bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of the unambiguous 2‐dimensional horizontal wave number spectrum of OH intensity perturbations
Authors: Gardner Chester S. , Coble Mark , Papen George C. , Swenson Gary R.
Published: February 2004

The unambiguous 2‐D horizontal wave number spectrum of gravity wave perturbations in OH emission intensities is computed from imager observations acquired on 2 Feb 1995 at the Starfire Optical Range, NM. The technique involves computing the (ω, k, l) = (ω, h, ϕ) spectrum of OH image collected over a several hour period, where k is the zonal wave number, l is the meridional wave number, h = (k² + l²)1/2, ϕ = tan−1 (k/l), and ω is the temporal frequency. The data are used to study the azimuthal distribution of wave energy propagation and the separability of the (h, ϕ) spectrum. The h spectral indices, computed from power‐law fits of the 2‐D spectrum over the wave number range 2π/(200 km) < h < 2π/(20 km), vary between −1.6 and −3.6 as a function of ϕ with a mean of −2.58. Consequently, the image spectrum is not separable in h and ϕ. We show that nonseparability is most likely associated with distortion of the wave perturbations caused by the viewing geometry at large zenith angles.
Gravity and seismicity over the Guerrero Seismic Gap, Mexico
Authors: Kostoglodov V. , Bandy W. , Domínguez J. , Mena M.
Published: February 2004

Four detailed (average station interval = 5 km) gravity transects were recently conducted in the Pacific coastal region of Mexico. A differential GPS technique was used to determine the elevation and coordinates of the gravity stations. The profiles are oriented northeast‐southwest and extend from the coast up to ∼60 km inland. The Bouguer gravity anomaly is decreasing consistently along every profile from 60–80 mGal at the coast with an approximately constant regional gradient of −2.2 mGal/km normal to the trench. A plot of the gravity anomaly against the distance from the trench axis demonstrates that the regional slope in the gravity anomaly is shifting gradually (20–25 mGal) inland along the coast of Guerrero from the southeast (Atoyac) to the northwest (Petatlán ‐ Zihuatanejo). A model cross section of the Mexican subduction zone (MSZ) based on the tomography inversion for the Guerrero region shows that the gravity anomaly values and the regional anomaly trend can be explained mostly by the effect of the density contrast between the slab and the continental crust. The upper surface of the subducted slab (USS) and the seismogenic contact zone between the upper plate and the slab is traced clearly in several seismicity cross sections based on the data of the regional seismic network in Guerrero. The depth and shape of the USS revealed from the seismicity and gravity anomaly data for the same profiles are in good agreement. This correlation may be fairly useful when applied to gravity profiles in order to estimate the depth of the USS and the seismogenic contact in other parts of the MSZ which lack reliable seismicity data.
Comment of “VLF signatures of ionospheric disturbances associated with sprites” by Inan et al.
Authors: Dowden Richard L.
Published: February 2004

The events described by Inan et al. [1995b] in the paper under comment here, and earlier as “VLF sprites” [ Inan et al., 1995a], refer to VLF scattering confined to a very narrow range about the forward direction. This must be produced by horizontally extensive (few hundred km) ionospheric disturbances with a Gaussian shape as suggested by them. Such scattering is quite at odds with the very wide angle VLF scattering (even to 180°) observed by Dowden et al. [1995]. The reason for this is not due to differences in receiving equipment or data interpretation, but due to the scattering sources being quite different in size and shape. The narrow angle scattering sources may be the recently discovered “elves” (Emissions of Light and VLF perturbations from EMP) produced by the electromagnetic pulse (EMP) of lightning. The wide angle scattering sources have been identified as the thin (∼ 1 km) vertical columns of plasma as predicted by Dowden et al. [1994] and seen as the luminous columns of sprites.
New stable isotope results from a 173‐year coral from Espiritu Santo, Vanuatu
Authors: Quinn Terrence M. , Crowley Thomas J. , Taylor Frederick W.
Published: February 2004

Long coral records provide valuable information on pentadal‐decadal scale climate variability. Previously we have reported on a preliminary analysis of a 173‐year record (1806–1978) from the island of Espiritu Santo (Republic of Vanuatu). Although this record contained information in the pentadal‐decadal band, low sampling resolution raised questions about the validity of the results. Herein we report new data from the Santo record, in which the sampling resolution has been doubled and the validation database expanded. Variations in coral δ18O at Santo record the combined effects of variations in SST and rainfall‐induced salinity changes. The most prominent spectral peaks in the time series are at the annual cycle and 14–15 and 7.4 years. The robust occurrence of the 14–15 year peak provides additional support to the importance of this oscillation as a cause of decadal‐scale climate variability. Overall, our new results provide greater credibility to the conclusions raised in the earlier study and indicate that the Santo record can more confidently be incorporated into the still‐small network of multicentury coral records of climate change.
Heat flow and mineralogy of TAG Relict High‐Temperature Hydrothermal Zones: Mid‐Atlantic Ridge 26°N, 45°W
Authors: Rona Peter A. , Petersen Sven , Becker Keir , Von Herzen Richard P. , Hannington Mark D. , Herzig Peter M. , Naka J. , Lalou C. , Thompson G.
Published: February 2004

Submersible investigations employing heat flow measurements (12 stations), sampling and imagery of the two relict high‐temperature hydrothermal zones of the TAG field, the Alvin and Mir sulfide zones, elucidate relations between heat sources and mineralization including an active sulfide mound that has been the focus of prior studies. Values of heat flow in the Mir zone and at the margin of the active mound are inversely proportional to distance from adjacent volcanic centers. This observation supports the hypothesis that intrusions at volcanic centers adjacent to the high‐temperature hydrothermal zones supply the heat to drive hydrothermal activity. The chronology of hydrothermal deposits in the different zones indicates that the intrusions are episodic with field‐wide high‐temperature hydrothermal events recurring at intervals of tens of thousands of years, while activity at individual zones may recur at intervals of hundreds to thousands of years. A sequence of hydrothermal deposits ranges to at least 140,000 years ago from massive sulfides forming at the active mound, to recrystallization of sulfides in the active and relict zones, to pyritization of an inactive mound in the Alvin zone; low‐temperature mineral phases precipitate before, during and after the sulfides.
Slopes of Western Galapagos volcanoes from airborne interferometric radar
Authors: Mouginis‐Mark Peter J. , Rowland Scott K. , Garbeil Harold
Published: February 2004

The distribution of slopes on the six basaltic shield volcanoes in the Western Galapagos Islands is investigated using a digital elevation model derived from airborne interferometric radar (TOPSAR) data. These measurements have a spatial sampling of 10 m/pixel, a vertical accuracy of 3 to 5 m, and constitute the highest resolution, most complete, topographic data set available for the islands. Volcano heights are determined to range from 1,124 m (Sierra Negra) to 1,710 m (Wolf). Over extensive areas of each volcano, slopes exceed 25°, with the highest slopes being ∼37° on Wolf and ∼36° on Fernandina. We confirm that two morphologic subgroups exist: Cerro Azul, Fernandina, and Wolf, with deep calderas (depth between 40–60% of the subaerial height of the volcano) and steep (>20°) maximum slopes at elevations between ∼60 and 80% of the volcano height; and Alcedo, Darwin, and Sierra Negra, with shallow calderas (depth <25% of subaerial height) and slopes that remain <15° until ∼90% of the total height is reached. Our data show that steep slopes are not uniquely correlated with the occurrence of arcuate fissures at the summit, leaving the origin of the steep slopes unresolved.
D/H kinetic isotope effect in the reaction CH4+Cl
Authors: Saueressig G. , Bergamaschi P. , Crowley J. N. , Fischer H. , Harris G. W.
Published: February 2004

The D/H kinetic isotope effect (KIEClD) in the reaction of CH4 with Cl has been investigated using a tunable diode laser absorption spectrometer (TDLAS) to measure 12CH3D/12CH4 ratios. A KIEClD of 1.508±0.041 was obtained at room temperature (296 K). Experiments employing Fourier transform infrared measurements of 12CH3D/12CH4 ratios confirmed this result. An investigation of the temperature dependence showed an increase of KIEClD (1.592±0.057 at 223 K) with decreasing temperature. This very large D/H fractionation effect affects the overall isotope fractionation in the middle and upper stratosphere, where Cl contributes substantially to the total removal of CH4.
Lidar observations of a large high‐altitude sporadic Na layer during active aurora
Authors: Collins R. L. , Hallinan T. J. , Smith R. W. , Hernandez G.
Published: February 2004

A large sporadic Na layer was observed near 109 km during active aurora at Poker Flat, Alaska, on the night of 12–13 January 1996. This sporadic layer is significantly denser and broader than previously reported thermospheric sporadic layers; having a peak concentration equal to the peak concentration of the background Na layer, a FWHM of 4.7 km, and a column abundance of 40% of the background Na layer. The appearence of the Nas layer coincided with a significant OH temperature gradient across the sky but did not coincide with the presence of a discrete arc overhead nor a distinct signature in the local magnetometer data. The presence of such a large amount of neutral Na at these altitudes contradicts the current models of Nas layer formation. The observations are discussed in the light of other observations, current models, layered auroral phenomena, and meteoric ablation.
Effects of static non‐hydrostatic stress on the R lines of ruby single crystals
Authors: Chai Mu , Michael Brown J.
Published: February 2004

The R‐line fluorescence spectroscopy of ruby (Cr:Al2O3) single crystals has been investigated under controlled deviatoric stresses at high confining pressures in a diamond anvil cell. Full stress tensors of the ruby samples were determined based on free‐slip boundary condition and cylindrical symmetry. The shift of the R2 line is insensitive to deviatoric stress and represents local mean stress (pressure). When ruby is loaded along the c‐axis, the R1‐R2 splitting changes linearly with deviatoric stress at a rate of −0.241±0.012 Å/GPa and does not depend on the confining pressure. The splitting increases non‐linearly for ruby loaded along the a‐axis. The dependencies of the splittings on deviatoric stress are in excellent agreement with theoretical calculations constrained by shock‐wave data. The widths of the fluorescence lines increase with growing inhomogeneity in pressure, but are not affected by the deviatoric stresses.
Climate change from increased CO2 and direct and indirect effects of sulfate aerosols
Authors: Meehl Gerald A. , Washington Warren M. , Erickson David J. , Briegleb Bruce P. , Jaumann Peter J.
Published: February 2004

A global coupled ocean‐atmosphere general circulation model without flux correction is integrated in a set of 75 year sensitivity experiments where the indirect forcing effect of sulfate aerosols is included for the first time in combination with transient greenhouse gas forcing and the direct effect of sulfate aerosols. Sulfate aerosol forcing increases from zero to present‐day estimates in the first 30 years of the integrations while equivalent CO2 forcing increases by 1% per year relative to the control experiment, similar to the rate of increase of observed greenhouse gas forcing over the period 1960–1990. Annual mean averages around year 30, analogous to present‐day conditions, indicate better agreement with recent observed geographic and zonal mean temperature anomaly patterns in the sulfate aerosol experiments and less warming in northern summer than winter. Sulfate aerosols then are increased following the IS92a scenario, while CO2 continues to increase at 1% per year. Averages around year 65, analogous to conditions roughly 35 years in the future, indicate warming almost everywhere in the troposphere over the globe as the CO2 forcing overwhelms the negative radiative forcing from the sulfate aerosols. There is also a general indication of weakening of the south Asian monsoon in the sulfate aerosol experiments. There is qualitative agreement in the patterns of the temperature changes, both geographic and zonal, between the different sulfate aerosol experiments, with the magnitude of the changes a function of the size of the forcing.
Balancing the Cenozoic carbon and alkalinity cycles: Constraints from isotopic records
Authors: Goddéris Y. , François L. M.
Published: February 2004

A simple model of the carbon, alkalinity and strontium cycles is built up and used to interpret the carbon and strontium isotopic evolution of seawater over the Cenozoic as recorded in marine limestones. The idea that weathering fluxes have increased globally over the Cenozoic in response to the Himalayan uplift is critically examined. It is shown that such an increase of global weathering, postulated to explain the sharp increase in the 87Sr/86Sr ratio, is not compatible with the carbon isotopic record, if the budgets of carbon, alkalinity and strontium are considered simultaneously. Another scenario of the Cenozoic evolution of global weathering consistent with the carbon isotopic record is proposed in which the global weathering flux varies only slightly over the Cenozoic with a small increase between 15 Ma and the present. This second scenario appears more consistent with the reconstructed carbonate sedimentation rate and the evolution of the carbon isotopic fractionation in continental and oceanic environments.
A process‐based model to derive methane emissions from natural wetlands
Authors: Walter B. P. , Heimann M. , Shannon R. D. , White J. R.
Published: February 2004

A process‐based model has been developed in order to calculate methane emissions from natural wetlands as a function of the hydrologic and thermal conditions in the soil. The considered processes in the model are methane production, methane consumption and transport of methane by diffusion, ebullition and through plants. The model has been tested against data from a three‐year field study from a Michigan peatland. The interannual and seasonal variations of the modelled methane emissions and methane concentration profiles are in good agreement with the observations. During the growing season the main emission pathway proceeds through plants. Ebullition occurs whenever the water table is above the soil surface, while diffusion is only significant in the first 15 days after a drop of the water table below the peat surface.
A relic proterozoic subduction zone in western Canada: New evidence from seismic reflection and receiver function data
Authors: Eaton David W. , Cassidy John F.
Published: February 2004

Lithoprobe seismic‐reflection profiles in the Western Canada Sedimentary Basin reveal a prominent band of dipping reflections that appear to extend into the upper mantle. The NNE strike of these reflections, determined from intersecting profiles, is parallel to the trend of the ca. 1.8 Ga Rimbey domain, an overlying magmatic belt. Receiver‐function analysis of teleseismic data recorded at a nearby site shows evidence for a low‐velocity zone (LVZ) beneath the Rimbey domain between 13 and 16 km depth. The unusually low S‐wave velocity of the LVZ (2.9±0.2 km/s) suggests a composition similar to serpentinized peridotites that occur in ophiolites. Taken together, our data support a model of Proterozoic subduction beneath the Hearne province. We interpret the dipping reflections observed on the seismic profiles as a shear zone that developed during subduction of a SE‐dipping slab, and the LVZ as a fragment of oceanic lithosphere that was obducted to the overriding plate.
Acceleration of solar flare ³He at the Earth's bow shock
Authors: Dwyer J. R. , Mason G. M. , Mazur J. E. , von Rosenvinge T. T.
Published: February 2004

The STEP instrument on the WIND spacecraft observed several upstream events in coincidence with ³He and Fe‐rich solar energetic particle events in late 1994. The distinctive ion composition of the impulsive solar energetic particle population provides an opportunity to study mechanisms that produce upstream events. We report energy spectra and mass composition of both the upstream event ions and SEP ions as well as the first detection of ³He ions in an upstream event. The detection of ³He unambiguously demonstrates that solar energetic particles can undergo further acceleration near the Earth.
The role of HOx in super‐ and subsonic aircraft exhaust plumes
Authors: Hanisco T. F. , Wennberg P. O. , Cohen R. C. , Anderson J. G. , Fahey D. W. , Keim E. R. , Gao R. S. , Wamsley R. C. , Donnelly S. G. , Del Negro L. A. , Salawitch R. J. , Kelly K. K. , Proffitt M. H.
Published: February 2004

The generation of sulfuric acid aerosols in aircraft exhaust has emerged as a critical issue in determining the impact of supersonic aircraft on stratospheric ozone. It has long been held that the first step in the mechanism of aerosol formation is the oxidation of SO2 emitted from the engine by OH in the exhaust plume. We report in situ measurements of OH and HO2 in the exhaust plumes of a supersonic (Air France Concorde) and a subsonic (NASA ER‐2) aircraft in the lower stratosphere. These measurements imply that reactions with OH are responsible for oxidizing only a small fraction of SO2 (2%), and thus cannot explain the large number of particles observed in the exhaust wake of the Concorde.
Vertical distribution of nighttime stratospheric NO2 from balloon measurements: Comparison with models
Authors: Renard Jean‐Baptiste , Pirre Michel , Robert Claude , Lefevre Franck , Lateltin Eric , Nozière Barbara , Huguenin Daniel
Published: February 2004

Vertical distributions of NO2 and O3 mixing ratios and the aerosol extinction coefficient were measured by night between 18 and 32 km altitude (9–80 hPa) at mid latitudes using the balloon‐borne instrument AMON on March 24, 1994. The NO2 profile is compared with results of simulations involving the REPROBUS 3D Chemical‐Transport Model and a Lagrangian model in which the ozone mixing ratio and the aerosol surface area are initialized using AMON measurements (other mixing ratios are initialized with REPROBUS). As confirmed by Lidar observations, the surface areas were larger than the monthly and zonally averaged SAGE 2 data available for March 1994 at 45°N. The Lagrangian model shows relatively good agreement with AMON results for pressure higher than 40 hPa and smaller than 15 hPa, but the computed NO2 value is too high between 15 and 40 hPa. This seems to indicate that heterogeneous reactions involving the NOY species in the aerosol layer are still incompletely understood.
Effects of the equatorial electrojet on magnetotelluric surveys: Field results from northwest Brazil
Authors: Padilha Antonio L. , Vitorello Ícaro , Rijo Luiz
Published: February 2004

The distortion effect that the nonuniform equatorial electrojet (EEJ) currents might have on the plane‐wave assumption in magnetotelluric (MT) applications is investigated under experimental conditions. A broad‐band (0.0006–2048 s) MT survey, consisting of 8 stations aligned at right angles to the magnetic equator and probing diverse geologic terrains, was carried out during the daytime, under the presence of the EEJ nonuniform source, and at nighttime, when the EEJ flux almost vanishes. A comparison between the daytime and nighttime soundings did not show any significant divergence, contrary to expectation from some theoretical calculations. Thus, the results assure that the traditional plane‐wave formula employed in MT can provide reliable subsurface conductivity structures even under the influence of the EEJ, at least up to periods of 2048 s. An additional modelling exercise considering the two models most commonly used to simulate the EEJ sources was also performed. It indicates that the line‐current model generates departures from the plane‐wave results not observed in practice and that the Gaussian model only agrees with the experimental data, within their error bars, if the geoelectric section approaches its most conductive limit defined from a static shift evaluation.
Origins of porosity and velocity variations at Cascadia Accretionary Prism
Authors: Jarrard Richard D.
Published: February 2004

At Cascadia accretionary prism, log‐based porosities exhibit complex lateral changes in response to variations in cementation, horizontal compression, and fracturing. Porosities at the prism toe are lower than in reference sediments, whereas porosities landward of the toe are slightly higher than those of reference sediments. Ideally, a porosity/velocity relationship can be used to determine two‐dimensional porosity structure from seismic velocities. Porosity is the major control on velocity at Cascadia, but variations in grain framework strength (frame bulk modulus) cause deformed and undeformed rocks to have quite different velocity/porosity relationships. Within the prism, cementation can raise velocities substantially above those for unconsolidated reference sediments of the same porosity; also, compression‐induced compaction and fracturing may affect this velocity/porosity transform. Because these factors vary both within and between prisms, velocity/porosity transforms are local, not universal.
High time resolution study of the hemispheric power carried by energetic electrons into the ionosphere during the May 19/20,1996 auroral activity
Authors: Lummerzheim D. , Brittnacher M. , Evans D. , Germany G. A. , Parks G. K. , Rees M. H. , Spann J. F.
Published: February 2004

The ultraviolet imager (UVI) on board the POLAR satellite offers the opportunity to obtain high time resolution global auroral images. The spectral discrimination of the imager is sufficient to separate the auroral far ultraviolet emissions from the scattered sunlight, even when the entire auroral zone is sunlit. The energy flux of the precipitating electrons is derived from the surface brightness through the LBH‐long filter. Global images which have the dayglow removed are spatially integrated to yield the total rate of energy input into the northern hemisphere. This parameter, the hemispheric power, has found much application in ionospheric modeling. It can also be derived from electron spectra measured along the track of the NOAA/TIROS satellites that are combined with average empirical auroral precipitation patterns. We show that the hemispheric power derived from the two‐dimensional images represents a substantial improvement in the temporal variability of this parameter. We present an example for the period of 19/20 May 1996 by comparing the hemispheric power derived from NOAA/TIROS measurements with those derived from the UVI images.
North northeast Brazil rainfall and its decadal‐scale relationship to wind stress and sea surface temperature
Authors: Wainer Ilana , Soares Jacyra
Published: February 2004

In this study low‐frequency variability of rainfall data for the North part of Northeast (NNE) Brazil is examined. The time scale dependence of NNE Brazil rainfall and the associated sea surface temperature (SST) field is also described. Area averaged monthly rainfall data for the NNE Brazil from 1911 to 1989 exhibits not only a distinct interannual behavior but also significant decadal variability. Examination of a 30 year data set of tropical Atlantic SST and wind stress show similar behavior. It was found that the relationships between SST, NNE Brazil rainfall and meridional pseudo‐wind stress on the interdecadal time scale are very similar to those on the interannual and seasonal time scale. In both time scales, SST in the Tropical Atlantic are in phase with the meridional displacement of the Intertropical Convergence Zone and associated meridional pseudo‐wind stress and out of phase with rainfall.
Microphysics of the Venusian and Martian mantles
Authors: Quest K. B. , Shapiro V. D. , Szegö K. , Dobe Z.
Published: February 2004

The planetary ionospheres around the nonmagnetic planets Mars and Venus are directly exposed to the shocked solar wind. An interaction between the solar wind protons and the ionospheric oxygen takes place in a narrow turbulent region referred to as the plasma mantle. In this letter the microphysics of the dayside mantle is investigated numerically using a one‐dimensional hybrid code that retains the inertia of the electron species. It is shown that lower hybrid waves propagating perpendicular to the magnetic field are destabilized. Wave saturation is caused by electrostatic trapping of the proton species, and the saturated amplitudes are shown to be in reasonable agreement with Pioneer‐Venus observations. Oxygen pick‐up and acceleration is found to be dominated by wave effects, resulting in significant ion heating, consistent with Phobos observations.
Anisotropic Pn tomography in the western United States
Authors: Hearn Thomas M.
Published: February 2004

Pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy. To investigate uppermost mantle anisotropy, a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy, and it was applied to Pn travel time data from the western United States. Results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations. A detailed resolution study examined the trade‐off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage. Pn anisotropy beneath the western United States has maximum amplitudes of ±0.3 km/s (7.5%) when resolved on a length scales of around 3°. The orientations of Pn anisotropy often correlate well with those inferred from shear‐wave splitting results. This correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions. The orientation of the Pn anisotropy can be correlated with some of the tectonic processes which occur within the western United States. For example, the fast direction of Pn anisotropy parallels the northeast direction of subduction of the Juan de Fuca Plate beneath the northwest Pacific coast, suggesting that there the anisotropy is related to subduction‐driven deformation. The fast direction of Pn anisotropy also parallels the strike of the San Andreas Fault system in central California, indicating that shear strains along the plate boundary may be responsible for the anisotropy there. Within the Great Basin, the Pn anisotropy varies substantially, and both partial melting and small‐scale convection within the uppermost mantle could be responsible for these anisotropy variations.
Constraints on global phase velocity maps from long‐period polarization data
Authors: Laske G. , Masters G.
Published: February 2004

Global phase velocity maps of long‐period surface waves are an essential ingredient in modeling 3‐D shear wave velocity and are capable of particularly good lateral resolution of upper mantle structure. Unfortunately, even recently derived maps disagree for harmonic degrees greater than about 6 so that further improvement is required. The resolution can be dramatically improved by adding both amplitude and polarization data to the inversion process. Both amplitude and polarization depend on the lateral gradients of phase velocity and hence constrain the short‐wavelength structure of the resulting models. Amplitude, polarization, and phase are readily determined for each arriving wave packet using multitaper techniques and can be interpreted using linear perturbation theory. The size of our phase and polarization data sets obtained from seismograms of the global seismic broadband networks GEOSCOPE, IDA/IRIS (International Deployment of Accelerometers/Incorporate Research Institutions for Seismology) and IRIS/USGS (U.S. Geological Survey) justifies inversion for phase velocity expanded in spherical harmonics up to l = 24. While the phase data between 3 and 15 mHz do not require structure beyond about l = 8, small‐amplitude structure of harmonic degree greater than 8 is needed to fit the polarization data. Checkerboard tests show that the resolution of phase velocity is greatly improved when polarization data are added to the inversion. Since amplitude data also depend on 3‐D anelastic structure of the mantle, these data need a more comprehensive interpretation, and we cannot expect to fit them with a purely elastic model. However, in this paper we show that a good fraction of the amplitude signal is consistent with our phase velocity maps and that it is possible to obtain maps which simultaneously explain both amplitude and polarization data.
Crack velocities in freshwater and saline ice
Authors: Petrenko Victor F. , Gluschenkov Oleg
Published: February 2004

This paper presents experimental results on both mode 1 and 2 crack velocities measured in a wide variety of ice types, columnar sea ice, columnar lake ice, laboratory‐grown columnar saline ice, and freshwater columnar and granular ice, in the temperature range from −5° to −35°C. Measurements of ice electrical conductance, electrical capacitance, and electromagnetic emissions from cracks as a function of time were used to determine crack velocities in samples with dimensions ranging from 0.05 to 30 m. In laboratory‐grown freshwater ice and in lake ice, average crack velocities varied from a few hundred to 1320 m/s. In contrast, in natural sea ice and laboratory‐grown saline ice, crack velocity was very low at about 10 m/s. This remarkable difference in the velocity of cracks growing in freshwater and saline ice is probably due to the dynamic resistance of unfrozen water in brine pockets and/or the large size of a crack tip process zone in saline ice. It was also found that cracks propagate discontinuously in saline ice owing to the strong interaction with microstructural elements such as drainage channels.
Earthquake‐induced flooding and paleoseismicity of the El Asnam, Algeria, fault‐related fold
Authors: Meghraoui Mustapha , Doumaz Fawzi
Published: February 2004

Late Quaternary activity of the El Asnam thrust fault (responsible for the October 10, 1980, MS = 7.3 earthquake and 36‐km coseismic faulting) is explored through diverse paleoseismological field methods. We trenched through the main and secondary 1980 coseismic scarps, made an analysis of earthquake‐induced flood deposits, conducted a geomorphic study of leveling profiles and offset geological units across thrust and normal scarps, and considered the geomorphology of the active fold. Twenty‐five radiocarbon ages from colluvial and alluvial deposits, along with conventional (fossils and artefacts) dating of geological units, show the timing of successive net vertical displacements on the thrust‐related fold and bending‐moment faults. Previous work showed that eight pre‐1980 earthquakes occurred in clusters during the late Holocene. Cumulative net vertical slip that amounts to 3 times the 1980 displacement is observed on multiple thrust fault scarps and offsets of geological units on bending‐moment faults. Maximum cumulative vertical displacements, which are visible in different zones along the fault segments, can reach 10 to 25 times the 1980 coseismic displacements. The late Quaternary uplift rate on the El Asnam fault is 0.6 mm/yr, and taking into account slip variation along the fault, this rate may range between 0.25 – 1.6 mm/yr, from which we obtain a shortening rate of 0.17 – 1.2 mm/yr. Because the Tell Atlas mountains exhibit comparable seismogenic fault‐related folds with parallel and sequential structures of active faults, a total average shortening rate of 2.05 mm/yr during the late Quaternary represents part of the convergence accommodated along the Africa‐Eurasia plate boundary.
Velocity weakening friction: A renormalization approach
Authors: Schmittbuhl Jean , Vilotte Jean‐Pierre , Roux Stéphane
Published: February 2004

The frictional response when a two‐dimensional elastic body, under a steady antiplane shearing, slides over a rigid frictional surface is solved using a real‐space renormalization technique. The model is mapped onto iterations of the classical Burridge‐Knopoff chain model. A velocity weakening friction law is assumed for the interactions between the rigid interface and the elastic slider. Through the renormalization approach, we specifically study the coupling between the bulk elastic response of the slider and the friction at the interface level. The frictional system is characterized by its mechanical response in terms of mean friction force opposed to a prescribed constant velocity. The response is shown to differ strongly from the interface friction law. Under renormalization, the response converges toward an apparent dynamical Coulomb's friction law, with distinct static and dynamic coefficients. This result provides some new insight in the “ubiquity” of Coulomb's friction law. On the base of this model, the experimental observability of velocity weakening friction laws and their scaling to seismically active system are discussed. The main result is that velocity weakening friction law is scale dependent. Within the assumptions of the model, the friction law at the scale of the system appears to be rather insensitive to the exact form of the friction at local scale on the interface.
Estimation of seismic waveform governing parameters with neural networks
Authors: Langer Horst , Nunnari Giuseppe , Occhipinti Luigi
Published: February 2004

We investigate the application of multilayer perceptron neural networks on the inversion of waveform governing parameters related to the seismic source and the propagation medium. These parameters are given by the size of the source, thicknesses and velocities of the layers, and a parameter κ describing the whole path attenuation of the wave due to absorption. Synthetic SH waves radiated from a circular source model are used for this study. The neural network returns a mapping function which can be used for an entire class of signals, provided that the parameters are within the limits of the model space explored during the training. The application of the mapping function to a set of signals is mathematically simple and fast. This can be a considerable advantage over systematic search techniques, such as simulated annealing or genetic algorithms, since the stability of the results that are found with the neural network can be tested easily with examples not used for the estimation of the mapping function. The use of an appropriate transform of the signal (i.e., spectra or autocorrelation function) gives slightly better results than the crude waveforms. The seismic waveform‐governing parameters can be identified with a reasonable accuracy if an appropriate network topology is chosen and if the number of examples used for the training phase is sufficiently large. Even in the case where 16 parameters of the models are searched and the global error remained somewhat unsatisfying, important parameters, such as the source radius or the velocity of the uppermost layers, are still recognized with a fair accuracy. The error is, at least to some degree, an effect of the nonuniqueness of the inversion problem. Performing a search with simulated annealing 31 times for an example seismogram, we obtain 31 solutions with a scatter for the different parameters which is of the same order as the errors obtained with the network.
Models of ocean ridge lithospheric deformation: Dependence on crustal thickness, spreading rate, and segmentation
Authors: Shaw William J. , Lin Jian
Published: February 2004

We use three‐dimensional (3‐D) temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid‐ocean ridge lithospheric structure. We find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along‐axis gradients in crustal temperature and a “pita‐pocket”‐shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle. In contrast, fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis. The overthickened crust produced by ridge‐hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling, creating an extremely weak lithosphere in a steady state model. We apply a simple two‐dimensional (2‐D) cyclic faulting model to across‐axis sections of temperature and rheological structure. Slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing. In contrast, fast spreading and hotspot‐affected segments are predicted to have much smaller variations in faulting styles, except in the immediate vicinity of a major transform fault. The presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness, spreading rate, and the ability of rift‐bounding normal faults to propagate along axis. For fast spreading and hotspot‐affected segments, the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma‐chamber‐supported isostatic features. This is in contrast to most slow spreading segments, where axial rift valleys are predicted to be a dominant morphological feature. For a slow spreading segment of great length, however, the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center.
Vertical magnetic structure of ocean crust determined from near‐bottom magnetic field measurements
Authors: Tivey Maurice A.
Published: February 2004

Consensus on the source of marine magnetic anomaly stripes has remained one of the most elusive aspects of the Vine and Matthews seafloor spreading model which has been of such fundamental importance to global plate tectonics. The first attempt to measure the vertical magnetic structure of oceanic crust exposed at a scarp face using near‐bottom magnetic measurements is reported along with the theoretical basis for the reduction and analysis of these measurements. Magnetometers mounted to deep‐tow and submersible platforms were used to determine the horizontal and vertical variation of crustal magnetization of the upper 2 km of young oceanic crust exposed by a steep submarine escarpment at the Blanco fracture zone in the northeast Pacific Ocean. Results show a large‐amplitude magnetic anomaly is consistently found at the extrusive to intrusive transition indicating that the extrusives are strongly magnetized compared to the weakly magnetized intrusive crust. Profiles also show the systematic presence of more than one polarity with depth indicating the possibility of dipping polarity boundaries within the extrusives. Forward models find that a dipping polarity boundary model correctly predicts the amplitude and shape of the overlying sea surface anomaly, while a block model with vertical polarity boundaries overestimates the amplitude. These in situ results are used to directly assess the contribution made by the extrusive section to the overlying magnetic anomaly signal, in essence, to define the magnetic source layer. It is concluded that the extrusive lavas constitute the main magnetic source for magnetic anomalies in oceanic crust less than 2 m.y. in age.
Viscosity of Himalayan leucogranites: Implications for mechanisms of granitic magma ascent
Authors: Scaillet Bruno , Holtz Francois , Pichavant Michel , Schmidt Michael
Published: February 2004

The viscosity of the Gangotri Himalayan leucogranite has been experimentally determined between 800° and 1100°C, 300 and 800 MPa, for meltwater contents of 3.98 and 6.66 wt %. The melt viscosity is independent of pressure and shows an Arrhenian behavior relative to temperature within the range of conditions investigated. We present an empirical relation that can be used to determine leucogranite magma viscosities knowing their meltwater content and temperature. This relation together with phase equilibria experiments constrain the viscosity of the Himalayan leucogranites to be around 104.5 Pa s during their emplacement. These viscosities and the widths of dikes belonging to the feeder system are consistent with the theoretical relationship relating these two parameters and show that the precursor magma of the leucogranite was at near liquidus conditions when emplaced within host rocks with preintrusion temperatures around 350°C. Calculated terminal ascent rates for the magma in the dikes are around 1 m/s. Magma chamber assembly time is, on this basis, estimated to be less than 100 years (for a volume of 150 km3). In addition, the dynamical regime of the magma flow in the dikes was essentially laminar, thus allowing preservation of any chemical heterogeneity acquired in the source. These results constrain the viscosity of melts formed during the first steps of crustal anatexis, those involving muscovite breakdown, to be also around 104.5 Pa s. Thus compaction may not be the only mechanism of melt segregation in partially melted crustal rocks in view of the very short timescale inferred for magma ascent and emplacement.
Crustal structure of a transform plate boundary: San Francisco Bay and the central California continental margin
Authors: Holbrook W. Steven , Brocher Thomas M. , ten Brink Uri S. , Hole John A.
Published: February 2004

Wide‐angle seismic data collected during the Bay Area Seismic Imaging Experiment provide new glimpses of the deep structure of the San Francisco Bay Area Block and across the offshore continental margin. San Francisco Bay is underlain by a veneer (<300 m) of sediments, beneath which P wave velocities increase rapidly from 5.2 km/s to 6.0 km/s at 7 km depth, consistent with rocks of the Franciscan subduction assemblage. The base of the Franciscan at 15–18 km depth is marked by a strong wide‐angle reflector, beneath which lies an 8‐ to 10‐km‐thick lower crust with an average velocity of 6.75 ± 0.15 km/s. The lower crust of the Bay Area Block may be oceanic in origin, but its structure and reflectivity indicate that it has been modified by shearing and/or magmatic intrusion. Wide‐angle reflections define two layers within the lower crust, with velocities of 6.4–6.6 km/s and 6.9–7.3 km/s. Prominent subhorizontal reflectivity observed at near‐vertical incidence resides principally in the lowermost layer, the top of which corresponds to the “6‐s reflector” of Brocher et al. [1994]. Rheological modeling suggests that the lower crust beneath the 6‐s reflector is the weakest part of the lithosphere; the horizontal shear zone suggested by Furlong et al. [1989] to link the San Andreas and Hayward/Calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust. A transect across the continental margin from the paleotrench to the Hayward fault shows a deep crustal structure that is more complex than previously realized. Strong lateral variability in seismic velocity and wide‐angle reflectivity suggests that crustal composition changes across major transcurrent fault systems. Pacific oceanic crust extends 40–50 km landward of the paleotrench but, contrary to prior models, probably does not continue beneath the Salinian Block, a Cretaceous arc complex that lies west of the San Andreas fault in the Bay Area. The thickness (10 km) and high lower‐crustal velocity of Pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby Pioneer seamount. The Salinian Block consists of a 15‐km‐thick layer of velocity 6.0–6.2 km/s overlying a 5‐km‐thick, high‐velocity (7.0 km/s) lower crust that may be oceanic crust, Cretaceous arc‐derived lower crust, or a magmatically underplated layer. The strong structural variability across the margin attests to the activity of strike‐slip faulting prior to and during development of the transcurrent Pacific/North American plate boundary around 29 Ma.
Near‐bottom magnetic survey of the Mid‐Atlantic Ridge axis, 24°–24°40′N: Implications for crustal accretion at slow spreading ridges
Authors: Hussenoeder Stefan A. , Tivey Maurice A. , Schouten Hans , Searle Roger C.
Published: February 2004

Near‐bottom magnetic field measurements provide increased spatial resolution over sea surface magnetic data and allow a detailed analysis of the accretionary and tectonic processes at work in a slow spreading environment. Through the use of magnetic inversion methods and forward modeling, we investigate the fine‐scale magnetic structure of young oceanic crust along four near‐bottom profiles that cross the two bathymetric segments immediately north of the Kane transform on the Mid‐Atlantic Ridge. Our results show the presence of a narrow, peaked central anomaly magnetic high (CAMH) located over the zone of most recent volcanism. We hypothesize that the pattern of high magnetization at segment ends and relatively low values at their centers is largely the result of increased iron and titanium content away from segment centers. Magnetization lows, which are not observed in the sea surface data, flank the CAMH and are associated with the axial valley walls. These lows may locate areas of intense magnetic source layer disruption. Blocks of relatively high magnetization found outside the valley walls suggest a cyclic faulting process, whereby discrete blocks of relatively unaltered crust are transported out of the rift valley by adjacent zones of faulting. These observations of fine‐scale magnetic structure show that while the emplacement of crust at slow spreading ridges is highly focused, subsequent tectonic disruption and alteration at the rift valley walls degrades the crustal magnetization signal and may be the primary reason for Atlantic magnetic anomaly variability.