changes in the Mississippi-Alabama barrier-island chain and the roles of extreme storms, sea level, and human activities

Barrier-island chains worldwide are undergoing substantial changes, and their futures remain uncertain. An historical analysis of a barrier-island chain in the north-central Gulf of Mexico shows that the Mississippi barriers are undergoing rapid systematic land loss and translocation associated with: (1) unequal lateral transfer of sand related to greater updrift erosion compared to downdrift deposition; (2) barrier narrowing resulting from simultaneous erosion of shores along the Gulf and Mississippi Sound; and (3) barrier segmentation related to storm breaching. Dauphin Island, Alabama, is also losing land for some of the same reasons as it gradually migrates landward. The principal causes of land loss are frequent intense storms, a relative rise in sea level, and a sediment-budget deficit. Considering the predicted trends for storms and sea level related to global warming, it is certain that the Mississippi-Alabama (MS-AL) barrier islands will continue to lose land area at a rapid rate unless the trend of at least one causal factor reverses. Historical land-loss trends and engineering records show that progressive increases in land-loss rate correlate with nearly simultaneous deepening of channels dredged across the outer bars of the three tidal inlets maintained for deep-draft shipping. This correlation indicates that channel-maintenance activities along the MS-AL barriers have impacted the sediment budget by disrupting the alongshore sediment transport system and progressively reducing sand supply. Direct management of this causal factor can be accomplished by strategically placing dredged sediment where adjacent barrier-island shores will receive it for island nourishment and rebuilding.

Alabama, Mississippi↗

USGS2022

Underwater hearing in sea ducks with applications for reducing gillnet bycatch through acoustic deterrence

As diving foragers, sea ducks are vulnerable to underwater anthropogenic activity, including ships, underwater construction, seismic surveys and gillnet fisheries. Bycatch in gillnets is a contributing source of mortality for sea ducks, killing hundreds of thousands of individuals annually. We researched underwater hearing in sea duck species to increase knowledge of underwater avian acoustic sensitivity and to assist with possible development of gillnet bycatch mitigation strategies that include auditory deterrent devices. We used both psychoacoustic and electrophysiological techniques to investigate underwater duck hearing in several species including the long-tailed duck ( Clangula hyemalis ), surf scoter ( Melanitta perspicillata ) and common eider ( Somateria mollissima ). Psychoacoustic results demonstrated that all species tested share a common range of maximum auditory sensitivity of 1.0–3.0 kHz, with the long-tailed ducks and common eiders at the high end of that range (2.96 kHz), and surf scoters at the low end (1.0 kHz). In addition, our electrophysiological results from 4 surf scoters and 2 long-tailed ducks, while only tested at 0.5, 1 and 2 kHz, generally agree with the audiogram shape from our psychoacoustic testing. The results from this study are applicable to the development of effective acoustic deterrent devices or pingers in the 2–3 kHz range to deter sea ducks from anthropogenic threats.

Journal of Experimental Biology↗

USGS2020

Increasing rates of carbon burial in southwest Florida coastal wetlands

Rates of organic carbon (OC) burial in some coastal wetlands appear to be greater in recent years than they were in the past. Possible explanations include ongoing mineralization of older OC or the influence of an unaccounted‐for artefact of the methods used to measure burial rates. Alternatively, the trend may represent real acceleration in OC burial. We quantified OC burial rates of mangrove and coastal freshwater marshes in southwest Florida through a comparison of rates derived from 210 Pb, 137 Cs, and surface marker horizons (MHs). Age/depth profiles of lignin: OC were used to assess whether down‐core remineralization had depleted the OC pool relative to lignin, and lignin phenols were used to quantify the variability of lignin degradation. Over the past 120 years, OC burial rates at seven sites increased by factors ranging from 1.4 to 6.2. We propose that these increases represent net acceleration. Change in relative sea‐level rise is the most likely large‐scale driver of acceleration, and sediment deposition from large storms can contribute to periodic increases. Mangrove sites had higher OC and lignin burial rates than marsh sites, indicating inherent differences in OC burial factors between the two habitat types. The higher OC burial rates in mangrove soils mean that their encroachment into coastal freshwater marshes has the potential to increase burial rates in those locations even more than might be expected from the acceleration trends. Regionally, these findings suggest that burial represents a substantially growing proportion of the coastal wetland carbon budget.

Florida↗

USGS1970

Metamorphosis of the landlocked sea lamprey, Petromyzon marinus

The external metamorphosis of the sea lamprey was divided into four stages, based primarily on the condition of the mouth: mouth reduced, mouth fused, mouth enclosed, and mouth elongated. During metamorphosis, the eye enlarged greatly, the snout and mouth region changed from a fleshy hood enclosing a sieve apparatus to a large sucking disc, the nasopore membrane and the branchial area shrank, the branchiopores changed in shape, the general color changed from dark brown and yellow to an intense blue-black dorsally and white ventrally, and the total length increased. Metamorphosis began in early to mid-July and did not take place after August. The duration of external metamorphosis was about 3 months for lampreys transforming under natural conditions. The mean lengths of metamorphosing lampreys from tributaries of lakes Superior and Michigan were 145 and 136 mm, respectively.

Journal of the Fisheries Research Board of Canada↗