bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleomagnetic study along the southeastern edge of the Altiplano ‐ Puna Plateau: Neogene tectonic rotations
Authors: Aubry Laurent , Roperch Pierrick , de Urreiztieta Marc , Rossello Eduardo , Chauvin Annick
Published: February 2004

In northwestern Argentina the transition zone between the Puna and the Sierras Pampeanas was deformed in Late Miocene and Pliocene time. Structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression. This study contributes new paleomagnetic results from four sites in Cretaceous rocks and 19 sites in Neogene sedimentary sequences. When compared with reference paleomagnetic data for stable continental South America, the results show an average inclination shallowing error of 10.5° and a pattern of clockwise rotations up to 29°. Study of the anisotropy of magnetic susceptibility (AMS) on 278 specimens provided, for most sites, AMS tensors with an oblate shape controlled by sedimentation and compaction, explaining the observed inclination shallowing. A slight AMS lineation was measured and we argue that this magnetic lineation reflects Late Miocene‐Pliocene compression. A correlation observed between magnetic lineations, deduced from the AMS analysis, and rotation estimates from remanent magnetizations, demonstrates that the inferred shortening directions have also been rotated clockwise.
Three‐dimensional models of deformation near strike‐slip faults
Authors: ten Brink Uri S. , Katzman Rafael , Lin Jian
Published: February 2004

We use three‐dimensional elastic models to help guide the kinematic interpretation of crustal deformation associated with strike‐slip faults. Deformation of the brittle upper crust in the vicinity of strike‐slip fault systems is modeled with the assumption that upper crustal deformation is driven by the relative plate motion in the upper mantle. The driving motion is represented by displacement that is specified on the bottom of a 15‐km‐thick elastic upper crust everywhere except in a zone of finite width in the vicinity of the faults, which we term the “shear zone.” Stress‐free basal boundary conditions are specified within the shear zone. The basal driving displacement is either pure strike slip or strike slip with a small oblique component, and the geometry of the fault system includes a single fault, several parallel faults, and overlapping en echelon faults. We examine the variations in deformation due to changes in the width of the shear zone and due to changes in the shear strength of the faults. In models with weak faults the width of the shear zone has a considerable effect on the surficial extent and amplitude of the vertical and horizontal deformation and on the amount of rotation around horizontal and vertical axes. Strong fault models have more localized deformation at the tip of the faults, and the deformation is partly distributed outside the fault zone. The dimensions of large basins along strike‐slip faults, such as the Rukwa and Dead Sea basins, and the absence of uplift around pull‐apart basins fit models with weak faults better than models with strong faults. Our models also suggest that the length‐to‐width ratio of pull‐apart basins depends on the width of the shear zone and the shear strength of the faults and is not constant as previously suggested. We show that pure strike‐slip motion can produce tectonic features, such as elongate half grabens along a single fault, rotated blocks at the ends of parallel faults, or extension perpendicular to overlapping en echelon faults, which can be misinterpreted to indicate a regional component of extension. Zones of subsidence or uplift can become wider than expected for transform plate boundaries when a minor component of oblique motion is added to a system of parallel strike‐slip faults.
Shear wave splitting observations and implications on stress regimes in the Los Angeles basin, California
Authors: Li Yong‐Gang
Published: February 2004

A systematic analysis of three‐component seismograms recorded at 15 stations from earthquakes occurring at depths of 5 to 18 km beneath the Los Angeles basin and adjacent areas during the period between 1988 and 1994 shows 20 to 160 ms shear wave splitting. Shallow events exhibit little splitting, while deeper events show progressively greater splitting with depth. The preferred polarization direction of the fast shear wave is nearly N‐S, consistent with the direction of the regional maximum horizontal compressive stress but independent of the azimuth between the event and station. We interpret that the shear wave splitting is caused by fluid‐filled crustal microcracks and macrofractures aligned in the N‐S direction. The shear wave splitting observations of 2.8 to 7.8 ms/km can be explained in terms of an anisotropic crust containing vertical cracks with the apparent crack density of 0.023–0.08. On a regional basis, the crack density may vary from station to station, but we find that the apparent crack density in the strike‐slip region of the Newport‐Inglewood fault and the Whittier fault is higher than in the reverse‐thrusting Santa Monica Mountains and Palos Verdes Hills. No systematic change of shear wave splitting in the Los Angeles basin is found in this study.
A migratory mantle plume on Venus: Implications for Earth?
Authors: Chapman Mary G. , Kirk Randolph L.
Published: February 2004

A spatially fixed or at least internally rigid hotspot reference frame has been assumed for determining relative plate motions on Earth. Recent 1:5,000,000 scale mapping of Venus, a planet without terrestrial‐style plate tectonics and ocean cover, reveals a systematic age and dimensional progression of corona‐like arachnoids occurring in an uncinate chain. The nonrandom associations between arachnoids indicate they likely formed from a deep‐seated mantle plume in a manner similar to terrestrial hotspot features. However, absence of expected convergent “plate” margin deformation suggests that the arachnoids are the surface expression of a migratory mantle plume beneath a stationary surface. If mantle plumes are not stationary on Venus, what if any are the implications for Earth?
Strombolian explosions: 1. A large bubble breaking at the surface of a lava column as a source of sound
Authors: Vergniolle S. , Brandeis G.
Published: February 2004

Strombolian activity consists of a series of explosions caused by the breaking of large overpressurized bubbles at the surface of the magma column. Acoustic pressure has been measured for 36 explosions at Stromboli. We propose that sound is generated by the vibration of the bubble before it bursts. Oscillations are driven by an initial overpressure inside the bubble, assumed to be initially at rest, just below the magma‐air interface. Inertia effects cause the bubble to overshoot its equilibrium radius. Then the bubble becomes underpressurized and contracts because of gas compressibility. These oscillations are only slightly damped by viscous effects in the magma layer above the bubble. The bubble cannot complete more than one cycle of vibration because of instabilities developing on the magma layer that lead to its breaking, near the minimum radius. Assuming a simple geometry, we model this vibration and constrain the radius and length of the bubble and the initial overpressure by fitting a synthetic waveform to the measured acoustic pressure. The fit between synthetic and observed waveforms is very good, both for frequency, ≈60 rad s−1, and amplitude. The initial bubble radius is ≈1 m, and the length varies between several and a few tens of meters. From the initial overpressure, approximately 105 Pa, we calculate the maximum radial velocity of ejecta, ≈30 m s−1. The generally good agreement between data and predictions of our model leads us to suggest that acoustic measurements are a powerful tool for the understanding of eruption dynamics.
Recent tectonic, magmatic, and hydrothermal activity on the East Pacific Rise between 17°S and 19°S: Submersible observations
Authors: Auzende Jean‐Marie , Ballu Valérie , Batiza Rodey , Bideau Daniel , Charlou Jean‐Luc , Cormier Marie Hélène , Fouquet Yves , Geistdoerfer Patrick , Lagabrielle Yves , Sinton John , Spadea Piera
Published: February 2004

The objective of the Naudur cruise (December 1993) of the submersible Nautile was to study the interaction among magmatic, tectonic, and hydrothermal processes at a very fast spreading mid‐ocean ridge axis. Twenty‐three dives were completed, both along and across the axis, in four areas located between 17°10′ and 18°45′S on the East Pacific Rise. Rock, sulfides, water, and biological samples have been collected along each of the segments. Two main types of segments have been distinguished, characterized either by the predominance of present‐day volcanic activity or by predominant tectonic activity. Linked to both types of activity, 69 hydrothermal sites have been discovered and sampled. They comprise four types, interpreted as successive evolutionary stages. The first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna. The second, in areas dominated by recent volcanic activity, have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys. The third stage is represented by more mature hydrothermal vents and deposits, along the faults bounding the eastern side of the axial graben in tectonic‐dominated areas. The associated fauna is well developed. The fourth stage corresponds to the reactivation of volcanic activity with lava flows, young black smokers, and diffuse venting associated with the faults bounding the axial graben. Fluids collected range from 200° to 340°C and show a wide variability in chemical and gas composition. Within each of the explored areas, evidence of recent volcanic activity has been observed.
Hess Deep rift valley structure from seismic tomography
Authors: Wiggins Sean M. , Dorman LeRoy M. , Cornuelle Bruce D. , Hildebrand John A.
Published: February 2004

We present results from a seismic refraction experiment conducted across the Hess Deep rift valley in the equatorial east Pacific. P wave travel times between seafloor explosions and ocean bottom seismographs are analyzed using an iterative stochastic inverse method to produce a velocity model of the subsurface structure. The resulting velocity model differs from typical young, fast spreading, East Pacific Rise crust by approximately ±1 km/s with slow velocities beneath the valley of the deep and a fast region forming the intrarift ridge. We interpret these velocity contrasts as lithologies originating at different depths and/or alteration of the preexisting rock units. We use our seismic model, along with petrologic and bathymetric data from previous studies, to produce a structural model. The model supports low‐angle detachment faulting with serpentinization of peridotite as the preferred mechanism for creating the distribution and exposure of lower crustal and upper mantle rocks within Hess Deep.
Sound velocity and elasticity of single‐crystal forsterite to 16 GPa
Authors: Zha Chang‐Sheng , Duffy Thomas S. , Downs Robert T. , Mao Ho‐Kwang , Hemley Russell J.
Published: February 2004

As an abundant component of Earth's upper mantle, forsterite (Mg2SiO4) has been the subject of extensive investigation for many decades. The pressure dependences of sound velocity and elasticity are of great importance for modeling the composition and evolution of Earth. For this reason, room temperature Brillouin scattering experiments were performed on single‐crystal forsterite at pressures up to 16 GPa in a diamond anvil cell. The nine elastic moduli increase linearly over the pressure range studied. Together with previous ambient pressure data, the pressure variation of the bulk modulus is Ks(GPa) = 128.8(5) + 4.2(2)P, and the shear modulus is G(GPa) = 81.6(2) + 1.4(1)P, where the numbers in parentheses are one standard deviation uncertainties in the last digit. Combining with previous ultrasonic data for polycrystalline β‐Mg2SiO4 to 3 GPa, the sound velocity contrasts between α‐ and β‐Mg2SiO4 were found to be largely pressure independent at room temperature. If, on the other hand, the pressure dependences of the bulk and shear modulus of the β phase of Mg2SiO4 are assumed to be identical to those for the α phase, the room temperature velocity contrasts between the two phases decrease by 20% and 12% from 1 bar to 13.8 GPa for compressional and shear waves, respectively.
Global mantle shear velocity model developed using nonlinear asymptotic coupling theory
Authors: Li Xiang‐Dong , Romanowicz Barbara
Published: February 2004

We present a three‐dimensional shear velocity model of the whole mantle developed using S H waveform data. The model is expressed horizontally in terms of spherical harmonics up to degree 12, and vertically in terms of Legendre polynomials up to degrees 5 and 7 in the upper and lower mantle, respectively. What distinguishes this model from other tomographic models published to date is (1) the theoretical normal mode‐based wave propagation approach, where we include across branch mode coupling terms in order to model the body wave sensitivity to structure along the path more accurately; (2) the wave‐packet weighting scheme which allows to balance contributions from high‐amplitude and low‐amplitude phases, increasing the resolution in some parts of the mantle. We also relax the constraints on the Moho depth, which is allowed to vary in the inversion, thus absorbing some uncertainties in crustal structure. The resulting model is generally in good agreement with other recent global mantle S velocity models and with some regional models. The rms profile with depth has more power than other models in the upper mantle/lower mantle transition region and the zone of increased power and low degree structure near the base of the mantle is confined to the last 500 km in depth. This model provides a particularly good fit to the non‐hydrostatic geoid through harmonic degree 12 (79% variance reduction), as well as good fits to observed splitting functions of S velocity sensitive mantle modes, indicating that both large‐scale and small‐scale features are really well constrained.
Stability of a frictional, cohesive layer on a viscous substratum: Variational formulation and asymptotic solution
Authors: Leroy Y. M. , Triantafyllidis N.
Published: February 2004

This contribution is concerned with the stability of a frictional, cohesive material layer, called the overburden, resting on a viscous, incompressible layer of lower density referred to as the substratum. The viscous layer is either perfectly bonded or free to slip with no friction on a rigid basement. The in situ stress is assigned a realistic gradient in the overburden and is assumed to be purely hydrostatic in the substratum. A general variational formulation of the linearized stability problem for the stratified system is obtained in which the viscous response of the substratum provides the characteristic time. To a given state of stress and perturbation corresponds a rate of growth or decay which is calculated by an asymptotic method for small overburden thickness compared to the perturbation wavelength. It is found that the substratum's thickness influences the rate of growth of the instability if its product by the perturbation wavenumber is smaller than 3 or 4, depending on the type of boundary condition at the basement. However, these conditions and the substratum thickness have no influence on neutral stability. Furthermore, the conditions for stability depend on the tectonic stress distribution in the overburden and not solely on the density contrast, as is the case for viscoelastic systems. It is also shown that the classical flow theory of plasticity adopted for the overburden, which successfully captures the onset of folding, fails to predict initiation of faulting for similar stress conditions.
Paleomagnetism and tectonics of the Southern Tarim Basin, northwestern China
Authors: Gilder Stuart , Zhao Xixi , Coe Robert , Meng Zifang , Courtillot Vincent , Besse Jean
Published: February 2004

We report Late Carboniferous, Permian, and early Tertiary paleomagnetic data from the southern Tarim basin. Prefolding magnetizations were isolated in each case. The Late Carboniferous–Permian and early Tertiary poles lie at 64.6°N, 166.5°E, A95 = 6.3° and 58.1°N, 202.0°E, A95 = 12.7°, respectively. The Late Jurassic to early Tertiary (J3–E1) paleolatitudes of Tarim and several basins throughout central Asia are similar, yet significantly (10° to 20°) shallower than those predicted by the Eurasian apparent polar wander path. Resolving this discrepancy remains a major problem in Asian paleomagnetism. Discordance of the late Paleozoic poles from Tarim and Siberia suggest that Tarim has rotated about 30° counterclockwise with respect to Siberia since the Permian. Where paleomagnetic samples of both Late Carboniferous to Early Triassic (C3–T1) and J3–E1 ages were collected from the same area of Tarim, a great circle passes through the means of the poles and the sampling locality. This suggests that (1) only a difference in inclination (and not declination) distinguishes the two data sets, and (2) vertical axis block rotations of the C3–E1 strata occurred after E1. Although based on data of lesser quality, the mean Early to Middle Jurassic (J1–2) pole from Tarim differs significantly from the Eurasian reference pole, requiring radical tectonic solutions to resolve them. The Tarim J1–2 pole is indistinguishable from both the mean J3–E1 and C3–T1 poles. The similarity of all the poles and the analogous tectonic setting of present‐day central Asia to that of the late Paleozoic in eastern North America raises the question whether all the data from Tarim are overprinted.
Imaging scattering structures in the lower mantle by migration of long‐period S waves
Authors: Lay Thorne , Young Christopher J.
Published: February 2004

A simplified wave field migration method is applied to long‐period tangential component recordings to image lower mantle heterogeneity beneath Alaska. Observed wave field complexities are dominated by coherent arrivals between direct S (or sS) and the core‐reflected ScS (or sScS) phases. A relative waveform inversion method is used to objectively extract the time and amplitude of the three main spikes in each wavetrain, removing the effects of source time function, receiver, and instrument responses. The timing of the intermediate arrivals relative to S or ScS is used to define the volumetric position of scattering ellipsoids in the lower mantle, with intersections of ellipsoids illuminating possible regions of isotropic scattering. While a complete waveform migration is not viable due to the very limited ray path coverage and the band‐limited data, the kinematics and relative amplitudes of the scattered arrivals are still valuable for assessing the origin of the waveform complexity. In this region, which displays particularly uniform existence of an extra arrival in the data, the simplest interpretation is that a scattering surface with lateral dimensions exceeding 1500 km at a depth near 2600 km in the mantle is responsible for the extra phase. This is consistent with previous interpretations involving a one‐dimensional model with a rapid shear velocity increase or discontinuity 240–280 km above the core‐mantle boundary. The proposed methodology can potentially image both heterogeneities or layering in the D″ region as well as scattering from midmantle structure, although data limitations will continue to be a major obstacle.
Failure in laboratory fault models in triaxial tests
Authors: Savage J. C. , Lockner D. A. , Byerlee J. D.
Published: February 2004

A model of a fault in the Earth is a sand‐filled saw cut in a granite cylinder subjected to a triaxial test. The saw cut is inclined at an angle α to the cylinder axis, and the sand filling is intended to represent gouge. The triaxial test subjects the granite cylinder to a constant confining pressure and increasing axial stress to maintain a constant rate of shortening of the cylinder. The required axial stress increases at a decreasing rate to a maximum, beyond which a roughly constant axial stress is sufficient to maintain the constant rate of shortening. Such triaxial tests were run for saw cuts inclined at angles α of 20°, 25°, 30°, 35°, 40°, 45°, and 50° to the cylinder axis, and the apparent coefficient of friction μa (ratio of the shear stress to the normal stress, both stresses resolved onto the saw cut) at failure was determined. Subject to the assumption that the observed failure involves slip on Coulomb shears (orientation unspecified), the orientation of the principal compression axis within the gouge can be calculated as a function of μa for a given value of the coefficient of internal friction μi. The rotation of the principal stress axes within the gouge in a triaxial test can then be followed as the shear strain across the gouge layer increases. For μi ∼ 0.8, an appropriate value for highly sheared sand, the observed values μa imply that the principal axis of compression within the gouge rotates so as to approach being parallel to the cylinder axis for all saw cut angles (20° < α < 50°). In the limiting state (principal compression axis parallel to cylinder axis) the stress state in the gouge layer would be the same as that in the granite cylinder, and the failure criterion would be independent of the saw cut angle.
Possible upper mantle reflection fabric on seismic profiles from the Tethyan Himalaya: Identification and tectonic interpretation
Authors: Alsdorf Douglas , Brown Larry , Nelson Douglas
Published: February 2004

Upper mantle structures of the Himalaya collision system are not well‐defined, yet are integral to our understanding of the lithospheric scale processes which have built the Tibetan plateau. A 100‐km‐long deep seismic reflection profile (Tib‐1) and a short cross line (Tib‐2) were recorded within the Tethyan Himalaya to help constrain the geometry of the crust and mantle. At long travel times, the returned seismic energy is especially weak compared to the noise; therefore the data were reprocessed with signal‐to‐noise enhancing techniques to identify coherent events at mantle travel times. Visual comparisons of shot gathers, recordings of the ambient noise, common midpoint stacks, and synthetic sections suggest that a dipping reflection “fabric” is present in the upper mantle. On the basis of geometrical and velocity constraints, the fabric is not likely related to sideswipe, P‐to‐S phase conversions, multiples, or surface waves. The fabric is most likely within the underthrust Indian lithosphere beneath the peaks of the High Himalaya. The fabric strikes ∼N63°E, significantly oblique to the ∼E‐W strike of crustal deformation; however, the fabric dip direction (∼44°, N27°W) is essentially the same as the fast direction of polarization inferred from a previous SKS shear wave splitting study. The crust versus mantle strike discordance suggests that the fabric may predate the collision of India with Tibet and mark previous deformation of the Indian lithosphere (e.g., rifting episodes that formed the Tethyan ocean). Conversely, if the fabric is the result of collision, the discordance would suggest that mantle deformation is decoupled from crustal deformation within the Himalayas.
Non‐double‐couple focal mechanisms in an oceanic, intraplate earthquake swarm: Application of an improved method for comparative event, moment tensor determination
Authors: Hung Shu‐Huei , Forsyth Donald W.
Published: February 2004

Between February 1991 and May 1992, an earthquake swarm including 33 teleseismically recorded events occured about 300 km west of the axis of the southern East Pacific Rise. Joint hypocentral relocations show that the best located events were in a 10‐km‐wide band between and parallel to two seamount chains. New, simulated‐annealing inversion procedures were developed to find the moment tensors of three of the largest events that then served as master events for focal mechanism determination of the smaller events. Path/station corrections were applied to the spectra of surface waves from the smaller events, and the corrected spectra were linearly inverted for the moment tensors. The new procedures made it possible to derive focal mechanisms for 32 of the 33 earthquakes, ranging in size from Mw 4.2 to 5.9. All of the events have double‐couple components that are nearly pure normal faulting with a wide range of strikes of the nodal planes. Most have a non‐double‐couple component that may have been caused by simultaneous slip on nearly randomly oriented fault planes. The individual mechanisms and net horizontal extension that is nearly equal in all directions are consistent with the release of thermal stresses in the cooling oceanic lithosphere.
Compressive and tensile failure at high fluid pressure where preexisting fractures have cohesive strength, with application to the San Andreas Fault
Authors: Fournier Robert O.
Published: February 2004

In thrusting and strike‐slip situations, when the maximum principal horizontal stress S1 acts nearly normal to a fault (a misoriented fault, such as the San Andreas), pore‐fluid pressure > the lithostatic load, Pf > Sv, is required to reactivate movement on that fault. Pf > Sv may be achieved without causing hydraulic tensile fracturing if (1) previously existing cracks have regained cohesive strength by chemical processes, (2) subcritical crack growth has been blunted, and (3) the least principal horizontal stress S3 nearly equals Sv. Where Pf > Sv has been attained within a misaligned fault, increasing the stress difference (S1 – S3) at constant Pf > Sv will not lead to shear failure, while a decrease in (S1 – S3) can lead to shear failure of that fault. However, where the cohesive strength of material in a broad misaligned fault zone is less than that of the surrounding intact rock, increasing (S1 – S3) while Pf > Sv can result in shear failure of fractures at near optimum angles to S1, but confined within this weak fault zone. If this faulting results in the local short‐lived attainment of Pf > Sv (cataclastic deformation and frictional heating overcoming dilation) and a simultaneous decrease in (S1 – S3), this combination of effects can trigger movement along the main trace of the misaligned fault. When increasing Pf results in hydraulic failure, anisotropy in tensile strength or fracture toughness resulting from foliation within faults allows fractures to propagate along the planes of weakness rather than across the foliation perpendicular to S3.
A high‐resolution P wave model for the top 1200 km of the mantle
Authors: Zhou Hua‐wei
Published: February 2004

This work attempts to map aspherical velocities in the top 1200 km of the mantle that are illuminated by the International Seismological Centre P wave data. The processing includes summary ray sorting and hypocentral redetermination. To cope with the extreme variation of ray coverage, a multicell inversion is used to simultaneously constrain anomalies in overlapping cells. The multicell solutions are summed after inversion to form the final model, named P1200, at cell size of 1° × 1° and about 50 km in thickness. The lateral resolution of the model is about 1°–2° in major subduction zones and about 5°–15° in most places. Major features in the model withstood tests on signal coherency and different reference models. The long‐wavelength parts of the P1200 model correlate well with previous global models, especially the S12 model of shear wave. Among the large features in the transition zone and below, there are two fast bands, one from Canada to South America and another from Siberia, through East Asia and the Philippine Sea, to western Australia. There are also slow patches under the central Pacific and from India to eastern Africa. Though most subduction zones have slab‐like high‐velocity anomalies, they are sandwiched by broad slow velocities at shallow depths. Some of the slabs appear to penetrate the 660‐km discontinuity, and stagnant slabs are also seen in the transition zone. Plume‐like slow anomalies exist below some prominent hot spots like Hawaii, Iceland, Yellowstone, and French Polynesia.
Simulation and analysis of 1991 U.S. Geological Survey refraction data from the New Madrid Seismic Zone
Authors: Liao Qingbo , McMechan George A.
Published: February 2004

Two orthogonal wide‐aperture seismic refraction lines were acquired in the New Madrid Seismic Zone by the U.S. Geological Survey in 1991. These N‐S and E‐W oriented lines cross the Blytheville Arch and adjacent portions of the Reelfoot Rift. Ten shots were fired along the E‐W line; eight shots were fired along the N‐S line. Iterative numerical modeling, including scattering and viscoelasticity, has been performed for all shots on both lines using a new eighth‐order staggered‐grid (two dimensional) finite difference solution based on superposition of relaxation mechanisms. The main features in the data can be explained by the existence of a strong 15‐km‐wide low‐velocity cap lying beneath the center of the Blytheville Arch between 1.5 and 2.5 km depth; the data do not allow estimation of its extent along the axis of the arch, but it is at least 30 km long and appears to become less prominent toward the southwest. The arch itself has a low‐velocity background that extends to at least 5 km depth. An additional low‐velocity layer (the Elvins shale) lies generally between 3 and 4 km depth, is upwarped at the edges of the arch, and thins toward the edges of the Reelfoot Rift. There is no evidence for anomalously low intrinsic Q, but the data may be interpreted to indicate strong local scattering that occurs west of the low‐velocity cap of the Blytheville Arch at depths that are similar to, and slightly shallower than, the cap. Additional strong scattering occurs at the western edge of the Reelfoot Rift. An alternative interpretation attributes the apparent scattering at the Blytheville Arch to instrument noise.
Excitation and propagation of Lg from earthquakes in central Asia with implications for explosion/earthquake discrimination
Authors: Cong Lianli , Xie Jiakang , Mitchell B. J.
Published: February 2004

Broadband Lg records from 52 shallow earthquakes that occurred near the Balapan and Lop Nor nuclear test sites have been used to study spectral characteristics of the excitation and propagation of the Lg phase in central Asia. Using a source model that falls off as ω2 at high frequencies and a nonlinear inversion method, we simultaneously inverted for Lg seismic moments (M0), corner frequencies (fc), and path‐variable values of Q0 and η (QLg at 1 Hz and its power law frequency dependence). The logarithms of M0 obtained using Lg correlate linearly with logarithms of fc, the slope being about −3.56 when all data are used and −4.04 when data were sufficient that full inversions could be performed. These slopes differ only slightly from the slope of −3.83 obtained in our earlier studies using underground nuclear explosions. The fc values in this study are systematically lower than those found for explosions at the same M0 level. A comparison of corner frequencies and Lg moment values for unknown events with the plots for known events of this study therefore provides means to discriminate between earthquakes and explosions. The mb values are systematically lower for earthquakes of this study than for previously studied explosions at the same seismic moment; moreover, the logarithms of Lg moments obtained in this study correlate linearly, with a slope of 1.04, with mb values when all data are used and 1.21 when only well‐recorded events are used. These slopes are close to the slope of 1.19 obtained for explosions. The average QLg values along the paths used in this study agree well with those obtained using explosion data in central Asia and with reported Lg coda Q values.
Does the mid‐ocean ridge propagate concurrently both on the seafloor and at depth? Implications from a gravity study of a large nontransform offset at 36.5°S, East Pacific Rise
Authors: Chen Y. John , Enriquez Kelly D. , Lonsdale Peter
Published: February 2004

Analyses of gravity data collected at a large nontransform offset (NTO) at 36.5°S on the East Pacific Rise indicate that the southward ridge propagation there does not occur concurrently both on the seafloor and at depth. A large negative axial mantle Bouguer anomaly commonly observed at the East Pacific Rise does not extend from the north into the overlapping zone following the ridge axis observed in seafloor bathymetry data. We speculate that the southward ridge propagation at this offset is probably an episodic process: it starts with rapid rise‐crest (dikes) propagation on the seafloor and is followed by slow propagation of the melt supply sources associated with the mantle upwelling. The gravity data collected at the 36.5°S NTO provide new insight on the tectonic evolution of a large propagating nontransform offset and also provide important constraints on ridge‐NTO models.
The Barents Sea Polar Front in summer
Authors: Parsons A. Rost , Bourke Robert H. , Muench Robin D. , Chiu Ching‐Sang , Lynch James F. , Miller James H. , Plueddemann Albert J. , Pawlowicz Richard
Published: February 2004

In August 1992 a combined physical oceanography and acoustic tomography experiment was conducted to describe the Barents Sea Polar Front (BSPF) and investigate its impact on the regional oceanography. The study area was an 80 × 70 km grid east of Bear Island where the front exhibits topographic trapping along the northern slope of the Bear Island Trough. Conductivity‐temperature‐depth, current meter, and acoustic Doppler current profiler (ADCP) data, combined with tomographic cross sections, presented a highly resolved picture of the front in August. All hydrographic measurements were dominated by tidal signals, with the strongest signatures associated with the M2 and S2 semidiurnal species. Mean currents in the warm saline water to the south of the front, derived from a current meter mooring and ADCP data, were directed to the southwest and may be associated with a barotropic recirculation of Norwegian Atlantic Water (NAW) within the Bear Island Trough. The geostrophic component of the velocity was well correlated with the measured southwestward mean surface layer flow north of the front. The frontal structure was retrograde, as the frontal isopleths sloped opposite to the bathymetry. The surface signature of the front was dominated by salinity gradients associated with the confluence of Atlantic and Arctic water masses, both warmed by insolation to a depth of about 20 m. The surface manifestation of the front varied laterally on the order of 10 km associated with tidal oscillations. Below the mixed layer, temperature and salinity variations were compensating, defining a nearly barotropic front. The horizontal scale of the front in this region was ∼3 km or less. At middepth beneath the frontal interface, tomographic cross sections indicated a high‐frequency (∼16 cpd) upslope motion of filaments of NAW origin. The summertime BSPF was confirmed to have many of the general characteristics of a shelf‐slope frontal system [Mooers et al., 1978] as well as a topographic‐circulatory front [Federov, 1983].
A river‐coastal sea ice interaction model: Mackenzie River Delta
Authors: Searcy Craig , Dean Kenneson , Stringer William
Published: February 2004

It has been observed that arctic coastal regions which receive river discharge show significant ice regression in advance of other coastal areas without this source of sensible heat. This phenomena has been particularly apparent on satellite imagery. The work reported here attempts (1) to quantify and assess the influence of springtime river discharge on the removal of nearshore ice and (2) to demonstrate the utility of satellite imagery as a source of data to “drive” a model assessing the influence of springtime river discharge on the removal of nearshore ice. Central to this work is a sequence of advanced very high resolution radiometer images of the Mackenzie Delta region, Northwest Territories, Canada. These images constitute a database establishing the temporal sequence of observable breakup events from which a thermodynamic model for the ice decay can be parameterized. The emphases of this model are the influence of the riverine sensible heat on the melting of sea ice and the ability of satellite imagery to monitor this process. For simplicity, climatological values rather than meteorological values for atmospheric and solar influences have been used. The results suggest that the river discharge supplies a significant amount of sensible heat to the base of the nearshore ice, accounting for nearly half of the energy required to melt the ice cover. As a result, the coastal ice is removed 7–14 days in advance of areas without significant river discharge.
Kinematic and dynamic evolution of deep water breaking waves
Authors: Griffin Owen M. , Peltzer Rodney D. , Wang Henry T. , Schultz William W.
Published: February 2004

Experiments were performed to exploit the dispersive properties of unsteady surface waves and to induce breaking by using a modified chirp pulse technique to focus the wave energy at a specific location in the Naval Research Laboratory deep water wave channel. The experiments have resulted in a highly resolved archive of breaking events ranging from wave steepening and incipient breaking to spilling and to plunging. The potential energy density, the crest front steepness, the horizontal asymmetry, and other geometric properties of an incipient breaker vary only within a moderate band about their mean values over the extent of these experiments. Thus the properties of an incipient unsteady breaker are well defined. The application of the phase‐time or Hilbert transform method to the data set provides new insights into the local properties of the unsteady wave breaking. Recently, spectral and piecewise‐linear algorithms for two‐dimensional potential flow were developed and used bySchultz et al.[1994] to compare the onset of breaking for several methods of energy input to the unsteady wave system. The computations show that steep plunging waves occur when energy input rates are large. The various energy input methods exhibit similar breaking trends in the limit as the energy input rate becomes small in that incipient spilling breakers form when the potential energy is approximately 52 to 54% of the energy for the most energetic Stokes wave, with the formation of a singularity immediately before the crest.
A test of the ability of TOPEX/POSEIDON to monitor flows through the Drake Passage
Authors: Woodworth P. L. , Vassie J. M. , Hughes C. W. , Meredith M. P.
Published: February 2004

TOPEX/POSEIDON (T/P) altimeter data from the Drake Passage (DP) area of the Southern Ocean, near to the highest latitudes accessible by the satellite, are found to agree as well, in general, to data from an extensive set of in situ instrumentation as has been observed for other parts of the ocean. Root‐mean‐square (rms) differences between subsurface pressures (SSP) measured by altimetry and bottom pressures (BP) measured by in situ devices are approximately 2 mbar in the northern DP and 4 mbar in the south, after application of a 20‐day low‐pass filter. Two mbar accuracy would suggest that altimetry could be capable of providing useful additional information on Antarctic Circumpolar Current (ACC) transport fluctuations through the DP on seasonal and interannual timescales. The larger rms differences on what is considered to be the more important southern side are explainable in terms of baroclinic variability at the southern end of the World Ocean Circulation Experiment (WOCE) SR1 section. The first WOCE conductivity‐temperature‐depth data sets from SR1 have confirmed the existence of a major baroclinic “eddy feature” in the south, which could account to a great extent for the larger rms values. Better southern DP locations could possibly have been chosen for WOCE. However, further recent in situ measurements from other locations near to the DP have shown that the southern DP ACC‐related signals in SSP or BP are only slightly larger than T/P accuracy, which suggests that, as yet, they cannot be adequately monitored by means of altimetry alone, whichever location is chosen. Even if the altimetric and in situ observations were perfect, would they provide information directly relatable to transport variability through the passage? This question has been studied by means of Fine‐Resolution Antarctic Model (FRAM) numerical modeling investigations, which have demonstrated the complexity of the correlation between the variability in DP transport and SSP (or sea level). Within FRAM, northern SSP (or BP) variations are virtually uncorrelated with transport fluctuations, and the modelling has confirmed the importance of southern measurements, as inferred from in situ measurements over a decade ago. It is evident that, for the foreseeable future, an extensive set of in situ instrumentation will be required, in addition to the altimetry, if further insight into circulation in this part of the Southern Ocean is to be obtained.
On the determination of wind direction using an upward‐looking acoustic Doppler current profiler
Authors: Zedel L. , Crawford G. B. , Gordon L.
Published: February 2004

Schott [1989] demonstrated that velocity estimates of the near‐surface ocean, made with an upward‐looking Acoustic Doppler Current Profiler (ADCP), could be used to estimate wind directions. Subsequent studies have attempted to repeat Schott's results with limited success [Brown et al., 1992; Ruiz, 1993; Visbeck and Fischer, 1995]. Considering the behavior of this near‐surface velocity with changes in wind speed and other observations of near‐surface acoustic scattering, we conclude that the acoustic backscatter is caused by subsurface bubbles generated by wave breaking. Data sets from both the equator and the North Pacific are examined. The North Pacific data were collected in a region where geostrophic flows are relatively small (typically a few centimeters per second). The ADCP measured surface currents in this region are shown to be directed, on average, at −12° ± 23° (i.e., to the right) of the wind direction. The amount of deflection increases with wind speed. We interpret this effect to be a consequence of Ekman dynamics and increased bubble concentration in the surface range bin at high wind speeds. In principle, the wind speed dependence of the surface current deflection can be accounted for, although additional data will be required to demonstrate that such a correction is generally applicable. At the equator, currents driven by large‐scale zonal winds contribute to the observed surface velocity. We estimate the regional current by using ADCP velocity observations from 24 m depth and calculate the locally wind‐driven surface current as a residual. The results obtained using this technique for equatorial data show an average difference between wind direction and wind‐driven surface current direction to be 1.4° ± 8°.
Frequency periodograms of altimetric sea level and their geographical variation from 2 years of TOPEX/POSEIDON data
Authors: Tai Chang‐Kou
Published: February 2004

Frequency periodograms are estimated from 2 years of TOPEX/POSEIDON altimetric sea levels at crossovers (XOs), where the height measurements are twice as abundant as at non‐XO points. It is shown that the extra set of height measurements is best utilized to reduce aliasing rather than to double the temporal resolution. Least squares fitting with Fourier series handles the problem of missing data and unequal spacing. A global average power spectrum (area‐weighted from spectra at XOs) and average spectra from low‐variability as well as high‐variability areas are obtained. In addition, regional average spectra are produced for various western boundary currents as well as the Antarctic Circumpolar Current and northeastern equatorial Pacific. All spectra are red and (with few exceptions) have a strong annual peak. Strong annual peaks are present for the northern hemisphere western boundary currents, where enhanced air‐sea interaction results in large annual heating/cooling effects. A mechanism of winter cooling proposed by Huang [1990] appears to be consistent with the seasonal movements and intensity of the surface jet, augmenting the annual peak. In contrast, the annual peak is less pronounced in the southern hemisphere western boundary currents, presumably because of smaller annual heating/cooling effects. Detrending of the height time series before the Fourier analysis shows the 2‐year sea level trends over the global ocean. The global average is about 4.5 mm/yr after correction for instrumental drift. The data have been treated with orbit and tide error reduction schemes. The frequency spectrum of the orbit correction is white (as expected) and proves that the orbit error reduction scheme utilized here does not attenuate the ocean signal.
A multibasin reduced model of the global thermohaline circulation: Paleoceanographic analyses of the origins of ice‐age climate variability
Authors: Sakai K. , Peltier W. R.
Published: February 2004

A new, multibasin reduced model of the global thermohaline circulation has been developed that builds upon the single‐basin Atlantic model recently described in Sakai and Peltier [1995]. The model comprises individual, two‐dimensional, Atlantic, Indian, and Pacific components which are linked via a circumpolar basin representative of the Southern Ocean. It also includes a complete seasonal cycle for sea surface temperature, a sea ice component, and an acceptably accurate representation of the influence of both wind stress and bottom topography. The circulation in the individual basins is described using a stretched coordinate system in order to allow the use of reduced vertical resolution where high resolution is unnecessary. Values for the control parameters of the new multibasin model are established in the same manner as was employed in analysis of the single‐basin model, and the basic conclusion of that study, that there exists a natural oscillatory mode of behavior of the deep circulation on a timescale ranging from a century to several millennia, is confirmed. The precise timescale of the internal variability is determined by the detailed nature of the hydrological cycle which is herein constrained far more realistically than was possible for the single basin model. The simulations performed with the model deliver a rather realistic facsimile of the millennium timescale Dansgaard‐Oeschger oscillations [Dansgaard et al., 1984] that were such a prominent feature of North Atlantic glacial climate according to climate proxy data from the Greenland Ice Core Project and Greenland Ice Sheet Project ice cores from Summit, Greenland, when boundary conditions are fixed to those appropriate to full glacial conditions. We also describe a sequence of paleoceanographic experiments that have been designed to explore the sensitivity of the deep circulation to the impact of the specific surface freshwater anomalies that are known to have developed during the last deglaciation event of the current ice age. The simulation of the response of the thermohaline circulation to such anomalies provides strong additional support for the notion that modulation of the strength of the overturning flow in the Atlantic basin played a very important role in the Younger‐Dryas climate reversal.