bluuebunny/crossref_metadata_2025 huggingface dataset:
Historic creep rate and potential for seismic slip along the Hayward Fault, California
Authors: Lienkaemper J. J. , Borchardt G. , Lisowski M.
Published: February 2008

The Hayward fault is considered the most likely source of one or more major earthquakes in the San Francisco Bay area in the next few decades. Historically, at least one, and probably two, major earthquakes (about M 6.8) occurred along the Hayward fault, one in 1836 and another in 1868. Little is known about the 1836 event, but the 1868 earthquake was accompanied by a surface rupture that extended as much as 41 km along the southern part of the fault. Although the amount of surface slip in 1868 is uncertain, right slip (including afterslip) reached at least several centimeters, and possibly several decimeters in places. This paper documents the spatial variation of creep rate along the Hayward fault since the 1868 earthquake. Creep (aseismic fault slip) occurs over at least 66 km and may extend over the fault's entire 82‐km length, of which about 13 km lies underwater. Creep rate seems nearly constant over decades, but short‐term variations occur. We derive creep rate mainly from our own systematic surveying of offset cultural features (curbs, fences, and buildings). On each feature we solve directly for accumulated creep by using multiple linear regression. Creep rate mostly falls in the range of 3.5–6.5 mm/yr; but systematic variation occurs along strike. Fault segments with distinctly higher and lower rates generally correspond to parts of the fault most salient from the overall average alinement of the fault. Most distinctive is a 4‐km‐long section near the south end of the fault that creeps at about 9 mm/yr. Such a high rate has occurred there at least since the 1920s and probably since the 1868 earthquake, as indicated by an offset railroad track built in 1869. We suggest that this 9 mm/yr slip rate may approach the long‐term or deep slip rate that controls average recurrence interval between major earthquakes. If so, assuming an elastic rebound model, the potential for slip in large earthquakes below the surficial creeping zone is now ∼1.1 m in the southern (1868) segment of the fault and ≥ 1.4 m in the northern (1836?) segment. Subtracting surface creep rates from a long‐term slip rate of 9 mm/yr gives present potential for surface slip in large earthquakes of up to 0.8 m, with an average of 0.6 m in the northern segment and 0.4 m in the southern segment. We present a simple hypothesis for rupture potential that is compatible with historic creep rate, microseismicity distribution, and geodetic data. If seismic rupture occurs on segments 41 km long by 10 km deep (7 km fully locked, 3 km creeping), today's potential for seismic moment release is 1.4 × 1019 and 1.1 × 1019 N m for both 1836? and 1868 segments, respectively, and 2.5 × 1019 N m for both segments jointly. Converting moment to magnitude gives ML 6.8 in the northern segment, ML 6.7 in the southern segment, and ML 7.0 for simultaneous rupture of both.
Comparison of vibroseis and explosive source methods for deep crustal seismic reflection profiling in the Basin and Range Province
Authors: Brocher Thomas M. , Hart Patrick E.
Published: February 2008

Direct comparison of low‐fold, high‐energy explosive and high‐fold, lower‐energy Vibroseis methods for acquiring deep crustal seismic reflection data in the Basin and Range Province suggests that the high‐fold common midpoint (CMP) method there does not provide the best possible image of lower crustal structure. During the recent acquisition of a Vibroseis profile in the Basin and Range Province we fired single deep shot holes to obtain a coincident single‐fold explosive section. Within the upper crust (upper 3 s) the explosive source and Vibroseis records are nearly equivalent. For record times below 3 s, however, comparison of the explosive source gathers and the coincident final 60‐fold Vibroseis section demonstrates that low‐fold explosive profiling provides a higher‐quality image of the midcrust to lower crust (3–10 s). The higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives, making them less sensitive to common noise sources. Whereas deeper than 4–5 s the Vibroseis energy levels on individual source efforts fall to that of ambient noise levels, the explosions provide signal‐generated energy exceeding ambient noise levels down to 18–19 s. Although individual reflections can be correlated on explosion and Vibroseis shot gathers, reflection events on the 60‐fold Vibroseis stack do not correlate to those on the single‐fold explosion profile, suggesting that the high‐fold CMP method in our study did not maintain the integrity of the weak lower crustal reflected arrivals. Reasons why the high‐fold CMP method apparently failed include complex, even time‐varying, statics, nonhyperbolic moveout at long offsets, and the difficulty in resolving stacking velocities with data having low signal‐to‐noise ratios. Reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower‐energy Vibroseis section.
Crustal structure of the western New England Appalachians and the Adirondack Mountains
Authors: Hughes Stephen , Luetgert James H.
Published: February 2008

We present an interpretation of the crustal velocity structure of the New England Appalachians and the Adirondack Mountains based on a seismic refraction/wide‐angle reflection experiment in eastern North America extending from the Adirondacks in New York State through the northern Appalachians in Vermont and New Hampshire to central Maine. Modeling of the eastern portion of the profile within the New England Appalachians shows a subhorizontal layered crust with upper crustal velocities ranging from 5.5 to 6.2 km/s, a midcrustal velocity of 6.4 km/s, and a lower crustal velocity of approximately 6.8 km/s. Crustal thickness increases from 36 km beneath Maine to 40 km in Vermont. Little evidence is seen for structures at depth directly related to the White Mountains or the Green Mountains. A major lateral velocity change in the upper and mid crust occurs between the Appalachians and the Adirondacks. This boundary, projecting to the surface beneath the Champlain Valley, dips to the east beneath the Green Mountains and extends to a depth of ∼25 km below the eastern edge of the Connecticut Valley Synclinorium in Vermont. The Tahawus Complex, a series of strong horizontal reflections at 18–24 km depth beneath the Adirondack Highlands, is seen to dip eastward beneath Vermont. Upper crustal rocks in the Adirondack Mountains have Poisson's ratios of 0.28±0.01 that can be correlated with the Marcy Anorthosite. Pois son's ratios of 0.24±0.01 calculated for rocks of the Connecticut Valley Synclinorium indicate a siliceous upper crust in Vermont. The lower crust is considered to be best represented by intermediate to mafic granulites; a high Poisson's ratio (0.26–0.27) tends to support a mafic lower crust in the New England Appalachians. This seismic refraction/wide‐angle reflection experiment provides further evidence for the obduction of the allochthonous western Appalachian units onto Grenvillian crust above a zone of detachment that penetrates at least to midcrustal depths and was the locus of successive Paleozoic thrusting.
Seismic Moment tensors of western U.S. earthquakes and implications for the tectonic stress field
Authors: Patton Howard J. , Zandt George
Published: February 2008

Using a linear inversion method applied to regional surface wave data, we have determined seismic moment tensors for 50 earthquakes in the western United States. Regional surface waves are well suited for studying moderate‐sized earthquakes (4 < M < 5) located outside of local seismic networks for which first‐motion solutions are difficult to obtain and seismic moment estimates are problematic due to clipping of local P and S wave recordings. We compared the inversion results to P wave first‐motion solutions for 17 of the 50 events, and our comparison shows much better agreement in the orientations of the T axis than for the P axis. This could be related to the sensitivity of both methods to focal depth estimates. The surface wave results yielded a very uniform and consistent data set of stress orientations. The T axis is uniformly horizontal for all areas sampled, with E‐W extension across northern California, northern Nevada, and into north central Utah; ESE‐WNW extension in southern Nevada; and ENE‐WSW extension in Idaho, north of the Snake River Plain. The P axis shows more variability than the T axis on local as well as regional distance scales. In the Great Basin, the results indicate a N‐S change in the relative magnitude of the vertical to maximum horizontal compressive stress, With most earthquakes showing normal faulting in the Battle Mountain region and strike‐slip faulting in southern Nevada. The direction of maximum horizontal compressive stress is mainly N‐S in northern California and western Oregon. Our results, along with the results of Zoback et al. (1981) for central California, indicate a 60° rotation of the maximum horizontal compressive stress between northern and central segments of the San Andreas fault system. A change in stress state along the northern segment could be related to the thermal and mechanical evolution of the Pacific‐North American plate boundary.
Tectonic implications of a remagnetization event in the Newark Basin
Authors: Witte William K. , Kent Dennis V.
Published: February 2008

The Newark basin red beds contain a secondary magnetization (the B component) acquired during the Middle Jurassic after the 5°–20° basin‐wide northwesterly dip was imparted to the strata of the basin and after most, if not all, of the limb rotation in the Jacksonwald syncline. The B component magnetization was most likely related to the same hydrothermal event which evidently remagnetized many of the igneous intrusions in the basin and reset their K/Ar systems at 175 Ma. The remagnetization of the red beds occurred over a few million years and was approximately coincident with the transition from continental rifting to seafloor spreading in the adjacent North Atlantic. The B component magnetization direction yields a paleomagnetic pole at 74°N, 96°E (K = 63, A95 = 2.6°, N = 50 sites) after structural correction for ⅓ of the Jacksonwald folding and none of the regional tilt. This pole supports recent evidence for a high‐latitude model of Jurassic apparent polar wander for North America.
The effect of silicate weathering on global temperature and atmospheric CO2
Authors: Brady Patrick V.
Published: February 2008

Models of the carbon cycle, used to calculate atmospheric CO2 levels and mean global surface temperatures over geologic time, rely heavily on estimates of CO2 consumed by chemical weathering. Weathering of calcium and magnesium silicates is the primary sink for atmospheric CO2, yet alkali feldspar dissolution rates or data from carbonate aquifers have generally been used as model inputs instead. The latter causes calculated CO2 levels and temperatures to be anomalously high in the model of Berner et al. (1983) (for example, temperatures of 24°C and 19°C are calculated for the Cretaceous and Eocene, respectively). When Ca and Mg silicate weathering rates are used as primary inputs instead, ambient temperatures and atmospheric CO2 levels are calculated to be significantly closer to recent preindustrial levels (global mean temperatures of ∼20.5°C and 17.5°C in the Cretaceous and Eocene, respectively), indicating an appreciably tighter coupling between chemical weathering and climate.
Origin of compositional zonation (high‐alumina basalt to basaltic andesite) in the Giant Crater Lava Field, Medicine Lake Volcano, northern California
Authors: Baker M. B. , Grove T. L. , Kinzler R. J. , Donnelly‐Nolan J. M. , Wandless G. A.
Published: February 2008

The Giant Crater lava field erupted high‐alumina basalt (HAB), basalt, and basaltic andesite (BA) lavas about 10,500 years ago on the south flank of Medicine Lake volcano. Lava flows from several adjacent vents constitute a compositionally zoned eruption of ≈4.4 km3. The earliest lavas are the most evolved (53.5 wt % SiO2 and 6.5 wt % MgO), and latest lavas are the most primitive (48 wt % SiO2 and 10.5 wt % MgO). The evidence from major and trace element chemistry, isotopes and mineral chemistry in lavas and inclusions indicates that fractional crystallization and assimilation of a granitic crustal component played important roles in producing the observed compositional variations. Models that involve fractionation of primitive HAB, assimilation of granitic crust, replenishment of the magma reservoir by primitive HAB and mixing of these components (FARM) are presented for the development of the compositionally zoned eruption. In these models, extensive fractionation of HAB and melting of silicic crust occurs with little or no chemical interaction between fractionated basalt and melted crust. The melted granitic crust is produced by heat supplied through fractionation of large volumes of basaltic magma. In an initial event, melted crust and highly fractionated basalt (ferrobasalt) mix with primitive HAB magma. Subsequent injections of primitive HAB mix with the ferrobasalt and the magma produced by the previous mixing event, and this sequence of mixing, reinjection, and mixing leads to the compositional zoning of the Giant Crater lava field.
Evidence for shallow and pervasive seismic anisotropy in the Wellington Region, New Zealand
Authors: Gledhill K. R.
Published: February 2008

The shear waves of local earthquakes were recorded during a 5‐month deployment of seven three‐component digital seismographs on the Wellington Peninsula, New Zealand. The seismographs were spaced an average of less than 5 km apart, and over 300 local earthquakes were recorded with phase arrivals within the shear wave window. A significant number (≈ 37%) of the earthquakes recorded showed clear evidence of shear wave splitting: identifiable fast and slow shear wave arrivals with similar pulse shapes. Consistent polarization directions at particular stations were also observed, even when poor signal‐to‐noise ratio or scattering meant that no slow shear wave arrival could be identified. However, there were large station‐to‐station differences in the polarization directions. Correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double‐couple focal mechanism. The cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy. Large station‐to‐station differences in the polarization alignments, ranging from 61°±24° to 137°±18°, suggest that most anisotropy is confined to the top 2–3 km of the crust. However, there is evidence from one station for a small amount of pervasive anisotropy; if such a trend existed on the other stations, it could not be identified because of the large scatter in the data points. The measured delay times between split shear waves vary from 0.02 to 0.22 s, with a mean value of 0.1±0.06 s. This translates to a near‐surface shear wave velocity anisotropy of about 10%, with up to 2% pervasive anisotropy possible throughout the crust. This data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important. There is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment.
An analysis of ridge axis gravity roughness and spreading rate
Authors: Small Christopher , Sandwell David T.
Published: February 2008

Fast and slow spreading ridges have radically different morphologic and gravimetric characteristics. In this study, altimeter measurements from the Geosat Exact Repeat Mission (Geosat ERM) are used to investigate spreading rate dependence of the ridge axis gravity field. Gravity roughness provides an estimate of the amplitude of the gravity anomaly and is robust to small errors in the location of the ridge axis. We compute gravity roughness as a weighted root mean square (RMS) of the vertical deflection at 438 ridge crossings on the mid‐ocean ridge system. Ridge axis gravity anomalies show a decrease in amplitude with increasing spreading rate up to an intermediate rate of ∼60–80 mm/yr and almost no change at higher rates; overall the roughness decreases by a factor of 10 between the lowest and highest rates. In addition to the amplitude decrease, the range of roughness values observed at a given spreading rate shows a similar order of magnitude decrease with transition between 60 and 80 mm/yr. The transition of ridge axis gravity is most apparent at three relatively unexplored locations on the Southeast Indian Ridge and the Pacific‐Antarctic Rise; on these intermediate rate ridges the transition occurs abruptly across transform faults.
A quantitative comparison of two paleomagnetic records of the Cobb Mountain Subchron from North Atlantic deep‐sea sediments
Authors: Clement Bradford M. , Martinson Douglas G.
Published: February 2008

We present a new paleomagnetic record of the Cobb Mountain Subchron obtained from deep‐sea sediments cored at Ocean Drilling Program site 647 in the southern Labrador Sea. The details of the transitional field behavior documented by this record appear to be very similar to those recorded in a previously published record of this subchron obtained at Deep‐Sea Drilling Project (DSDP) site 609 in the North Atlantic (Clement and Kent, 1987). We used a quantitative correlation technique (Martinson et al., 1982) to establish statistically the degree of similarity between these two records and thereby constrain the spatial variability in these transitional fields. The error in the alignments is reduced significantly by aligning records of virtual geomagnetic pole positions rather than directions, indicating that these records document such large scale changes in the fields that we can not distinguish them from dipolar changes, given the proximity of these two sites. These replicate records of the Cobb Mountain Subchron provide evidence that deep‐sea sediments are capable of providing high resolution records of geomagnetic field behavior. A reexamination of the sequence of polarity transitions recorded at DSDP site 609 in light of these results suggests the presence of two preferred transitional field configurations. The field appears to change from one configuration to the other for several reversals and then back to the original configuration, suggesting that a geographical influence on the reversal process persists through this sequence. The variability in these reversal records provides insights into the response of the geodynamo to this geographical influence.
Joint inversion of fault zone head waves and direct P arrivals for crustal structure near major faults
Authors: Ben‐Zion Yehuda , Katz Simeon , Leary Peter
Published: February 2008

A newly recognized class of seismic phases, namely fault zone head waves arising from refraction at a transfault velocity contrast, provides additional constraints to direct P waves in inversions of near‐fault arrival time data for earthquake location and velocity structure. Incorporation of fault zone head waves in the inversion process increases the available data and broadens the spatial coverage of ray paths between sources and receivers. Equally important, the explicit recognition of fault zone head waves eliminates a source of error resulting from misidentifying head wave first arrivals as direct P waves. A simple algorithm for the joint inversion of fault zone head waves and direct P phases is illustrated with observed and synthetic data. Application to a small data set recorded by the borehole seismic network at Parkfield, California, provides separate seismic velocity depth profiles for the crustal blocks on the two sides of the San Andreas fault. For depths less than 3 km the obtained velocity contrast is 10–20%. For greater depths it decreases to 3–7%. Numerical tests with synthetic data show that for sources with known locations and for well located local earthquakes (randomized mislocation errors of up to 600 m) proper use of fault zone head waves can significantly improve the accuracy of the fitted velocity structure. The results of this work motivate the inclusion of fault zone head waves in state‐of‐the‐art joint structure‐hypocenter inversions using high quality near‐fault data.
Geosat‐derived geoid anomalies at medium wavelength
Authors: Cazenave A. , Houry S. , Lago B. , Dominh K.
Published: February 2008

Geosat profiles of the Exact Repeat Mission have been averaged over a 1‐year period and high‐pass‐filtered using inverse method techniques. The geoid surface constructed with both ascending and descending profiles shows at medium wavelengths band‐shaped anomalies preferentially elongated in the east‐west direction. These anomalies have an average amplitude of ∼30 cm and dominant wavelengths to 750km and 1100 km. We have performed numerous tests to show that the lineations are not artefacts created by the filtering process. Moreover, two‐dimensional (2‐D) filtering with the inverse method applied on a regional basis over the Pacific gives essentially similar results, indicating that the filtered geoid is not affected by directional bias. Seafloor topography in the Pacific filtered by 2‐D inverse method also shows east‐west trending depth anomalies positively correlated to medium‐wavelength geoid lineations. Along the East Pacific Rise, there is a clear correlation between geoid lineations and regional variations in axial depth. Cross‐spectral analysis carried out on geoid and topography data over the Pacific area gives coherence maxima at 750‐km and 1100‐km wavelengths and admittance values of 2–3 m/km. Observed admittance matches the magnitude and shape of admittance predicted by lithospheric cooling models, suggesting that the lineations are related to regional variations in the plate cooling process. Convection models produce much higher admittances than observed unless a low‐viscosity layer is assumed so that dynamical support cannot be completely discarded. In most instances, however, the position and direction of the lineations seem to be controlled by major fractures zones which is in favor of a lithospheric origin. In the south central Pacific where the lineations appear parallel to absolute plate motion, there may be a combination of both lithospheric and sublithospheric processes.
Positioning results with doris on SPOT2 after first year of mission
Authors: Cazenave A. , Valette J. J. , Boucher C.
Published: February 2008

A new space system named DORIS has been developed by the Centre National d'Etudes Spatiales in France with the collaboration of the Institut Geographique National for precise orbit determination and positioning. It is a Doppler‐based system with a worldwide ground network of transmitting beacons and an onboard receiver. The SPOT2 satellite launched in January 1990 carries DORIS as an experimental instrument. In this study, we present positioning results obtained from the analysis of the first 10 months of DORIS data through a semi‐dynamical approach (i.e. the satellite orbit is improved simultaneously with the beacon positions). External calibration with satellite laser ranging (SLR) and very long baseline interferometry (VLBI) positions indicates that DORIS accuracy from a few months of data is presently 15–20 cm rms in absolute and relative (baseline length > 1000 km) positioning. For very short baselines (< 100 km), external comparison with ground‐based geodetic measurements indicates a few cm accuracy. Month to month repeatability of both absolute and relative (baseline length > 1000 km) positioning results is in the range 10–20 cm rms. For the shortest baselines (< 100 km), the month to month repeatability is better than 5 cm in cartesian coordinates. This repeatability is only ∼ 2 cm in the north and east components whereas it is 12 cm in the vertical component, suggesting that tropospheric errors are important. It should be possible to improve these results in the future by reducing the residual orbit error, especially the atmospheric drag perturbation which was particularly troublesome in 1990, a year having very large solar activity and by improving modeling of the tropospheric correction.
Laboratory, numerical, and oceanic fossil turbulence in rotating and stratified flows
Authors: Gibson Carl H.
Published: February 2008

Turbulent eddy motions are produced, and characterized, by inertial‐vortex forces. Growth of the energy, or Obukhov, scale L0 is constrained by buoyancy and Coriolis forces at the stratified fossil Ozmidov LR0 and rotational fossil Hopfinger LΩ0 length scales, and constrained at all scales by viscous, buoyancy, and Coriolis forces at sizes proportional to the stratified fossil Kolmogorov LKF and rotational fossil Kolmogorov LKƒ length scales, respectively. Universal constants of proportionality between turbulence wavelengths and these fossil turbulence length scales at their respective buoyant‐inertial, Coriolis‐inertial, buoyant‐inertial‐viscous, and Coriolis‐inertial‐viscous transitions are inferred from available laboratory, numerical, and field studies. It is important to distinguish between turbulence, which entrains, and these nonturbulent, or fossil turbulence, internal waves, Coriolis‐inertial eddies, and viscous flow regimes, which are nonlinear remnants of turbulence but do not entrain and have other properties different from turbulence. Hydrodynamic phase diagrams based on the universal constants of transition may be used to classify microstructure and mesostructure as actively turbulent, active‐fossil, fossil, or nonturbulent. One advantage of making these distinctions is to avoid undersampling errors from data sets consisting only of fossil turbulence microstructure and mesostructure. This may improve the reliability of dissipation methods used for estimating average heat, mass, and momentum fluxes in the ocean and atmosphere from microstructure and mesostructure data.
Film drop distributions from bubbles bursting in seawater
Authors: Resch François , Afeti George
Published: February 2008

The liquid droplets produced from the breaking up of the film cap of oceanic bubbles have long been considered as constituting a significant fraction of the marine aerosol. The size and number distributions of these film drops are investigated for single bubbles that burst in seawater under laboratory conditions. Optical particle detection and counting techniques are used to size the droplets generated from bubbles varying in diameter from about 1 to 6 mm. Parallel measurements are also made using a laser holographic setup for bubbles up to about 10 mm in diameter. Taken together, a film drop size spectrum extending from 0.8 to 500 μm is covered. From these results, an experimental correlation between the number of film drops generated and the parent bubble diameter is proposed. Based on this correlation and published data concerning the number of jet drops produced per bubble, a crossover point for bubbles that produce predominantly either film drops or jet drops is located at a bubble diameter of about 1.8 mm. In other words, bubbles larger than this threshold diameter produce more film drops than jet drops, while smaller bubbles generate more jet drops. A couple of film drops as large as 500 μm for a 10 mm bubble are detected. Although these maximum sizes diminish with decreasing bubble diameter, they happen to be much larger than previously thought.
A study of the inertial‐dissipation method for computing air‐sea fluxes
Authors: Edson J. B. , Fairall C. W. , Mestayer P. G. , Larsen S. E.
Published: February 2008

The inertial‐dissipation method has long been used to estimate air‐sea fluxes from ships because it does not require correction for ship motion. A detailed comparison of the inertial‐dissipation fluxes with the direct covariance method is given, using data from the Humidity Exchange Over the Sea (HEXOS) main experiment, HEXMAX. In this experiment, inertial‐dissipation packages were deployed at the end of a 17 m boom, in a region relatively free of flow distortion; and on a mast 7 m above the platform (26 m above the sea surface) in a region of considerable flow distortion. An error analysis of the inertial‐dissipation method indicates that stress is most accurately measured in near‐neutral conditions, whereas scalar fluxes are most accurately measured in near‐neutral and unstable conditions. It is also shown that the inertial‐dissipation stress estimates are much less affected by the flow distortion caused by the platform as well as by the boom itself. The inertial‐dissipation (boom and mast) and boom covariance estimates of stress agree within ±20%. The latent heat flux estimates agree within approximately ±45%. The sensible heat flux estimates agree within ±26% after correction for velocity contamination of the sonic temperature spectra. The larger uncertainty in the latent heat fluxes is due to poor performance of our Lyman‐α hygrometers in the sea spray environment. Improved parameterizations for the stability dependence of the dimensionless humidity and temperature structure functions are given. These functions are used to find a best fit for effective Kolmogorov constants of 0.55 for velocity (assuming a balance of production and dissipation of turbulent kinetic energy) and 0.79 for temperature and humidity. A Kolmogorov constant of 0.51 implies a production‐dissipation imbalance of approximately 12% in unstable conditions.
Lagrangian observations in the Iberian coastal transition zone
Authors: Haynes R. , Barton E. D.
Published: February 2008

Satellite‐tracked drifters deployed in the Iberian coastal transition zone were gradually carried northward in a poleward flowing continental slope current between September 1986 and March 1987 in convoluted trajectories which revealed the ubiquitous presence of mesoscale eddies. We estimate the rate of dispersion from the Lagrangian statistics of the drifting buoys and present a comparison of the results with those obtained from other oceanic regions. The dispersion of the drifters is well modelled by a simple description of eddy diffusion assuming homogeneous isotropic turbulence (Taylor's theory). The assumption of homogeneity and stationarity facilitated the derivation of stable Lagrangian statistics. After correction for non stationarity, the diffusivities obtained were k11 = 3.4 × 106 cm2 s−1 in the zonal direction and K22 = 2.5 × 106 cm2 s−1 in the meridional direction. The Lagrangian integral time scales were respectively T11 = 1.25 days and T22 = 1.9 days. Our results and those of other authors, in environments ranging from truly oceanic to eastern boundaries, have all indicated that Taylor's theory fits the observations well. As a first order approximation the assumption of a homogeneous and stationary field would appear quite robust.
Water mass subduction and the transport of phytoplankton in a coastal upwelling system
Authors: Washburn Libes , Kadko David C. , Jones Burton H. , Hayward Thomas , Kosro P. Michael , Stanton Timothy P. , Ramp Steve , Cowles Timothy
Published: February 2008

Observations during the Coastal Transition Zone (CTZ) experiment in summer 1988 reveal the presence of deep phytoplankton layers in a coastal upwelling system. The layers occur throughout the CTZ study area, including a strong baroclinic jet which was present over the period of the experiment. On the basis of a variety of bio‐optical, hydrographic, and geochemical indicators, it is concluded that the water masses associated with the layers result from subduction processes. Criteria are developed for identification of subducted water masses based on the beam attenuation coefficient, chlorophyll fluorescence, and distribution of light in the water column. Temperature‐salinity characteristics are consistent with two source regions for the subducted layers, one nearshore and a second farther offshore. Most of the layers correspond to the inshore source which is apparently distributed alongshore. Subducted water masses are found in all six grid surveys of the CTZ experiment and probably result from a variety of physical processes. One of these is flow along sloping isopycnal surfaces due to advection and mixing processes. Advection time scales for flow out the axis of the jet range from a few days to a few weeks, depending on the depth of a particular surface, and the bio‐optical indicators for subduction processes persist over these time scales.
The structure of the transition zone between coastal waters and the open ocean off northern California, winter and spring 1987
Authors: Kosro P. Michael , Huyer Adriana , Ramp Steven R. , Smith Robert L. , Chavez Francisco P. , Cowles Timothy J. , Abbott Mark R. , Strub P. Ted , Barber Richard T. , Jessen Paul , Small Lawrence F.
Published: February 2008

Physical and biological fields in the coastal transition zone off northern California were measured during February, March, May and June 1987 in an extended alongshore region between 60 km and 150 km offshore. The spring transition, as seen in coastal sea level and winds, occurred in mid‐March. Surface variability during the two spring cruises was stronger and of larger scale than that seen during the two winter cruises. An equatorward‐tending current, flowing along the boundary between low steric sea level inshore and high steric sea level offshore, dominated both the directly‐measured (acoustic Doppler current profiler) and geostrophic current fields during spring. Current jets of comparable strength directed both offshore and onshore were seen off Cape Mendocino and Point Arena; these evolved significantly in the 3 weeks between cruises. Inshore of the current, properties associated with upwelled water were found near the surface, including low temperature and high salinity, nutrients and chlorophyll; offshore of the current, waters were warmer, less saline, lower in nutrients and more oligotrophic. Geostrophic and directly measured volume transports in the current were about 2–3 Sv. Isopycnals inshore of the spring upwelling front were displaced vertically by O(40–80 m) from their depths during the winter survey; these displacements extended deep into the water column and were largely independent of depth between 100 and 400 m. Surface mixed layers tended to be deep in winter and shallower inshore of the upwelling front in spring. A connection between the equatorward‐tending frontal jet off northern California and the more well‐studied California Current further south is suggested by the similarity of their transports and of their dynamic height values.
The meandering Gulf Stream as seen by the Geosat altimeter: Surface transport, position, and velocity variance from 73° to 46°W
Authors: Kelly Kathryn A.
Published: February 2008

The surface geostrophic velocity field for the Gulf Stream region was analyzed for the position, structure and surface transport of the Gulf Stream for 2.5 years of the Geosat altimeter Exact Repeat Mission. Synthetic data using a Gaussian velocity profile were generated and fit to the sea surface residual heights to create a synthetic mean sea surface height field and profiles of absolute geostrophic currents. Most of cross‐track velocity variance was due to the meandering of the Gulf Stream, rather than to rings or complicated velocity profiles, which suggested that the simple model parameters are an efficient description of the Gulf Stream fluctuations. An analysis of the model parameters and the actual geostrophic velocity profiles revealed two different flow regimes for the Gulf Stream connected by a narrow transition region coincident with the New England Seamount Chain. The upstream region was characterized by relatively straight Gulf Stream paths, long Eulerian time scales and eastward propagating meanders. The downstream region had more large meanders, no consistent propagation direction and shorter Eulerian time scales. A 25% reduction in surface transport occurred in the transition region, with a corresponding reduction in current speed and no change in Gulf Stream width. A significant anomaly in surface transport near 69°W suggested the existence of small‐scale recirculation gyres there. The highest‐variance empirical orthogonal functions of position and surface transport were significantly correlated, with larger surface transports leading more northerly positions by a month.
Tidal and atmospheric forcing of the upper ocean in the Gulf of California: 1. Sea surface temperature variability
Authors: Paden Cynthia A. , Abbott Mark R. , Winant Clinton D.
Published: February 2008

Two years of satellite infrared imagery (1984–1986) are used to examine the sea surface temperature (SST) variability in the northern Gulf of California. Empirical orthogonal functions (EOFs) of the temporal and spatial SST variance for 20 monthly mean images show that the dominant SST patterns are generated by spatially varying tidal mixing in the presence of seasonal heating and cooling. These patterns are modified in the fall and winter when shelf temperatures south of Tiburon Island cool in response to upwelling‐favorable winds. These same winds bring cold, dry air from off the continental United States, causing local cooling of the shallow northern shelf. During the rest of the year, the broad, shallow shelves are warmer than offshore. The seasonally reversing temperature patterns are consistent with recent hydrographic observations which show a cyclonic surface circulation in the summer and a weaker anticyclonic circulation during the rest of the year. Atmospheric forcing of the northern gulf appears to occur over large spatial scales. Area‐averaged SSTs for the Guaymas Basin, island region, and northern basin show significant fluctuations which are highly correlated. These fluctuations in SST correspond to similar fluctuations in the air temperature which are related to synoptic weather events over the gulf. During periods of particularly low wind speeds, the air temperature over the gulf increases dramatically. By afternoon, intense heating of the sea surface results in the appearance of warm SST anomalies in the satellite data. These SSTs are approximately 2°C warmer than surrounding SSTs and most likely occur as a result of a spatially varying wind field. A regression analysis of the SST relative to the fortnightly tidal range shows that tidal mixing occurs over the sills in the island region as well as on the shallow norther shelf. Mixing over the sills, however, occurs as a result of large breaking internal waves or internal hydraulic jumps which mix water over the upper 300–500 m. This mixing pumps heat away from the surface, deep into the water column, there by maintaining the cool SSTs. Since mixing occurs over greater depths in the island region, the temperatures there are much colder than those generated by tidal mixing on the shallow shelves, resulting in the persistent pool of cool water evident in the satellite data. This cooler water is mixed horizontally by the basin‐scale circulation, lowering the SSTs over much of the northern gulf. These reduced SSTs have a large impact on the surface heat flux by lowering the saturation vapor pressure of the air. As a result, the amount of heat lost to the atmosphere through the latent or evaporative heat flux is reduced. This may explain why the Gulf of California gains heat on an annual average rather than losing heat as occurs in the Mediterranean and Red seas where tidal mixing is not significant.
Impact of jittered sampling on conventional spectral estimates
Authors: Moore M. I. , Thomson P. J.
Published: February 2008

We examine the effect of jittered sampling (perturbations about a regular sampling scheme) on spectral analysis from a theoretical point of view, developing a general model for the effect in one dimension and then specializing to examples of Gaussian jitter. We find that the spectral integral or process variance is unaffected by jitter, but weight is cut off the spectrum according to a roll‐off function which is the squared modulus of the jitter characteristic function (Fourier transform of the distribution function). This weight is redistributed evenly across the spectrum for uncorrelated jitter and nearly so for broadband correlated jitter. However, the redistributed weight can be concentrated by strong correlation or periodicity in the jitter. The paper concentrates on the effects of jitter correlation, but we note that the distribution function, which decides the roll‐off, is also of key importance.
A multitracer study of radionuclides in Lake Zurich, Switzerland: 1. Comparison of atmospheric and sedimentary fluxes of 7Be, 10Be, 210Pb, 210Po, and 137Cs
Authors: Schuler C. , Wieland E. , Santschi P. H. , Sturm M. , Lueck A. , Bollhalder S. , Beer J. , Bonani G. , Hofmann H. J. , Suter M. , Wolfli W.
Published: February 2008

Atmospheric and sedimentary fluxes of natural ( i.e., 210Pb, 210Po, 7Be, and 10Be) and artificial (i.e., Chernobyl 137Cs ) radionuclides and particles through Lake Zurich (at 50 m and 130 m depth) between 1983 and 1987 were compared in order to establish trace metal pathways and their kinetics. Atmospheric fluxes averaged 0.83 dpm cm−2 yr−1 for 210Pb and 16.0 dpm cm−2 yr−1 for 7Be during 1984 to 1987. Vertical fluxes of 210Pb and 7Be, associated with settling particles, were measured in sediment traps deployed at 50 m and 130 m depth at a station near the deepest part of Lake Zurich. Average fluxes of 210Pb and 7Be at 50 m depth were 0.94 and 4.90 dpm cm−2 yr−1, respectively, and 16.5×107 atoms m−2 d−1 for 10Be. Sediment traps at 50 m quantitatively collected atmospherically deposited 210Pb falling through the water column. At 130 m depth, immediately above seasonally anoxic bottom waters, 210Pb, 10Be, and 137Cs fluxes were higher than at 50 m by up to 60% at times during the summer stagnation period. Sediment inventories of 210Pb and 10Be are in agreement with atmospheric nuclide fluxes, suggesting only moderate recycling of these nuclides in the bottom waters, whereas 137Cs inventory in the sediments is lower than expected from sediment trap fluxes, indicating immobilization of this nuclide.
Modeling the calcite lysocline
Authors: Archer David
Published: February 2008

A numerical model of calcite dissolution in contact with sediment pore water is used to predict the depth and shape of the calcite lysocline in the deep sea. Model results are compared with lysocline data from 13 regions in the Atlantic, Pacific, and Indian Oceans. The model lysocline shape is sensitive to the calcite dissolution rate constant, the calcite, organic carbon, and refractory material rain rates, and the rates of oxic versus anoxic organic carbon degradation in the sediment. The model is able to reproduce the observed lysocline, within the constraints of the sediment trap and calcite accumulation data, using a calcite dissolution rate constant of 30–100%d−1, a molar ratio of organic carbon to calcite rain rates of 0.5–1.0, and an initial CaCO3 fraction of 90% (excluding organic carbon). This rate constant is consistent with microelectrode results presented by Archer et al. [1989]. The model predicts that 30–50% of the calcite rain to the sea floor at the saturation horizon dissolves in response to organic carbon respiration, consistent with previous modeling studies. The range in the best fit value of the organic‐inorganic carbon rain rates arises from model sensitivity to uncertainty in the rate of anoxic carbon degradation in the sediment and in the rain rate of refractory material, rather than from scatter in the data. Lysocline data from the western equatorial Atlantic are anomalous to the rest of the data; this anomaly may be explained by high rates of refractory material sedimentation from the Amazon River.
High‐resolution measurement of surface‐active film redistribution in ship wakes
Authors: Peltzer R. D. , Griffin O. M. , Barger W. R. , Kaiser J. A. C.
Published: February 2008

Surface‐active films are found in all parts of the ocean. They are easily detected visually on the ocean surface when the wind speed is less than 5 m/s (10 knot). They are generally of biological origin and consist mainly of fulvic and humic acids, carbohydrates, proteins, and lipids. Their molecules have hydrophilic (partially water soluble) heads and hydrophobic (insoluble carbon chain) tails. Surface‐active films that have been concentrated at the edges of the centerline wake generated by the passage of a surface ship strongly influence the propagation of the short gravity and capillary waves which interact with electromagnetic waves at both radar and visible wavelengths. An accurate method for detecting the presence of surface films and measuring the fine‐scale surface film pressure and surface elasticity distributions on a water surface was developed at the Naval Research Laboratory (NRL). These measurements, together with model‐scale flow visualization experiments, have allowed us to develop a better understanding of the role these ship‐generated surface‐active film distributions play in the formation of the dark centerline and “railroad track” or dark lines which are aligned at some narrow angle to the ship's track in synthetic aperture radar (SAR) images of ship wakes.
Modeling the seasonal undercurrents in the Somali Current system
Authors: Jensen Tommy G.
Published: February 2008

Seasonal changes in the Somali Current system are studied using a four‐layer model where the lowest layer is at rest. The results suggest that barotropic instability is likely to cause the generation of the Great Whirl in early June. We find a very good agreement between the observed undercurrents and the simulations in the model. Equatorial onshore flow below the thermocline in June is associated with the disappearance of the undercurrent below the Somali Current. The early return of the southward undercurrent in the fall is caused by baroclinic instability of the Great Whirl followed by a reversal to eastward undercurrents along the equator. Remote winds are suggested to control the intermediate depth flow along the Somali coast acting through equatorial waves and Rossby waves generated along the west coast of India. Experiments where the duration of the summer monsoon is extended show that the initial decrease in the magnitude of the Great Whirl is due to eastward and downward energy transfer rather than due to relaxation of the wind. The model solutions suggest that baroclinic instability plays an important role in the decay of the Great Whirl.
Hydrographic patterns and vertical mixing in the equatorial Pacific along 150°W
Authors: Carr Mary‐Elena , Oakey Neil S. , Jones Burton , Lewis Marlon R.
Published: February 2008

The WEC88 cruise sampled along a meridional transect from 15°N to 15°S along 150°W from February 17 to March 18, 1988, with a 6‐day time series at the equator. The large‐scale hydrographic patterns were typical for boreal spring. Equatorial maxima in dissipation of turbulent kinetic energy ∈, and of thermal variance χ, were found between 2°N and 2°S for the top 60 m. The equatorial time series coincided with a shift from southward to northward velocity, which returned the zonal current system to the equator. This led to a decrease in temperature, and increases in salinity, nutrient, and chlorophyll concentrations in the surface layer. Vertical diffusivity as well as ∈ and χ increased with the observed intensification of the Equatorial Undercurrent. Maximum values of ∈ and χ were observed at around 55 m, and the temporal trends occurred first at depth. Turbulent heat flux out of the mixed layer was the same order of magnitude as the penetrative irradiance at that depth. Maximum vertical heat flux occurred at depth in response to large diffusivity coefficients. The Richardson number was useful in predicting the regions of enhanced mixing in the meridional transect. However, for the equatorial time series, where the Ri was less than 0.45, intensity of dissipation was not proportional to Richardson number.