bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation relationships of stratospheric molecular constituents from high spectral resolution, ground‐based infrared solar absorption spectra
Authors: Rinsland Curtis , Goldman Aaron , Connor Brian J. , Stephen Thomas M. , Jones Nicholas B. , Wood Stephen W. , Murcray Frank J. , David Shelle J. , Blatherwick Ronald D. , Zander Rodolphe , Mahieu Emmanuel , Demoulin Phillipe
Published: September 2002

Comparisons of chemically active species with chemically inert tracers are useful to quantify transport and mixing and assess the accuracy of model predictions. We report measurements of chemically active species and chemically inert tracers in the stratosphere derived from the analysis of infrared solar absorption spectra recorded with a ground‐based Fourier transform spectrometer operated typically at 0.005‐ to 0.01‐cm−1 spectral resolution. The measurements were recorded from Kitt Peak in southern Arizona (latitude 31.9°N, 111.6°W, 2.09 km altitude). Time series of N2O, CH4, O3, and HNO3 vertical profiles have been retrieved from measurements in microwindows. From these results, correlations between N2O and CH4 stratospheric mixing ratios and between O3 and HNO3 lower stratospheric mixing ratios have been derived. The measured correlations between N2O versus CH4 mixing ratios are compact and show little variability with respect to season in quantitative agreement with Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) spring and autumn measurements recorded near the same latitude. Lower stratospheric O3 versus HNO3 mixing ratios measured during low to moderate aerosol loading time periods also show a compact relations though the HNO3/O3 slope is a factor of 2 lower than obtained from November 1994 ATMOS measurements near the same latitude. We also compare Kitt Peak and ATMOS N2O versus CH4 and O3 versus HNO3 relations obtained by averaging the measurements over two broad stratospheric layers. This comparison avoids bias from the a priori profiles and the limited vertical resolution of the ground‐based observations.
An inverse model for three‐dimensional flow in variably saturated porous media
Authors: Hughson Debra L. , Yeh T.‐C. Jim
Published: September 2002

Geostatistical theory has shown promise in dealing with issues of stability, uniqueness, and identity of estimates inherent in inverse problems of subsurface flow. Here the geostatistical method is extended to three‐dimensional, unsteady flow in variably saturated porous geological media (the vadose zone) that are modeled using the Richards equation and the van Genuchten‐Mualem constitutive equations. The saturated hydraulic conductivity, α, and n parameters of this relationship are treated as spatially correlated, statistically independent, stochastic processes for representing heterogeneity of porous media. For given covariance functions of the parameters the adjoint‐state sensitivity method is used to calculate first‐order approximations for covariances of capillary pressure and moisture content and cross covariances between capillary pressure, moisture content, and the hydraulic properties. These covariances and cross covariances are then used in a successive linear estimator (SLE) to estimate the conditional means of the heterogeneous hydraulic property fields based on measurements of pressure and moisture content data. A sequential conditioning approach for our SLE was also applied to data sets collected at different sampling times during a transient infiltration event. This approach has the benefit of reducing the size of the matrices and so helps avoid numerical stability problems. On the basis of our study, pressure and moisture content data sets collected at later times of an infiltration event or during steady state flow were found to provide better estimates (smaller mean‐square error compared to the true field) of the hydrological parameters of the vadose zone than data from very early times.
A seasonal fractional ARIMA Model applied to the Nile River monthly flows at Aswan
Authors: Montanari Alberto , Rosso Renzo , Taqqu Murad S.
Published: September 2002

A seasonal fractional autoregressive integrated moving average (ARIMA) model is introduced, with both short‐ and long‐term persistent periodic components. The estimation of the parameters is carried out by applying the Whittle's approximation to the Gaussian maximum likelihood function, which yields asymptotically consistent estimates. This method is particularly useful for hydrological time series. It is applied here to the Nile River monthly flows at Aswan in order to detect whether long memory is present. The results are compared with ones obtained by applying heuristic procedures, some of which were developed recently, in order to see how these perform on seasonal data.
Sources of chloride and implications for 36Cl dating of old groundwater, Southwestern Great Artesian Basin, Australia
Authors: Love A. J. , Herczeg A. L. , Sampson L. , Cresswell R. G. , Fifield L. K.
Published: September 2002

We investigate the relative importance of a number of processes that affect Cl and 36Cl distribution in the southwestern flow systems of the Great Artesian Basin, Australia, to provide more precise estimates of age and flow and recharge rates. In situ production of 36Cl , secular variations in the rate of production of 36Cl , and its subsequent fallout as well as rates of diffusion of stable Cl from adjacent aquitards mask the interpretation of rates of decay of 36Cl/Cl along hydraulic gradients and can preclude estimating absolute groundwater ages. However, mean flow velocities calculated from absolute Cl concentrations are 0.24±0.03 m/yr and are a more reliable parameter than calculated ages because fewer assumptions and approximations are required. Recharge rates to the main Cadna‐owie Formation and Algebuckina Sandstone aquifer of the Great Artesian Basin estimated from Cl mass balance in the unconfined aquifers are 0.16±0.08 mm/yr and were probably up to a factor of 3 higher >30 kyr ago as indicated by decreasing Cl and δ18O concentrations along the head gradient.
Flow in unsaturated fractured porous media: Hydraulic conductivity of rough surfaces
Authors: Or Dani , Tuller Markus
Published: September 2002

The general trend in models for flow in unsaturated fractured porous media is to regard desaturated fractures as nonparticipating elements that impede flow. Mounting experimental and theoretical evidence shows that fractures retain and conduct liquid in the form of film and partially filled corner flow to a relatively low degree of saturation. A simple geometrical model for rough fracture surfaces is developed offering a tractable geometry for calculations of surface liquid storage due to adsorbed films and capillary menisci. Assuming that under slow laminar flow the equilibrium liquid configurations on the fracture surface are not modified significantly, the average hydraulic conductivities for film and corner flows were derived and used as building blocks for a representative fracture roughness element and an assemblage of statistically distributed surface roughness elements. Calculations for a single representative element yielded excellent agreement with surface storage and unsaturated hydraulic conductivity measurements of Tokunaga and Wan [1997]. A statistical representation of surface roughness using a gamma distribution of pit depths resulted in closed‐form expressions for unsaturated hydraulic conductivity averaged across the fracture length (transverse to flow) or weighted by the liquid cross section occupying the fracture surface. An important attribute of the surface roughness model is the direct link between fracture surface and matrix processes unified by the matric potential. The proposed model represents a first step toward development of a comprehensive approach for liquid retention and hydraulic conductivity of unsaturated fractured porous media based on details of liquid configuration for different matric potentials.
Modeling the thermal‐hydrologic processes in a large‐scale underground heater test in partially saturated fractured tuff
Authors: Birkholzer J. T. , Tsang Y. W.
Published: September 2002

The Drift Scale Test (DST) is being conducted in an underground facility at Yucca Mountain, Nevada, to probe the coupled thermal, hydrological, mechanical, and chemical processes likely to occur in the fractured rock mass around a potential high‐level nuclear waste repository. Thermal‐hydrological processes in the DST have been simulated using a three‐dimensional numerical model. The model incorporates the realistic test configuration and all available site‐specific measurements pertaining to the thermal and hydrological properties of the unsaturated fractured tuff of the test block. The modeled predictions were compared to the extensive set of measured data collected in the first year of this 8‐year‐long test. The mean error between the predictions and measurement at 12 months of heating for over 1600 temperature sensors is about 2°C. Heat‐pipe signature in the temperature data, indicating two‐phase regions of liquid‐vapor counterflow, is seen in both the measurements and simulated results. The redistribution of moisture content in the rock mass (resulting from vaporization and condensation) was probed by periodic air‐injection testing and geophysical measurements. Good agreement also occurred between the model predictions and these measurements. The general agreement between predictions from the numerical simulations and the measurements of the thermal test indicates that our fundamental understanding of the coupled thermal‐hydrologic processes at Yucca Mountain is sound. However, effects of spatial heterogeneity from discrete fractures that are observed in the temperature data are not matched by simulations from the numerical model, which treat the densely spaced fractures as a continuum.
The fractional‐order governing equation of Lévy Motion
Authors: Benson David A. , Wheatcraft Stephen W. , Meerschaert Mark M.
Published: September 2002

A governing equation of stable random walks is developed in one dimension. This Fokker‐Planck equation is similar to, and contains as a subset, the second‐order advection dispersion equation (ADE) except that the order (α) of the highest derivative is fractional (e.g., the 1.65th derivative). Fundamental solutions are Lévy's α‐stable densities that resemble the Gaussian except that they spread proportional to time1/α, have heavier tails, and incorporate any degree of skewness. The measured variance of a plume undergoing Lévy motion would grow faster than Fickian plume, at a rate of time2/α, where 0 < α ≤ 2. The equation is parsimonious since the parameters are not functions of time or distance. The scaling behavior of plumes that undergo Lévy motion is accounted for by the fractional derivatives, which are appropriate measures of fractal functions. In real space the fractional derivatives are integrodifferential operators, so the fractional ADE describes a spatially nonlocal process that is ergodic and has analytic solutions for all time and space.
Regional flood quantile estimation under linear transformation of the data
Authors: Arsenault Michel , Ashkar Fahim
Published: September 2002

We examine some performance indices (PIs) that are used to compare regional and at‐site flood quantile estimation methods. These include the relative bias, the regional average root‐mean‐square‐error (RMSE), the regional average relative root‐mean‐square‐error (RRMSE), and the average RMSE and RRMSE ratios of quantile estimators. We study the dependence of these PIs on the relative variability (coefficient of variation) of the data. This is done by examining the effect of a location shift in the data on these PIs. The aim is to bring awareness to the fact that when comparing hydrological quantile estimators, some PIs are more greatly affected than others by data shifts in location. Among the PIs considered, we identify those that are invariant to a location shift in the data and those that are not. This is done under both assumptions of homogeneous and heterogeneous hydrological region. The generalized extreme value distribution is used to demonstrate some of the results, but the conclusions are applicable to other distributions with a location parameter. It is argued that because of the lack of invariance to location shift of certain quantile estimation methods and PIs, additional precautions need to be taken when comparing these methods. Although we focus discussion around flood frequency analysis, the points raised should be viewed within the broader context of hydrological frequency analysis.
Valuing discrete improvements in drinking water quality through revealed preferences
Authors: McConnell Kenneth E. , Rosado Marcia A.
Published: September 2002

We show how nonmarginal benefits from higher drinking water quality can be measured doing a standard welfare analysis, where the parameters for the analysis are obtained through a nested logit model structured according to the defensive inputs available for drinking water treatment.
Satellite thermal analyses of lava dome effusion rates at Unzen Volcano, Japan
Authors: Wooster M. J. , Kaneko T.
Published: September 2002

This paper presents analyses of infrared remote sensing data of the 1992–1994 lava dome growth episode of Unzen Volcano, Japan. The primary aim is to determine whether such data can indicate trends in the rate of magma supply to the dome, since this is one of the principal factors controlling the frequency of pyroclastic flow. Spaceborne, low‐spatial‐resolution infrared radiance data correctly delimit the two discrete phases of magma supply to the dome, and during phase 1 the short‐wavelength data are found to be strongly correlated with ground‐based estimates of lava effusion rate. The correlation is, however, significantly weaker during phase 2. Analysis of high‐spatial‐resolution data from airborne and satellite based systems allows us to determine the principal source of shortwave infrared flux as fumarolically heated dome surfaces, these being significantly hotter than the actively growing lava lobes. Variations in the low‐spatial‐resolution shortwave infrared signal are therefore directly attributed to fluctuations in the size and temperature of the fumarolically heated area. These fumarolic variations are largely dependent upon the flux rate of magmatic gas, which in turn is partly dependent upon the rate of magma supply. Gas fluxes measured using correlation spectrometer (COSPEC) measurements of SO2 concentration indicate that the relationship between fumarolic discharge and the effusion rate of lava was relatively stable during phase 1 but highly variable during phase 2. We suggest these variations as an explanation for the differing correlations observed between the emitted shortwave infrared flux and the effusion rate of lava during phases 1 and 2. We conclude that appropriate analysis of high‐ and low‐spatial‐resolution shortwave infrared remote sensing data may assist in elucidating trends in lava supply at volcanoes exhibiting similar dome growth episodes to that experienced at Unzen.
An eddy‐permitting coupled physical‐biological model of the North Atlantic: 1. Sensitivity to advection numerics and mixed layer physics
Authors: Oschlies Andreas , Garçon Veronique
Published: September 2002

Physical influences on biological primary production in the North Atlantic are investigated by coupling a four‐component pelagic ecosystem model with a high‐resolution numerical circulation model. A series of sensitivity experiments demonstrates the important role of an accurate formulation of upper ocean turbulence and advection numerics. The unrealistically large diffusivity implicit in upstream advection approximately doubles primary production when compared with a less diffusive, higher‐order, positive‐definite advection scheme.This is of particular concern in the equatorial upwelling region where upstream advection leads to a considerable increase of upper ocean nitrate concentrations. Counteracting this effect of unrealistically large implicit diffusion by changes in the biological model could easily lead to misconceptions in the interpretation of ecosystem dynamics. Subgrid‐scale diapycnal diffusion strongly controls biological production in the subtropical gyre where winter mixing does not reach the nutricline. The parameterization of vertical viscosity is important mainly in the equatorial region where friction becomes an important agent in the momentum balance.
Laminar cirrus observed near the tropical tropopause by LITE
Authors: Winker D. M. , Trepte C. R.
Published: September 2002

The Lidar In‐space Technology Experiment (LITE) provided near‐global observations of optically thin clouds during a 10‐day Space Shuttle mission in September 1994. We report here on layers of cirrus occurring in thin sheets, which we refer to as ‘laminar’ cirrus, observed near the tropical tropopause. The layers were observed to have thicknesses generally between a few hundred meters and one kilometer and to be unusually homogeneous in the horizontal, with extents of up to 2700 km. Layers were observed near and possibly above the mean tropical tropopause, both in clear air and above intense tropical thunderstorms, but only in the tropics (35°N to 20°S). Thin layers near the tropopause were found to be common, but not pervasive, throughout the tropics including regions characterized by large scale subsidence in the middle troposphere.
Ground deformation measurements at the Soufriere Hills Volcano, Montserrat: II: Rapid static GPS measurements June 1996–June 1997
Authors: Shepherd John B. , Herd Richard A. , Jackson Paul , Watts Robert
Published: September 2002

We describe measurements of ground deformation around the erupting Soufriere Hills volcano made between June 1996 and June 1997 using Rapid Static GPS. The measurements define a lateral displacement field which is approximately radially symmetrical about the growing dome. No significant sustained vertical displacements have been identified. Radial displacements have generally increased steadily during this period of observation. The rate of displacement decreases from 43 cm per year at a distance of 0.6 km from the centre of the dome to about 1 cm/year at a distance of 3.8 km. Strains within one km of the dome itself are greater than 10−3. The rate of variation of strain with distance indicates a causative centre of pressure at a depth of not greater than 750 meters below the summit of the dome
Geometrical path length probability density functions of the skylight transmitted by midlatitude cloudy skies: Some case studies
Authors: Veitel H. , Funk O. , Kurz C. , Platt U. , Pfeilsticker K.
Published: September 2002

Examples of inferred probability density functions of geometrical path lengths (PDF‐GP) for zenith‐scattered skylight transmitted by cloudy skies at midlatitude (50°N, 8.2°E) are reported. The method relies on high resolution spectroscopy of the O2 A‐band (760nm–780nm) absorption in the transmitted skylight, and an·inverse Laplace transform to recover the PDF‐GP from the absorption strengths of a set of A‐band absorption lines. Some examples of a larger set of measurements are presented and discussed in detail. These examples include PDF‐GPs for typical midlatitude summer (fair weather Cumulus) and winter (Stratus) cloudiness. Moreover, two cloud observations with largely enhanced O2 A‐band absorptions (up to a factor of 7 implying a mean geometrical path length of about 116 km) are discussed, namely the PDF‐GPs of thunderstorm clouds (Cumulonimbus, Cb) and of clouds formed in a cold front (Stratocumulus, Sc; Altostratus, As; Nimbostratus, Ns; Cirrus, Ci).
Seasonal climatology of the nighttime tidal perturbation of temperature in the midlatitude mesopause region
Authors: Williams Bifford P. , She C. Y. , Roble Raymond G.
Published: September 2002

We present a seasonal‐average climatology of the nocturnal variation of temperature and sodium density in the mesopause region (80–110km) based on 300 nights of observation from 1990–1997 by the sodium resonance lidar in Fort Collins, Colorado (40.6°N, 105°W). Waves with downward phase progression were observed during all seasons, with properties (coherence over many nights, observed half‐period, etc.) consistent with the upward‐propagating semidiurnal tide. The semidiurnal tide was strong (10K at 90km) and stable from November to March with a 30 km vertical wavelength. In April and September, the vertical wavelengths were longer and the amplitude increased rapidly with height above 95km. From May–August, there is a change in wave structure and low amplitudes at 90 km, possibly due to the diurnal tide. The Thermosphere‐lonosphere‐Mesosphere‐Electrodynamics General Circulation Model (TIME‐GCM) predicts a dominant semidiurnal tide in good agreement with observations, except for a difference in the phase at equinox.
Coherent Pc 3 pulsations in the prenoon sector observed along the 210° magnetic meridian
Authors: Tanaka Y.‐M. , Yumoto K. , Shinohara M. , Kitamura T.‐I. , Solovyev S. I. , Vershinin E. F. , Fraser B. J. , Cole D.
Published: September 2002

We studied the coherence and phase structure of Pc 3 pulsations in the H and D components of the magnetic field observed at latitudinally distributed stations along the 210° magnetic meridian by using the H component at GUA (L = 1.01) as the reference. At GUA, which is located near the magnetic equator, field line resonance does not occur because of the small vertical component of the ambient magnetic field, so pulsations observed there are likely produced by fast mode waves propagating from sources external to the magnetosphere. We found that the D‐component Pc 3's in the prenoon sector 0900–1200 LT at L = 1.2–5.5 show a coherent, in‐phase (out‐of‐phase) oscillation in the northern (southern) hemisphere with respect to the H component Pc 3's at GUA. This result indicates that the D‐component Pc 3's in the prenoon sector at L = 1.2–5.5 are closely related to the upstream source waves.
Mechanism of ELF radiation from sprites
Authors: Pasko Victor P. , Inan Umran S. , Bell Timothy F. , Reising Steven C.
Published: September 2002

Charge and current systems associated with sprites constitute a part of the large scale atmospheric electric circuit, providing a context for physical understanding of recently discovered ELF radiation originating from currents flowing within the body of sprites. It is shown that the impulse of the electric current driven in the conducting body of the sprite by lightning generated transient quasi‐electrostatic fields produces significant electromagnetic radiation in the ELF range of frequencies, comparable to that radiated by the causative lightning discharge.
Sediment resuspension in the wakes of Hurricanes Edouard and Hortense
Authors: Dickey T. D. , Chang G. C. , Agrawal Y. C. , Williams A. J. , Hill P. S.
Published: September 2002

A unique set of physical and optical observations of sediment resuspension was obtained during the passage of two hurricanes, Edouard and Hortense. The eyes of these hurricanes passed within approximately 110 and 350 km, respectively, of our study site on the continental shelf (100 km south of Cape Cod, Massachusetts) within a two‐week period in the fall of 1996. Sediments were resuspended to more than 30 m above the ocean bottom during both hurricanes. The sediment resuspension processes associated with the two hurricanes are shown to differ primarily because of the separation distances between the eyes of the hurricanes and the observational site (e.g., local versus remote forcing). Observed particle‐size distributions were shifted toward smaller scales during Hurricane Edouard because of flocculate disaggregation caused by high levels of localized shear and turbulence.
Global variations in droughts and wet spells: 1900–1995
Authors: Dai Aiguo , Trenberth Kevin E. , Karl Thomas R.
Published: September 2002

The Palmer Drought Severity Index (PDSI) was calculated globally using gridded monthly air temperature and precipitation. From 1900 to 1995, there are large multi‐year to decadal variations in the percentage areas in severe drought (PDSI < −3.0) and severe moisture surplus (PDSI > +3.0) over many land areas while secular trends are small. Since the late 1970s, however, there have been some increases in the combined percentage areas in severe drought and severe moisture surplus, resulting from increases in either the drought area (e.g., over the Sahel, eastern Asia and southern Africa) or both the drought and wet areas (e.g., over the U.S. and Europe). Although the high percentages of the dry and wet areas in the recent decades are not unprecedented during this century (except the Sahel), the recent changes are closely relate to the shift in El Niño‐Southern Oscillation (ENSO) towards more warm events since the late 1970s and coincide with record high global mean temperatures. Moreover, for any given value of ENSO indices, the PDSI anomalies tend to be larger than would be expected from previous records. These changes are qualitatively consistent with those expected from increased green‐house gases in the atmosphere.
GCM sensitivity test using increased rotation rate, reduced solar forcing and orography to examine low latitude glaciation in the neoproterozoic
Authors: Jenkins Gregory S. , Frakes Larry A.
Published: September 2002

During the Neoproterozoic period, most landmasses were assembled into the super‐continent, Rodinia, and parts experienced glaciation in low paleolatitudes. We examine possible causes of glaciation by increasing rotation rate, reducing the solar constant and carbon dioxide concentrations for an idealized super‐continent that is centered in the Northern Hemisphere tropics‐subtropics. Further, we introduce a 2 km north‐south mountain chain in the western regions of this super‐continent. A mixed layer ocean or prescribed time varying sea surface temperatures are used in these simulations. Our results show that neither an individual factor or a combination of these factors can cool temperatures enough to bring about glaciation on the tropical super‐continent. We conclude that other factors would be needed to initiate glaciation.
A model for pickup ion transport in the heliosphere in the limit of uniform hemispheric distributions
Authors: Schwadron N. A.
Published: September 2002

On the basis of recent observations that pickup ions are inefficiently scattered through 90° pitch angle, a pickup ion transport model has been developed which assumes that pickup ion distributions are uniform in the so‐called forward and reverse hemispheres, containing ions with pitch angles less than 90° and greater than 90°, respectively. A simple, fast numerical scheme is then described which solves the transport equations. Several examples are solved and comparisons are performed between the numerical solutions and previously used analytical solutions. Results from the model are then compared to pickup ion spectra obtained with the solar wind ion composition spectrometer on the Ulysses satellite.
The interaction of neutral interstellarHwith the heliosphere: A 2.5‐D particle‐mesh boltzmann simulation
Authors: Lipatov A. S. , Zank G. P. , Pauls H. L.
Published: September 2002

A nonstationary Boltzmann particle‐mesh method for neutral hydrogen originating from either the interstellar medium, the heliosheath, or the solar wind has been developed to simulate the detailed interaction of the solar wind with the local interstellar medium (LISM). Such a Boltzmann description for the hydrogen serves to capture the complicated neutral distribution. The neutral component is coupled to a hydrodynamic plasma through charge exchange. The distribution of the ionized component was taken from a two‐shock model [Zank et al., 1996b]. This approach is used (1) to elucidate the nature of the solar wind ‐ LISM interaction more carefully than has been done previously in a multifluid approach [Zank et al., 1996b] and (2) to identify precisely the role played by the three identifiably distinct neutral populations in determining the global structure and dynamics of the heliosphere. The differences in the global neutralHdistribution that exist between Boltzmann and multifluid gasdynamic models axe discussed briefly.
Self‐organization in the two‐dimensional magnetohydrodynamic transverse Kelvin‐Helmholtz instability
Authors: Miura Akira
Published: September 2002

For a two‐dimensional transverse configuration in a compressible plasma a magnetohydrodynamic simulation of the Kelvin‐Helmholtz (K‐H) instability has been performed for a subfast shear flow. The simulation shows that after the linear growth and the subsequent nonlinear saturation of the fastest growing vortices these vortices are susceptible to vortex pairings, which occur because of the growth of subharmonics. The total kinetic energy remains almost constant in the evolution of the instability, but the enstrophy decreases rapidly owing to the selective dissipation by an artificial viscosity, which is added to prevent mesh oscillations. Therefore the nonlinear evolution of the two‐dimensional transverse K‐H instability, in particular, the successive pairings of vortices, are well described as a self‐organization process resulting from the interplay of the nonlinearity and the dissipation. After the early stage of the instability development the kinetic energy and the squared vorticity cascade toward the long wavelength (inverse cascade) to form power law spectra in the wavenumber space. The inverse cascade in the wavenumber space corresponds, in the configuration space, to an emergence of a large isolated flow vortex and an associated eddy of inertia current out of trains of small‐scale vortices and current eddies in the early stage. At the end of the simulation run the power law exponents in the wavenumber space of the kinetic energy, the squared vorticity, and the magnetic energy, which are all integrated across the initial flow direction, become −3.89, −2.08, and −4.58, respectively, in the intermediate wavenumber subrange.
Phase velocity structure from Rayleigh and Love waves in Tibet and its neighboring regions
Authors: Griot Daphné‐Anne , Montagner Jean‐Paul , Tapponnier Paul
Published: September 2002

Deep geodynamic processes involved in the continental collision between India and Asia are still controversial. To address this issue, phase velocities of 619 Rayleigh waves and 254 Love waves have been inverted by using an anisotropic tomographic technique. Such a technique enables us not only to retrieve phase velocity distributions but also to map azimuthal anisotropy on a large scale. Only phase velocity anomalies, in the period range 50–200 s, are dealt with here, but the azimuthal anisotropy has been taken into account in the inversion process. Moreover, because of the important influence of shallow layers on the inversion results, an inversion of phase velocities corrected for shallow layers has been performed. To correct raw phase velocities, the 3SMAC crust (heterogeneous reference Earth model) has been used. The comparison of phase velocity distributions for Love and Rayleigh waves with and without corrections of surficial layers implies that much of the low‐velocity zone, visible at short periods, in central Tibet, can be explained by unusually thick crust. At periods longer than 100 s, Tibet is, overall, characterized by high‐velocity anomalies. Such anomalies seem to converge toward the center of the plateau as period increases. We conclude that the phase velocity structure beneath Tibet results, at short periods, from thick crust and possible partial melting in the uppermost lithosphere and, at long periods, from the inward plunge of the cold lithospheric mantle surrounding the high plateau.
New Cretaceous and Early Tertiary paleomagnetic results from Xining‐Lanzhou basin, Kunlun and Qiangtang blocks, China: Implications on the geodynamic evolution of Asia
Authors: Halim N. , Cogné J. P. , Chen Y. , Atasiei R. , Besse J. , Courtillot V. , Gilder S. , Marcoux J. , Zhao R. L.
Published: September 2002

We present the results of a paleomagnetic study of 360 cores, drilled at three different areas from the Tibetan Plateau: 13 sites from Lower Cretaceous red beds in the Xining‐Lanzhou basin, around Xining and Lanzhou cities, southwest of the Qilian mountains (36.2°N, 103.5°E); 13 sites from Cretaceous red beds in the Kunlun block, near Maqin (34.5°N, 100.1°); and 9 sites from Paleocene/lower Eocene red beds in the Qiangtang block, near Fenghuoshan (34.5°N, 92.8°E). Thermal demagnetization of the samples allowed us to isolate a high‐temperature component which passes both positive reversal and fold tests for all formations. The corresponding paleopoles lie at 50.3°N, 195.5°E (A95 = 4.6°) for Xining‐Lanzhou, 80.1°N, 281.2°E (dp = 7.8°, dm = 12.7°) for Maqin, and 62.6°N, 210.5°E (dp = 3.9°, dm = 6.8°) for Fenghuoshan. We discuss these in the frame of a new paleogeographic reconstruction of the Cretaceous paleoposition of the blocks forming the Asian mosaic. We conclude that the Xining‐Lanzhou area could not be part of the North China Block but rather was associated with the Tarim‐Qaidam assemblage. Paleomagnetic data argue in favor of a Qaidam‐Kunlun‐Tarim‐Junggar assemblage in the Cretaceous, significantly to the south of its current position with respect to the Asian continent (Siberia, Mongolia and North China). The large N‐S convergence (800±500 km) implied since the Cretaceous appears to be far larger than could be absorbed in the Altay range to the north and Qilian Shan to the east (of the order of 300 km). Part of this motion could have occurred along a large left‐lateral strike slip fault system, which may connect with the Mongol‐Okhotsk suture to the northeast. Verifying this hypothesis will require new geologic and paleomagnetic data from these remote regions.
Two‐dimensional travel time inversion for the crustal P and S wave velocity structure of the Ruby Mountains metamorphic core complex, NE Nevada
Authors: Stoerzel Andreas , Smithson Scott B.
Published: September 2002

Problems about the nature of extended crust in the Basin and Range include the role of plastic flow during extension, the possibility of fluids in the crust, and the amount of mantle‐derived material added to the crust. To address these problems, the University of Wyoming conducted a multicomponent wide‐angle seismic experiment in the Ruby Mountains of the Basin and Range. The northern Ruby Mountains expose upper and middle crustal rocks of a metamorphic core complex, whereas the southern Ruby Mountains consist of low‐grade miogeoclinal rocks. The wide‐angle experiment consisted of a 95 km long N‐S profile that extended from the southern to the northern Ruby Mountains. Travel time inversion of the wide‐angle reflection data reveals relatively high seismic velocities in the midcrust to lower crust which suggest that fluid‐filled pores, if they exist, do not reduce the seismic velocities significantly. Therefore the midcrustal to lower crustal porosity is probably much smaller than 1–2 vol %. The average P wave velocity for the crust is 6.4–6.5 km/s in the north and 6.1–6.2 km/s in the south. This difference suggests that larger amounts of mantle‐derived material were added during the Tertiary to the core complex crust of the northern Ruby Mountains than to the “normal” Basin and Range crust of the southern Ruby Mountains. The velocity profile is consistent with a maximum of 6–7 km of mantle‐derived material in the south and a maximum of 12–15 km of mantle‐derived material in the north. Thus volumetric mafic intrusions probably accommodated a maximum of 7 km of core complex exhumation. Shear wave splitting indicates that the upper to middle crust is seismically anisotropic probably due to superposed Tertiary plastic flow patterns. Doming and inflation of a midcrustal layer in the seismic model are compatible with Tertiary flow of material from the south to the north that compensated about 2–5 km of core complex exhumation. Precritical Moho reflections (0–70 km offset, 10–11 s normal two‐way travel time) are weaker and less continuous than midcrustal reflections (7–9 s normal two‐way travel time). Strong and continuous Moho reflections occur at postcritical (>80 km) offsets only. Modeling shows that the Moho depth increases from about 32.5 km in the south to about 34.5 km in the north.
High‐resolution electromagnetic imaging of the San Andreas Fault in central California
Authors: Unsworth Martyn , Egbert Gary , Booker John
Published: September 2002

Although there is increasing evidence that fluids may play a significant role in the earthquake rupture process, direct observation of fluids in active fault zones remains difficult. Since the presence of an electrically conducting fluid, such as saline pore water, strongly influences the overall conductivity of crustal rocks, electrical and electromagnetic methods offer great potential for overcoming this difficulty. Here we present and compare results from high‐resolution magnetotelluric (MT) profiles across two segments of the San Andreas Fault (SAF) which exhibit very different patterns of seismicity: Parkfield, which has regular small earthquakes and creep events, and in the Carrizo Plain, where the fault is seismically quiescent and apparently locked. In both surveys, electric fields were sampled continuously, with 100 m long dipoles laid end‐to‐end across the fault. From 100 to 0.1 Hz the data from both profiles are consistent with a two‐dimensional (2‐D) fault‐parallel resistivity model. When both transverse electric and magnetic (TE and TM) mode data are included in the interpretation, narrow (∼300–600 m wide) zones of low resistivity extending to depths of 2–4 km in the core of the fault are required at both locations. However, at Parkfield the conductance (conductivity thickness product) of the anomalous region is an order of magnitude larger than at Carrizo Plain, suggesting much higher concentrations of fluids for the more seismically active Parkfield segment. We also image structural differences between the two segments. At Carrizo Plain, resistive, presumably crystalline, rocks are present on both sides of the fault at depths below 3–4 km. In particular, we clearly image resistive basement extending ∼10 km or more east of the SAF, beneath the Elkhorn Hills and Temblor Range. At Parkfield the situation is quite different with a resistive block of Salinian granite west of the fault and an electrically conductive, presumably fluid rich Franciscan complex to the east. It is possible that these structural differences control the difference in mechanical behavior of the fault, either directly by affecting fault strength or indirectly by controlling fluid supply.
Sound velocities and elastic properties of Fe‐bearing wadsleyite and ringwoodite
Authors: Sinogeikin S. V. , Katsura T. , Bass J. D.
Published: September 2002

The sound velocities and single‐crystal elastic moduli of β phase (wadsleyite) and γ phase (ringwoodite) of (Mg,Fe)2SiO4 with Fe/(Fe+Mg) ratios of ∼0.075 and ∼0.09, respectively, have been determined at ambient conditions by Brillouin spectroscopy. Both compressional and shear wave aggregate velocities decrease with increasing Fe content in both phases, but the magnitude of this decrease is different for the two phases. The adiabatic bulk modulus, Ks, of Fe;‐bearing β‐Mg2SiO4 (Ks = 170±2 GPa) is indistinguishable from that of the Mg end‐member within experimental uncertainty, whereas Ks of γ‐(Mg,Fe)2SiO4 increases rapidly with increasing iron content. The shear moduli of both phases decrease with increasing Fe content. Our measurements indicate that the velocity and impedance contrasts between olivine and β‐(Mg,Fe)2SiO4 are independent of Fe content for Mg‐rich compositions, but the contrast for the β → γ‐(Mg,Fe)2SiO4 transition increases significantly with increasing Fe content. The new data support a previous estimate of 40±10% for the olivine content of the upper mantle and suggest that less than 50% (Mg,Fe)2SiO4 is sufficient to account for the observed impedance contrasts at depths of both 410 km and 520 km. Unless the effect of Fe on elastic properties is accounted for, it is difficult to account for both the 410 and 520 km discontinuities with a single olivine content.
Jurassic and Cretaceous apparent polar wander relative to South America: Some tectonic implications
Authors: Beck Myrl E.
Published: September 2002

Late Mesozoic apparent polar wander (APW) with respect to stable South America has been difficult to define. It appears that this observation has a simple explanation; very little APW occurred and none at all during a 50 m.y. period (165 to 115 Ma). It is possible to compute a “stillstand” pole for this interval (88.8°S, 72.4°E, K = 244.7, A95 = 4.9°) from five reliable studies comprising a total of 523 sites. A second stillstand of much shorter duration (70–80 Ma) is defined by four poles (52 sites); this is located at 80.8°S, 346.7°E (K = 457.7; A95 = 4.3°). APW of 9.2° occurred during the interval 115–80 Ma. A comparable stillstand is found in the North American record [e.g., Van Fossen and Kent, 1992]; this has a similar degree of scatter (K = 421) and spans at least 36 m.y. (124 to 88 Ma). Thus North America and South America both were rotating due westward during a large part of the Cretaceous; this has some important implications for global tectonics.
Measurement of surface currents using sequential synthetic aperture radar images of slick patterns near the edge of the Gulf Stream
Authors: Lyzenga David R. , Marmorino George O.
Published: September 2002

Two‐dimensional surface currents are estimated over an area of ∼100 km2 near the inshore edge of the Gulf Stream by correlating the surface slick patterns observed on two synthetic aperture radar (SAR) images collected about 20 min apart. The currents obtained from this analysis are found to agree well with shipboard acoustic Doppler current profiler (ADCP) measurements at 10 to 20‐m depths. The orientations of the long, linear slicks observed in this region differ from the mean current direction by about 16° but coincide with the direction of maximum radial deformation or strain, as calculated from the observed current field. The calculated current gradients indicate a mean surface convergence of about 2×10−5 s−1 and a cyclonic vorticity of about 1×10−4 s−1, values which fall within the range of previous estimates in the study area. These results suggest that slick patterns may be used to infer surface current magnitude and direction from a time sequence of SAR images but that care must be exercised in inferring current directions from the orientations of slick patterns within a single image.
Effects of intermediate‐scale waves on radar signatures of ocean fronts and internal waves
Authors: Lyzenga David R.
Published: September 2002

A new mechanism is proposed to explain the large changes in radar backscatter observed at high microwave frequencies (X band and C band) near ocean fronts and internal waves. The proposed mechanism involves the interaction of short “Bragg” waves with intermediate‐scale waves having wavelengths of the order of 1 m, as well as the interaction of both sets of waves with the large‐scale current gradients associated with the front or internal wave. Intermediate‐scale waves that are steepened by the large‐scale current field interact more strongly with the short Bragg waves, resulting in an increase in the mean spectral density of the short waves. Calculations of these effects, when combined with a simple Bragg model for the radar backscatter, are found to agree at least qualitatively with the backscatter modulations observed in SAR images collected near the edge of the Gulf Stream.
Subsurface, surface, and radar modeling of a Gulf Stream current convergence
Authors: Jansen Robert W. , Shen Colin Y. , Chubb Scott R. , Cooper Arnold L. , Evans T. E.
Published: September 2002

In this paper we investigate the underlying dynamics associated with a strong, line‐shaped submesoscale feature that was observed in radar imagery at the boundary between Gulf Stream (GS) and shelf water near Cape Hatteras during the first Naval Research Laboratory High‐Resolution Remote Sensing Experiment (HIRES 1). The line‐shaped feature, which appears as a pronounced (∼10 dB) increase in radar cross section, extends several kilometers in the east‐west direction. In situ current measurements have shown that this feature coincides with the boundary of a sharp current convergence front. These measurements also indicate that the frontal dynamics is associated with the subduction of denser GS water under lighter shelf water. Using the observation that the convergence can be attributed to a hydrodynamic instability at the water interface, we have modeled the resulting subsurface hydrodynamics on the basis of a rigid‐lid, two‐dimensional solution of the Navier Stokes equation. The calculations of subsurface current flow were used as input to a spectral (wave action) model of wave‐current interaction to obtain the surface wave field, which in turn was used to provide input for modeling of radar backscatter. The resulting description also includes the effects of surfactant‐induced wave damping on electromagnetic backscatter. Our predictions are compared with real aperture radar imagery and in situ measurements from the HIRES 1 experiment.
Diel variability of photosynthetic picoplankton in the equatorial Pacific
Authors: Vaulot Daniel , Marie Dominique
Published: September 2002

The diel variability in cell abundance, light scatter, and pigment fluorescence of three autotrophic picoplankton groups (Prochlorococcus, Synechococcus, picoeukaryotes) measured by flow cytometry was investigated in surface waters of the equatorial Pacific Ocean (5°S, 150°W) during 5 days with about 1 hour temporal resolution. The diel variability of vertical profiles was examined at the same station on days 2 and 4. Prochlorococcus division rate was also estimated from cell cycle measurements. A more limited data set was obtained at a station located in very oligotrophic waters (16°S, 150°W). All three picoplankton populations exhibited very marked diel variability. Cell division was highly synchronized but not phased identically for all three populations: Synechococcus divided first, followed 2 hours later by Prochlorococcus and 7 hours later by picoeukaryotes. Cells grew in size only once the sun had risen, but growth did continue in the dark for a short period. Growth processes occurred in parallel at the top and the bottom of the mixed layer, inducing uniform profiles for cell abundance and scatter. For chlorophyll fluorescence, in contrast, prokaryotes displayed opposite patterns during the light period between surface (decrease due to very strong quenching) and depth (increase). This created steep vertical gradients during the day that vanished at night because of convective mixing. In the top 25 m, strong light intensities (including UV radiation) had very pronounced detrimental effects on prokaryotes, especially on Prochlorococcus, inducing fluorescence quenching, slowed down growth, and retardation of DNA synthesis.