bluuebunny/crossref_metadata_2025 huggingface dataset:
Hillslope evolution by nonlinear, slope‐dependent transport: Steady state morphology and equilibrium adjustment timescales
Authors: Roering Joshua J. , Kirchner James W. , Dietrich William E.
Published: February 2004

Soil‐mantled hillslopes are typically convex near the crest and become increasingly planar downslope, consistent with nonlinear, slope‐dependent sediment transport models. In contrast to the widely used linear transport model (in which sediment flux is proportional to slope angle), nonlinear models imply that sediment flux should increase rapidly as hillslope gradient approaches a critical value. Here we explore how nonlinear transport influences hillslope evolution and introduce a dimensionless parameter ΨLto express the relative importance of nonlinear transport. For steady state hillslopes, with increasing ΨL(i.e., as slope angles approach the threshold angle and the relative magnitude of nonlinear transport increases), the zone of hillslope convexity becomes focused at the hilltop and side slopes become increasingly planar. On steep slopes, rapid increases in sediment flux near the critical gradient limit further steepening, such that hillslope relief and slope angle are not sensitive indicators of erosion rate. Using a one‐dimensional finite difference model, we quantify hillslope response to changes in baselevel lowering and/or climate‐related transport efficiency and use an exponential decay function to describe how rapidly sediment flux and erosion rate approach equilibrium. The exponential timescale for hillslope adjustment decreases rapidly with increasing ΨL. Our results demonstrate that the adjustment timescale for hillslopes characteristic of the Oregon Coast Range and similar steep, soil‐mantled landscapes is relatively rapid (≤50 kyr), less than one quarter of the timescale predicted by the linear transport model.
Multiparameter two‐dimensional inversion of scattered teleseismic body waves 2. Numerical examples
Authors: Shragge J. , Bostock M. G. , Rondenay S.
Published: February 2004

In this paper, we investigate the formal inversion of synthetic teleseismic P coda waves for subsurface elastic properties using a ray theoretic approach which assumes single scattering [Bostock et al., this issue]. We consider a model comprising an idealized lithospheric suture zone whose geometrical configuration is drawn from previous deep crustal seismic studies. Two‐dimensional, pseudospectral synthetic seismograms representing plane waves propagating through this model are preprocessed to extract an estimate of the scattered wave field generated by short‐wavelength structure. These data are employed in a series of numerical simulations which examine the dependence of multiparameter inversion results on a range of input parameters. In particular, we demonstrate (1) the contrasting sensitivity which forward and backscattered waves display to structural recovery, (2) the diminution of the problem null‐space accompanied by increased source coverage, (3) improvements in model reconstruction achieved through simultaneous treatment of multiple scattering modes, and (4) the robustness of the method for data sets with noise levels and receiver geometries that approach those of field experiments.
Generation and evolution of hydraulic underpressures at Wellenberg, Switzerland
Authors: Vinard Pascal , Bobet Antonio , Einstein Herbert H.
Published: February 2004

Pore pressures below hydrostatic (underpressures) have been measured at the Wellenberg site in Switzerland, that Nagra (Swiss National Cooperative for the Disposal of Radioactive Waste) has selected as a potential repository for short‐lived radioactive waste. The rock consists of overconsolidated Cretaceous marls and Tertiary shales, characterized by low to very low hydraulic conductivity. The maximum measured underpressures are between 4.8 and 7.1 MPa below hydrostatic and occur at depths of ∼ 800 m below ground. Results from this investigation show that mechanical unloading is capable of producing the observed underpressures. Unloading can be produced in conjunction with denudation (uniform erosion), differential erosion, and glaciation. The effect of these processes, of relevant material parameters, and of boundary conditions are investigated in two phases. In the first phase, simple one‐dimensional and two‐dimensional “column” models are used under uniform erosion (denudation) conditions. These investigations show that within the range of the investigated parameters and physical processes, the hydraulic conductivity, the stiffness, and the drainage boundary conditions are more important than erosion rate, horizontal stresses, and nonlinear material behavior. However, although underpressures are produced with parameters and conditions corresponding reasonably well to observations, uniform rates of denudation alone cannot account for the actually encountered magnitude of underpressure. The second phase in which differential erosion and, particularly, the effect of glaciation are considered produces very realistic results. This is established by examining a number of combinations of denudation, differential erosion, and glacial loading and unloading.
Deep crustal structure of the Chicxulub impact crater
Authors: Christeson Gail L. , Nakamura Yosio , Buffler Richard T. , Morgan Jo , Warner Mike
Published: February 2004

We present the results of a wide‐angle seismic survey conducted over the Chicxulub impact crater. Profile Chicx‐A/A1 is a chord across the offshore portion of the crater, and Chicx‐B/F is an onshore‐offshore profile through the center of the crater. We use travel times recorded by 32 ocean bottom seismograph and 19 land‐based receivers to model the crustal structure of the crater. The primary feature in the shallow velocity structure is a low‐velocity Tertiary basin, the edge of which correlates with a steep gravity gradient in some regions of our survey. Directly beneath the Tertiary basin at the center of the crater is a region of relatively low velocities and densities that is interpreted as a unit of melt rocks (either suevite breccias with isolated melt pods or suevite breccias overlying a coherent melt sheet). This unit has a thickness of ∼1 km and a diameter of ∼100 km. Central uplift with a diameter of 40–60 km is observed on Chicx‐B/F. A lower limit on the vertical extent of uplift is 9 km. Shallow basement is also observed along the northwest portion of Chicx‐B/F, associated with a prominent northwest trending high in the gravity field. Superimposed on a regional trend is Moho uplift of ∼1 km near the center of the Chicx‐A/A1 profile, with adjacent Moho deepening of ∼1.25–1.5 km. The Moho topography may be related to deformation processes associated with the formation of the outer ring or to the excavation and collapse of the transient cavity.
Upper mantle S velocity structure of central and western South America
Authors: van der Lee Suzan , James David , Silver Paul
Published: February 2004

We have used broadband seismograms recorded at portable and permanent seismic stations in South America to image the three‐dimensional S velocity structure beneath central and western South America. We have interactively fitted the waveforms of S and surface wave trains in >500 seismograms from 90 earthquakes and 60 different stations. The constraints on upper mantle S velocity structure provided by the waveform fits have been combined with independent estimates of depth to the Mohorovičić discontinuity, yielding model SA99. The S velocity structure of SA99 reveals a highly heterogeneous upper mantle with a maximum peak to peak difference of 1 km/s above 100 km. The heterogeneity seems to be correlated with tectonic and geologic domains. SA99 shows high lithospheric S velocities for the Precambrian cratons. The Amazon basin is also underlain by high S velocities, while three other intraplatform basins are underlain by low S velocities at various depths. SA99 shows extremely low velocities in the mantle wedge beneath the volcanically active Andes. We explain these low S velocities as originating from realistic amounts of volatiles, released by the subducting Nazca crust, and resulting amounts of partial melt within the peridotitic mantle wedge, without requiring elevated temperatures. While SA99 could be consistent with diffuse influx for corner flow in the mantle wedge, it does not support models that require influx of subcontinental upper mantle into the mantle wedge through a laterally uniform low‐viscosity channel.
Upper mantle anisotropy from long‐period P polarization
Authors: Schulte‐Pelkum Vera , Masters Guy , Shearer Peter M.
Published: February 2004

We introduce a method to infer upper mantle azimuthal anisotropy from the polarization, i.e., the direction of particle motion, of teleseismic long‐period P onsets. The horizontal polarization of the initial P particle motion can deviate by >10° from the great circle azimuth from station to source despite a high degree of linearity of motion. Recent global isotropic three‐dimensional mantle models predict effects that are an order of magnitude smaller than our observations. Stations within regional distances of each other show consistent azimuthal deviation patterns, while the deviations seem to be independent of source depth and near‐source structure. We demonstrate that despite this receiver‐side spatial coherence, our polarization data cannot be fit by a large‐scale joint inversion for whole mantle structure. However, they can be reproduced by azimuthal anisotropy in the upper mantle and crust. Modeling with an anisotropic reflectivity code provides bounds on the magnitude and depth range of the anisotropy manifested in our data. Our method senses anisotropy within one wavelength (250 km) under the receiver. We compare our inferred fast directions of anisotropy to those obtained from Pn travel times and SKS splitting. The results of the comparison are consistent with azimuthal anisotropy situated in the uppermost mantle, with SKS results deviating from Pn and Ppol in some regions with probable additional deeper anisotropy. Generally, our fast directions are consistent with anisotropic alignment due to lithospheric deformation in tectonically active regions and to absolute plate motion in shield areas. Our data provide valuable additional constraints in regions where discrepancies between results from different methods exist since the effect we observe is local rather than cumulative as in the case of travel time anisotropy and shear wave splitting. Additionally, our measurements allow us to identify stations with incorrectly oriented horizontal components.
Crustal deformation along the Altyn Tagh fault system, western China, from GPS
Authors: Shen Zheng‐Kang , Wang Min , Li Yanxing , Jackson David D. , Yin An , Dong Danan , Fang Peng
Published: February 2004

We collected GPS data from the southern Tarim basin, the Qaidam basin, and the western Kunlun Shan region between 1993 and 1998 to determine crustal deformation along the Altyn Tagh fault system at the northern margin of the Tibetan plateau. We conclude from these data that the Altyn Tagh is a left‐lateral strike slip fault with a current slip rate of ∼9 mm/yr, in sharp contrast with geological estimates of 20–30 mm/yr. This contrast poses an enigma: because the GPS data cover a wider region than the geologic data, they might be expected to reveal somewhat more slip. We also find that the Tarim and Qaidam basins behave as rigid blocks within the uncertainty of our measurements, rotating clockwise at a rate of ∼11 and ∼4.5 nrad/yr, respectively, with respect to the Eurasia plate. The rotation of the Tarim basin causes convergence across the Tian Shan, increasing progressively westward from ∼6 mm/yr at 87°E to ∼18 mm/yr at 77°E. Our data and other GPS data suggest that the Indo‐Asia collision is mainly accommodated by crustal shortening along the main Himalayan thrust system (∼53%) and the Tian Shan contractional belt (∼19%). Eastward extrusion of the Tibetan plateau along the Altyn Tagh and Kunlun faults accommodates only ∼23% of the Indo‐Asia convergence.
Fluvial incision and tectonic uplift across the Himalayas of central Nepal
Authors: Lavé J. , Avouac J. P.
Published: February 2004

The pattern of fluvial incision across the Himalayas of central Nepal is estimated from the distribution of Holocene and Pleistocene terraces and from the geometry of modern channels along major rivers draining across the range. The terraces provide good constraints on incision rates across the Himalayan frontal folds (Sub‐Himalaya or Siwaliks Hills) where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces. Farther north and upstream, in the Lesser Himalaya, prominent fill terraces were deposited, probably during the late Pleistocene, and were subsequently incised. The amount of bedrock incision beneath the fill deposits is generally small, suggesting a slow rate of fluvial incision in the Lesser Himalaya. The terrace record is lost in the high range where the rivers are cutting steep gorges. To complement the terrace study, fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate. This approach allows quantification of the effect of variations in channel slope, width, and discharge on the incision rate of a river; the determination of incision rates requires an additional lithological calibration. The two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared. In the Sub‐Himalaya, river incision is rapid, with values up to 10–15 mm/yr. It does not exceed a few millimeters per year in the Lesser Himalaya, and rises abruptly at the front of the high range to reach values of ∼4–8 mm/yr within a 50‐km‐wide zone that coincides with the position of the highest Himalayan peaks. Sediment yield derived from the measurement of suspended load in Himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range. The observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat‐ramp‐flat geometry of the Main Himalayan Thrust fault. The morphology of the range reflects a dynamic equilibrium between present‐day tectonics and surface processes. The sharp relief together with the high uplift rates in the Higher Himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the Tibetan Plateau driven by late Cenozoic climate change, or late Miocene reactivation of the Main Central Thrust.
Submersible study of an oceanic megamullion in the central North Atlantic
Authors: Tucholke Brian E. , Fujioka Kantaro , Ishihara Takemi , Hirth Greg , Kinoshita Masataka
Published: February 2004

Recently discovered megamullions on the seafloor have been interpreted to be the exhumed footwalls of long‐lived detachment faults operating near the ends of spreading segments in slow spreading crust. We conducted five submersible dives on one of these features just east of the rift valley in the Mid‐Atlantic Ridge at 26°35′N and obtained visual, rock sample, gravity, and heat flow data along a transect from the breakaway zone (where the fault is interpreted to have first nucleated in ∼2.0–2.2 Ma crust) westward to near the termination (∼0.7 Ma). Our observations are consistent with the detachment fault hypothesis and show the following features. In the breakaway zone, faulted and steeply backtilted basaltic blocks suggest rotation above a deeper shear zone; the youngest normal faults in this sequence are interpreted to have evolved into the long‐lived detachment fault. In younger crust the interpreted detachment surface rises as monotonously flat seafloor in a pair of broad, gently sloping domes that formed simultaneously along isochrons and are now thinly covered by sediment. The detachment surface is locally littered with basaltic debris that may have been clipped from the hanging wall. The domes coincide with a gravity high that continues along isochrons within the spreading segment. Modeling of on‐bottom gravity measurements and recovery of serpentinites imply that mantle rises steeply and is exposed within ∼7 km west of the breakaway but that rocks with intermediate densities prevail farther west. Within ∼5 km of the termination, small volcanic cones appear on the detachment surface, indicating melt input into the footwall. We interpret the megamullion to have developed during a phase of limited magmatism in the spreading segment, with mantle being exhumed by the detachment fault <0.5 m.y. after its initiation. Increasing magmatism may eventually have weakened the lithosphere and facilitated propagation of a rift that terminated slip on the detachment fault progressively between ∼1.3 m.y. and 0.7 m.y. Identifiable but low‐amplitude magnetic anomalies over the megamullion indicate that it incorporates a magmatic component. We infer that much of the footwall is composed of variably serpentinized peridotite intruded by plutons and dikes.
Near‐axis subsidence rates, hydrothermal circulation, and thermal structure of mid‐ocean ridge crests
Authors: Cochran James R. , Buck W. Roger
Published: February 2004

We systematically investigated near‐axis subsidence on the ridge flanks of intermediate and fast spreading mid‐ocean ridges using bathymetric data from well‐surveyed portions of the Southeast Indian Ridge (spreading at 72–76 mm/yr), the northern East Pacific Rise (91–96 mm/yr), and the southern East Pacific Rise (144 mm/yr). In all three regions, the mean subsidence rate of young (<1–1.4 Ma) seafloor is less than 220 m/m.y.1/2. The distribution of individual estimates shows a distinct peak at 180–220 m/m.y.1/2 with few profiles having subsidence rates greater than 275 m/m.y.1/2. The observed subsidence rates are significantly lower than both the worldwide average (∼350 m/m.y.1/2) and subsidence rates observed for older lithosphere at the same ridge segments. Intense hydrothermal circulation at the ridge axis can result in low subsidence rates on the adjacent ridge flanks provided the vigorous flow is confined to the immediate vicinity (<∼5 km) of the axis. According to our model, the extremely vigorous hydrothermal circulation ceases off‐axis, and conductive heat flow becomes a primary mechanism of vertical heat transport on the ridge flanks. The very low geothermal gradient within the cooled portion of the uppermost lithosphere retards conductive cooling, and the cooled area needs to be heated from below before a geothermal gradient can be established which permits significant heat to be conducted out of the lithosphere. As a result, subsidence in very young (∼0.1–1 Ma) lithosphere is suppressed. A simple one‐dimensional thermal model with a Nusselt number parameterization was used to estimate the effect of hydrothermal circulation. An upper layer with a high Nusselt number and the half‐space that it overlies are initially at a temperature of 1300°C and the surface is maintained at 0°C. After 0.1 m.y. of cooling (about 3.5 to 7 km from the spreading axis), the Nusselt number of the top layer is set to 1 so that normal conduction is simulated in the cooled layer. We used an explicit finite difference method to solve for the temporal changes in temperature with depth. This model produces subsidence rates in the range that we observe for Nu in the range of 15–30. Isotherms resulting from the modeling imply rapid lithospheric thickening very near the axis, which is incompatible with most current models for the formation of the bathymetric axial high observed at fast spreading mid‐ocean ridges.
A constitutive law for low‐temperature creep of water‐saturated sandstones
Authors: Ngwenya Bryne T. , Main Ian G. , Elphick Stephen C. , Crawford Brian R. , Smart Brian G. D.
Published: February 2004

An accurate predictive model for the long‐term strength of the continental lithosphere is important in a range of geophysical and geodynamic problems. While laboratory experiments are consistent with Mohr‐Coulomb brittle faulting in the cold, upper continental crust, there is increasing evidence that time‐dependent processes may also be important in these rocks, even at low temperature. However, there is some ambiguity as to the exact form of the constitutive law for describing time‐dependent behavior of upper crustal rocks. Here we present results of room temperature creep experiments on a suite of water‐saturated sandstones spanning a range of petrophysical and rheological properties and underlying deformation mechanisms. On physical and microstructural grounds our analysis suggests that a modified power law creep, of the form ε = A′(σd ‐ σf)n′, where σd is the differential stress and σf is the long‐term failure (fundamental) strength, provides a more complete description of the experimental data. In particular, the parameters can be used to differentiate between sandstone types, with A′, σf and η′ varying systematically with cementation state, rock rheology, and confining pressure. The fundamental strength (σf) for time‐dependent deformation varies much more than the other parameters of the distribution, making it a potentially sensitive indicator of underlying creep mechanisms. Further tests would be needed to prove the constitutive law on a wider range of rock types and to prove that the three‐parameter model is statistically better in the general case.
The static shape of yield strength fluids slowly emplaced on slopes
Authors: Osmond David I. , Griffiths Ross W.
Published: February 2004

A theoretical solution is presented for the static three‐dimensional shape of a fluid having a finite yield strength after it is extruded slowly from a small source on an inclined plane. The solution is compared to the shapes obtained in laboratory experiments in which slurries of kaolin in polyethylene glycol wax were ejected, slowly and in small incremental volumes, onto a slope. This study is aimed at understanding the shape of highly silicic lava flows, such as large rhyolite flows which occur on steep mountain sides or on gentle slopes, and the role of a finite yield strength in governing the slow spreading and the final static shape. The results indicate that it is possible to obtain an estimate of the yield strength of a fluid which is emplaced under such circumstances, using only horizontal aspect ratios and the slope of the underlying ground.
Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)
Authors: Van der Woerd J. , Xu Xiwei , Li Haibing , Tapponnier P. , Meyer B. , Ryerson F. J. , Meriaux A.‐S. , Xu Zhiqin
Published: February 2004

The western part of the Tanghe Nan Shan range southwest of Subei (western Gansu, China) is presently growing on thrust ramps splaying from the left‐lateral Altyn Tagh Fault. Late Cenozoic thrusting has folded and sliced Oligocene‐Miocence red beds into an imbricate wedge, capped by warped and uplifted Quaternary terraces that form a 2‐ to 5‐km‐wide ledge, north of the steeply faceted range front. Seismic scarps 1.5 to 4.5 m high cut young fans along the outer thrusts. Carbon 14 dating of organic remains collected on strath terraces constrains the chronology of deposition and incision by the streams. Most of the fans and terraces in the southern part of the Subei basin appear to have been emplaced after the last glacial maximum, particularly during the early Holocene optimum (9–5 ka). Measurements of the shapes of the warped terraces constrain minimum and maximum throws on the outer thrusts. The minimum vertical throw of 34 ± 2 m of a surface dated at 8411 ± 530 years B.P. at one site provides a minimum vertical uplift rate of 4.1±0.5 mm/yr. The maximum possible uplift (115±15 m) of the oldest terrace surface, whose probable age is 15 to 18 ka, places an upper bound on the uplift rate of 7±2 mm/yr. The thrust geometry at depth and the cumulative shortening (10–20 km) deduced from balancing sections logged across the imbricate thrust wedge are consistent with a shortening rate of about 5 mm/yr and an onset of thrusting at about 4±2 Ma. Such a shortening rate implies a significant northward decrease in slip rate along the Altyn Tagh Fault. The recent growth of the western Qilian mountain ranges thus appears to be intimately coupled with sinistral motion on the Altyn Tagh Fault and the extrusion of Tibet.
Modeling seismic reflection patterns from Ocean Drilling Program Leg 168 core density logs: Insight into lateral variations in physical properties and sediment input at the eastern flank of the Juan de Fuca Ridge
Authors: Zühlsdorff Lars , Spiess Volkhard
Published: February 2004

At the eastern flank of the Juan de Fuca Ridge, single‐channel and multichannel very high resolution seismic data were recorded with a water gun and two Generator‐Injector Guns, which provided different source frequencies. To ground truth the seismic records, traces were visually correlated with reflection patterns of synthetic seismograms calculated from closely spaced density measurements on Ocean Drilling Program Leg 168 drill cores. The quality of both core logging and seismic data is sufficiently high to reveal a relation between density variations and seismic reflections at higher source frequencies. This suggests that density approximates subbottom variations in seismic impedance. The core density data are also sufficient to model a frequency‐dependent decrease in seismic reflection amplitudes, which is observed above a buried basement ridge. This indicates that low reflectivity in recorded traces is independent of processing artifacts, rough topography, or seafloor inclination and can be related to locally low in situ sediment density and high porosity, which appear to be associated with fluid flow. Further, the part of the amplitude decrease, which is due to an overall reduction of density contrasts, appears to be frequency‐dependent. In addition, the comparison between seismic records and core data reveals distinct depositional environments, which are divided by the buried basement ridge. In the western part of the study area a transition from an older to a younger part of the sediment section is observed. The former is characterized by focused turbiditic input in the vicinity of ODP Site 1024 and a gradual decrease in grain size toward ODP Site 1025. In the younger part, however, much thinner turbidites were deposited more evenly between both sites. Only one seismic reflector can be traced throughout the study area, suggesting that only major turbiditic events are capable of affecting both sides of the ridge. The sediment thickness above this reflector indicates that average accumulation rates for the youngest part of the sediments east and west of the ridge are rather similar.
Fresh water delivery to the continental shelf and subsequent mixing: An observational study
Authors: Sanders Todd M. , Garvine Richard W.
Published: February 2004

We address two questions related to buoyant coastal discharge: (1) What agents control the delivery of estuarine fresh water to the shelf? (2) How is this fresh water mixed with shelf water? Our subject was the Delaware Coastal Current, the buoyancy driven current originating in the Delaware estuary. The delivery of freshwater to the shelf at subtidal frequencies is controlled by two forcing agents: upland freshwater discharge into the estuary and the component of the wind parallel to the estuarine axis. The mean current we observed in the source region showed a strong decline in speed and large counterclockwise veering with increased depth. These variations in the mean current are best explained by thermal wind shear. From the moored instrument records we found evidence that in the source region mixing events of several hours duration are common at tidal frequencies when the local Richardson number Ri drops to a minimum level below 1/4. Furthermore, there is a pronounced modulation of this minimum level for Ri controlled by the action of coastal Ekman circulation. Data from the satellite‐tracked drifters show the striking difference between the coastal current configuration during downwelling and upwelling events. During downwelling the flow is downshelf and weakly onshore with particle trajectories orthogonal to the mean horizontal salinity gradient. In contrast, during upwelling the flow is strongly offshore and somewhat upshelf with particle trajectories down the mean salinity gradient, implying rapid mixing of plume water with shelf water. Corresponding values for horizontal dispersion of plume water showed modest values for both the along‐shelf and across‐shelf directions under downwelling but a very large value for the across‐shelf dispersion under upwelling. We conclude that wind stress, acting through the mediation of the strain field produced by coastal upwelling circulation, is the primary means for completing the mixing of fresh water within the plume with shelf water.
On subinertial oscillations trapped by the Juan de Fuca Ridge, northeast Pacific
Authors: Lavelle J. W. , Cannon G. A.
Published: February 2004

Spectra of currents measured along the 2100 m deep Juan de Fuca Ridge in the northeast Pacific have prominent tidal, inertial, and weather‐band (3–7 day period) spectral peaks. The weather‐band peak, in particular, has a number of interesting features. It is strongest in close lateral proximity to the ridge and strongest near ridge depth; intensification near the ridge is characteristic of trapped motion. Spectral peak intensities vary seasonally, with largest amplitudes occurring in autumn and winter; seasonal variation suggests that surface weather is forcing flow at depth. Together, trapping and seasonality indicate opportunistic amplification of oscillatory motion at the ridge. At all frequencies, forcing, topography, and stratification together shape current and hydrographic distributions near the ridge. Effects of those interactions for subinertial motion are quantified here using a primitive equation numerical model. Forcing period (1–10 days) and friction are the principal dependencies examined. Results show that flow over the Juan de Fuca Ridge can be amplified by factors of 3–4 for diurnal and up to a factor of 7 for weather‐band frequencies. Amplification is ridge trapped, with effects extending many hundreds of meters upward and extending 5–10 km laterally for 5‐day‐period flow; the strength and area of amplification increase with increasing period over the 1‐ to 10‐day‐period band. Oscillatory weather‐band flow leads to vertical velocities on the order of 0.3 cm s−1 on ridge flanks, which in turn cause periodic temperature (T) and salinity (S) variations with amplitudes of as much as 0.05°C and 0.01 practical salinity units, respectively. The vertical motion and consequent vertical displacement (>100 m) lead to isotherms that plunge below the crest alternately each side of the ridge, a distribution observed in conductivity‐temperature‐depth transect data. Near the ridge crest, cross‐ridge baroclinic pressure gradients caused by cyclically plunging isopycnals are in near‐geostrophic balance with the Coriolis force associated with along‐ridge flow. Along‐ridge current amplification is therefore closely tied to isopycnal movement up and down ridge flanks. Since weather‐band oscillations lead to much larger cross‐ridge baroclinic pressure gradients than do diurnal motions of the same amplitude, weather‐band forcing causes greater along‐ridge current amplification. Results vary strongly with forcing period and direction of forcing with respect to the ridge but depend only moderately on friction and turbulent mixing coefficients.
Vertical structure of turbulence on the southern flank of Georges Bank
Authors: Burgett R. L. , Hebert D. , Oakey N. S.
Published: February 2004

Microstructure and acoustic Doppler current profiler measurements were taken at two sites on Georges Bank: in a shallow (45 m) well‐mixed region on top of the bank and in a deeper (75 m) more stratified region on the southern flank. Each site was visited twice, April 25 to May 3, 1995, and June 6–16, 1995. At both sites the dominant variability in the turbulent kinetic energy dissipation rate ε was related to the semidiurnal M2 tide. In the well‐mixed lower portions of the water column a phase delay was observed between ε and the current speed. The delay increased with height above the bottom. There was also a depth‐dependent phase delay in the square of the vertical shear of horizontal velocity, S2, of the M2 component. This phase lag has a depth dependence similar to that found for ε, which suggests that the turbulent mixing is locally generated by the current shear, as opposed to having propagated from the boundary.
Influence of data assimilation upon M2 tidal elevations and current profiles in the Irish Sea
Authors: Lee Jong Chan , Davies Alan M.
Published: February 2004

A three‐dimensional hydrodynamic model of the Irish Sea into which surface tidal elevations can be assimilated is used to examine the influence of blending elevation data upon tidal currents and energy flux in the region. The model uses a sigma coordinate in the vertical, with discretization accomplished using finite difference grids in all dimensions. An alternating direction implicit method is used to integrate the hydrodynamic equations through time, and an elevation specified open boundary condition is applied. Vertical eddy viscosity is computed using a Prandtl mixing length submodel. In an initial series of calculations the influence of the coefficient of bottom friction upon tidal elevations and currents is examined with a view to determining an optimal value, in the sense that the model can reproduce the extensive data set of elevations and currents in the region. In subsequent calculations the influence of assimilating elevation data for a range of bottom friction coefficients and weighting factors upon computed elevations and currents is examined. Calculations show the importance of using an optimal friction coefficient in a three‐dimensional model in order to reproduce surface elevation and current profiles. Although the assimilation of elevation data can improve computed elevations even when the bottom friction coefficient was not optimal, it can lead to significant physically unrealistic flows with an associated energy flux in the region of the blending point, which can lead to a deterioration in the accuracy of the computed currents in the region of the blending point.
Frontshear and backshear instabilities of the mean longshore current
Authors: Baquerizo Asunción , Caballeria Miquel , Losada Miguel A. , Falqués Albert
Published: February 2004

An analytical model based on Bowen and Holman [1989] is used to prove the existence of instabilities due to the presence of a second extremum of the background vorticity at the front side of the longshore current. The growth rate of the so‐called frontshear waves depends primarily upon the frontshear but also upon the backshear and the maximum and the width of the current. Depending on the values of these parameters, either the frontshear or the backshear instabilities may dominate. Both types of waves have a cross‐shore extension of the order of the width of the current, but the frontshear modes are localized closer to the coast than are the backshear modes. Moreover, under certain conditions both unstable waves have similar growth rates with close wave numbers and angular frequencies, leading to the possibility of having modulated shear waves in the alongshore direction. Numerical analysis performed on realistic current profiles confirm the behavior anticipated by the analytical model. The theory has been applied to a current profile fitted to data measured during the 1980 Nearshore Sediment Transport Studies experiment at Leadbetter Beach that has an extremum of background vorticity at the front side of the current. In this case and in agreement with field observations, the model predicts instability, whereas the theory based only on backshear instability failed to do so.
Mono Lake excursion recorded in sediment of the Santa Clara Valley, California
Authors: Mankinen Edward A. , Wentworth Carl M.
Published: February 2004

Two intervals recording anomalous paleomagnetic inclinations were encountered in the top 40 meters of research drill hole CCOC in the Santa Clara Valley, California. The younger of these two intervals has an age of 28,090 ± 330 radiocarbon years B.P. (calibrated age ∼32.8 ka). This age is in excellent agreement with the latest estimate for the Mono Lake excursion at the type locality and confirms that the excursion has been recorded by sediment in the San Francisco Bay region. The age of an anomalous inclination change below the Mono Lake excursion was not directly determined, but estimates of sedimentation rates indicate that the geomagnetic behavior it represents most likely occurred during the Mono Lake/Laschamp time interval (∼45–28 ka). If true, it may represent one of several recurring fluctuations of magnetic inclination during an interval of a weak geomagnetic dipole, behavior noted in other studies in the region.
The termination shock near 35° latitude
Authors: Whang Y. C. , Burlaga L. F. , Wang Y.‐M. , Sheeley N. R.
Published: February 2004

The termination shock moves outwards and inwards over timescales of a solar cycle in response to the variations in the average solar wind speed. The amplitude is greater than 50 AU near 35° latitude; the maximum (minimum) distance occurs during the rising (declining) phase of the solar cycle. Shock parameters are distinctly different when the shock moves outwards or inwards. During the period of high‐speed (low‐speed) solar wind, the shock moves outward (inward) and the shock is weaker (stronger). This study assumes that the first crossing of Voyager 1 with the termination shock occurred at 85.5 AU on 2002.6. If Voyager 1 did cross the shock in 2002.6, the spacecraft would likely cross the shock at least two more times before 2010, but no second crossing would occur close to 2003.1. If Voyager 1 did not cross the shock in mid‐2002, it might still do so before 2005.
Sensitivity of TOMS aerosol index to boundary layer height: Implications for detection of mineral aerosol sources
Authors: Mahowald Natalie M. , Dufresne Jean‐Louis
Published: February 2004

The TOMS aerosol index (AI) is proposed as a powerful tool in determining the sources of mineral aerosols. The sensitivity of the AI to the height of the aerosol layer has been noted previously, but the implications of this sensitivity for deducing sources has not been explicitly considered. Here, we present a methodology and sensitivity test to show the importance of spatial and temporal variations of the planetary boundary layer height to deducing sources using the AI. These results suggest that while dry topographic low sources may be large sources of desert dust, conclusions eliminating other sources may be premature, especially when these sources occur on the edges of deserts, where boundary layer heights are lower, and human influences potentially more important. The compounding problem of differentiating downwind transport and local sources suggests it may not currently be possible to use the AI to conclusively determine mineral aerosol source regions.
Anisotropy and Alfvénicity of hourly fluctuations in the fast polar solar wind
Authors: Neugebauer M.
Published: February 2004

Ulysses magnetic and plasma data obtained in the fast polar solar wind are used to study the nature of fluctuations in the frequency range between the turbulent inertial range and the lower‐frequency range dominated by quasi‐static structures originating at the Sun. For daily variations of hourly averages of the magnetic field components the anisotropy of the fluctuations is less than in the higher‐frequency range, and the minimum variance direction loses its alignment with the magnetic field in favor of a more radial direction as the solar distance increases. The anisotropy of the magnetic fluctuations also increases with solar distance, while the Alfvénicity, as measured by the correlation of the field and velocity components, decreases. Most of the observed frequency and distance trends are similar to those observed in fast solar wind streams in the ecliptic, despite the difference in stream structure between high and low latitudes. One major difference from higher‐frequency (inertial range) variations at high latitudes is the continuing decline of the correlation between velocity and field vectors between 2 and 4.5 AU. It is suggested that the relative contribution of discontinuities and planar magnetic structures grows with solar distance relative to the contribution of Alfvén waves.
Thruster firing effects in the shuttle environment: 1. Neutral gas composition
Authors: Hunton D. E.
Published: February 2004

The changes in the composition of the neutral gas in the space shuttle payload bay caused by the shuttle's vernier reaction control system rocket engines were measured with a quadrupole mass spectrometer aboard STS‐4. The average hydrogen level during thruster firings was approximately 25 times greater than nitric oxide, NO, which was previously found to be one of the largest components of the gas in the payload bay during firings. The attitude of the orbiter also influenced the effects of the vernier engines as measured in the payload bay. When the mass spectrometer was pointed into the velocity vector, thrusters whose exhaust plumes were directed approximately upstream into the flow of ambient neutral species caused decreases in atomic oxygen signals. NO was enhanced in the payload bay during thruster firings primarily when the shuttle was in a bay‐to‐ram attitude. The return flux from the left‐side, downward pointing vernier thruster was detected by the mass spectrometer only when the shuttle was in a belly‐to‐ram attitude. All of these results are consistent with earlier observations that the kinematics of gas phase and surface scattering are important in determining the effects of the thruster firings on the payload bay environment.
Quantitative measurements of vaporization, burst ionization, and emission characteristics of shaped charge barium releases
Authors: Hoch Edward L. , Hallinan Thomas J. , Stenbaek‐Nielsen Hans C.
Published: February 2004

Intensity‐calibrated color video recordings of three barium‐shaped charge injections in the ionosphere were used to determine the initial ionization, the column density corresponding to unity optical depth, and the yield of vaporized barium in the fast jet. It was found that the initial ionization at the burst was less than 1% and that 0% burst ionization was consistent with the observations. Owing to the Doppler shift, the column density for optical thickness in the neutral barium varies somewhat according to the velocity distribution. For the cases examined here, the column density was 2‐5 × 1010 atoms/cm². This value, which occurred 12 to 15 s after release, should be approximately valid for most shaped charge experiments. The yield was near 30% (15% in the fast jet) for two of the releases and was somewhat lower in the third, which also had a lower peak velocity. This study also demonstrated the applicability of the computer simulation code developed for chemical releases by Stenbaek‐Nielsen and provided experimental verification of the Doppler‐corrected emission rates calculated by Stenbaek‐Nielsen (1989).
A proposed neutral line signature
Authors: Doxas I. , Speiser T. W. , Dusenbery P. B. , Horton W.
Published: February 2004

An identifying signature is proposed for the existence and location of the neutral line in the magnetotail. The signature, abrupt density and temperature changes in the Earth‐tail direction, was first discovered in test particle simulations. Such temperature variations have been observed is ISEE data (Huang et al., 1992), but their connection to the possible existence of a neutral line in the tail has not yet been established. The proposed signature develops earlier than the ion velocity space ridge of Martin and Speiser (1988) but can only be seen by spacecraft in the vicinity of the neutral line, while the latter can locate a neutral line remotely.
Growth of a spike on a Gaussian radio wave in the lower ionosphere
Authors: Pandey H. D. , Tripathi Ghanshyam , Tripathi V. K.
Published: February 2004

A Gaussian radio wave, propagating through the ionosphere causes differential ohmic heating of electrons and subsequent quasi‐linear diffusion of the plasma. The plasma duct thus formed tends to self‐focus the radio wave. When the radio wave has an on‐axis spike in its intensity distribution, it causes stronger local density depression and attracts more electromagnetic energy from the neighborhood, enhancing the spike. The self‐focusing of the radio wave and the growth of the perturbation are coupled through the nonlinear permittivity. For RF powers corresponding to vo ∼ cs, the optimum radius of the fastest growing perturbation is ∼ 3c/ωpo and the amplification length is ∼ 10cω/ωpo2, where vo is the amplitude of electron oscillatory velocity, cs is the sound speed, ω is the frequency of the radio wave, and ωpo is the plasma frequency.
A test of the Hall‐MHD Model: Application to low‐frequency upstream waves at Venus
Authors: Orlowski D. S. , Russell C. T. , Krauss‐Varban D. , Omidi N.
Published: February 2004

Early studies suggested that in the range of parameter space where the wave angular frequency is less than the proton gyrofrequency and the plasma beta, the ratio of the thermal to magnetic pressure, is less than 1 magnetohydrodynamics provides an adequate description of the propagating modes in a plasma. However, recently, Lacombe et al. [1992] have reported significant differences between basic wave characteristics of the specific propagation modes derived from linear Vlasov and Hall‐MHD theories even when the waves are only weakly damped. In this paper we compare the magnetic polarization and normalization magnetic compression ratio of ULF upstream waves at Venus with magnetic polarization and normalized magnetic compression ratio derived from both theories. We find that while the “kinetic” approach gives magnetic polarization and normalized magnetic compression ratio consistent with the data in the analyzed range of beta (0.5 < beta < 5) for the fast magnetosonic mode, the same wave characteristics derived from the Hall‐MHD model strongly depend on beta arid are consistent with the data only at low beta for the fast mode and at high beta for the intermediate mode.