bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitation of magnetosonic waves in the undisturbed solar wind
Authors: Xue Song , Thorne Richard M. , Summers Danny
Published: February 2004

The growth and damping properties of linear magnetosonic (R‐mode compressional Alfvén) waves are compared for the idealized case of either generalized bi‐Lorentzian or bi‐Maxwellian fits to solar wind parameters within 1 AU. Measured proton temperature anisotropies (T∥,p, > T⟂,p) can provide significant local convective amplification, though only for small angles of propagation to the magnetic field. For more oblique propagation, waves are subject to Landau damping and cyclotron resonant absorption. Since it is unlikely that Waves will remain confined to a small cone of propagation over the appreciable distances (∼106 km) necessary for significant growth, this confinement could explain the apparent absence of magnetosonic waves in the undisturbed solar wind.
In‐situ detection of tropospheric OH radicals by folded long‐path laser absorption. Results from the POPCORN Field Campaign in August 1994
Authors: Dorn H.‐P. , Brandenburger U. , Brauers T. , Hausmann M. , Ehhalt D. H.
Published: February 2004

Ground based in‐situ measurements of tropospheric hydroxyl radicals were conducted by folded long‐path laser absorption as part of the field campaign POPCORN in August 1994. The OH instrument used an open optical multiple‐reflection cell of 38.5 m base length through which the laser beam was passed up to 80 times. The broadband emission of a short‐pulse UV laser together with a multichannel detection system allowed the simultaneous observation of six OH absorption lines in a spectral interval of Δλ≃0.24 nm at 308.1nm (A²Σ+,υ′ = 0← X²Π,υ″ = 0 transition). Along with the OH radicals, the trace gases SO2, HCHO, and naphthalene were measured by this technique. The large spectral detection range covered a multitude of rotational absorption lines of these trace gases which were all used for multicomponent analysis, thus allowing a specific and sensitive detection of tropospheric OH radicals. An average 2σ detection limit of 1.5 × 106 OH/cm³ for an integration time of 200 seconds and an absorption light path length of 1848 m was determined from the field measurements. In total, 392 OH data were obtained by long‐path absorption during 16 days of field measurements. The observed OH concentrations reached peak values of 13 × 106 cm−3 at noon.
Crustal thinning in the Southwestern Iberia Margin
Authors: González A. , Torné M. , Córdoba D. , Vidal N. , Matias L. M. , Díaz J.
Published: February 2004

The mode of crustal thinning in the southwestern margin of the Iberian Peninsula is investigated along a transect that extends from onshore Iberia to the eastern end of the Horseshoe Abyssal Plain. On onshore areas, the crustal structure has been deduced using wide‐angle seismic reflection data, whereas offshore we have used coincident steep and wide‐angle reflection data along a NE‐SW oriented seismic profile that extends from Cape San Vicente to the Horseshoe Abyssal Plain. In addition, 2D gravity modelling has been performed to validate the crustal structure deduced from seismic data. Our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore Iberia to less than 15 km in the Horseshoe Abyssal Plain and that thinning occurs over horizontal distances of about 120 km.
The influence of IMF By on the mapping between the Earth's magnetotail and its ionosphere
Authors: Kullen A. , Blomberg L. G.
Published: February 2004

It is well known that the dawn‐dusk component of the interplanetary magnetic field (IMF) creates asymmetries in the magnetotail. The main effects are a weak nonuniform penetration of IMF By into the tail and a rotation of the whole tail current sheet. In this study we modify the Tsyganenko (1989) model to simulate the IMF By induced tail effects. By field line mapping we examine how different regions of the magnetosphere and ionosphere are influenced by the IMF By tail effects. The plasma sheet is shown to rotate around the central axis of the magnetotail. The mapping from the plasma sheet to the ionosphere for nonzero IMF By shows a change in the geometry of the auroral oval. The whole oval rotates, a local bulge occurs near midnight and the thickness of the oval is different in the morning and the evening sector. Simple geometrical considerations show that the tilting of the field lines caused by IMF By leads to a rotation of the oval, whereas the rotation of the tail current sheet is reponsible for the thickness variations along the oval. These results fit very well with the observed auroral distribution for an IMF with a dawnward or duskward direction.
Environmental factors as complicating influences in the recovery of quantitative geomagnetic‐field paleointensity estimates from sediments
Authors: Schwartz Martha , Lund Steve P. , Johnson Thomas C.
Published: February 2004

We have recently recovered replicate records of the Earth's magnetic‐field (relative) paleointensity for 12–71,000 years BP from marine sediments of the western North Atlantic Ocean. Our records are remarkably similar to two other recently published relative paleointensity records from marine and lacustrine sediments in Europe. This suggests to us that, even over this wide region, sediments may serve as reasonable, correlatable recorders of fluctuations in the strength of the Earth's magnetic field. If this is true, then similar records from around the globe could eventually provide valuable information about the deep‐Earth processes which create the field. We note, however, that our intensity record is significantly correlated with the down‐core ratio of magnetic susceptibility (χ) to anhysteretic remanent magnetism (ARM). This ratio is a measure of the relative grain size of magnetite (the primary magnetic mineral in these sediments), and as such is a sediment magnetic property which is controlled only by the local depositional environment (and indirectly by global climate). We are concerned that the European records may be similarly biased by climatic or other environmental factors, possibly synchronous with ours. We caution that extreme care must be taken to understand and remove any such magnetic influences from sediment relative paleointensity records before they are used as quantitative estimators of the past intensity of the Earth's magnetic field.
A model of currents and electric fields in a discrete auroral arc
Authors: Blixt E. M. , Brekke A.
Published: February 2004

A discrete auroral arc is set up by precipitating electrons. Current continuity requires a return current fed by a horizontal current from the arc. In the closing of this current system E‐region plasma might escape along the return current, and if the source acts as a constant current generator the horizontal electric field can get enhanced. The existence and relative position of these features can be used to diagnose the state of the magnetospheric generator.
Unbiased multifractal analysis: Application to fault patterns
Authors: Ouillon G. , Sornette D.
Published: February 2004

We describe a method for systematically correcting finite size and irregular geometrical boundary effects in multifractal analysis. Our multiplicative scheme is tested on synthetic measures, as well as on real fault networks. We document the largely underestimated distorsions induced by the irregular geometry of the support of the measure. Our method allows us to test for the universality of multifractal spectra by comparing results at different magnifications with their respective adjusted corrections. Our result call for a reexamination of many fractal and multifractal analysis which have been carried out without taking these effects into account.
Dibromomethane (CH2Br2) measurements at the upper troposphere and lower stratosphere
Authors: Kourtidis Kostas , Borchers Reinhard , Fabian Peter
Published: February 2004

Stable brominated hydrocarbons, beside their role in the geochemical cycling of bromine, may enter the stratosphere, where they can efficiently catalyze ozone destruction through the release of inorganic bromine (Bry). Three vertical profiles of dibromomethane (CH2Br2) are reported here, derived from the analyses of air samples collected by the MPAE balloon‐borne cryogenic sampler at 17°N in 1990 and at 68°N in 1992 and 1995. Upper tropospheric volume tractions of this compound from below detection limit to 1.7 pptv were measured. Although CH2Br2 was already below the detection limit at the tropopause during one of those flights, it was present in the first 2–4 km of the stratosphere in measurable quantities during the other two flights. Concurrent CH3Br and Halons 1211 (CBrClF2) and 1301 (CBrF3) measurements suggest that CH2Br2 could contribute up to 20% of Bry released in this altitude region while it could deliver around 10% of the total bromine entering the stratosphere.
Ocean circulation variations associated with the Antarctic Circumpolar Wave
Authors: Jacobs G. A. , Mitchell J. L.
Published: February 2004

Altimeter data analysis indicates persistent sea surface height (SSH) anomalies propagating eastward about the Antarctic continent. The spatial and temporal characteristics match variations in the atmosphere and sea surface temperature (SST) that are associated with the Antarctic Circumpolar Wave (ACW). During the observation time period, the SSH appears quasiperiodic with a dominant 4 year period and 180° longitude wavelength. Thus, the SSH signature of the ACW appears as two anomalies on opposite sides of the Antarctic continent propagating eastward at 10 cm/s. The SSH response to observed wind forcing agrees in terms of amplitude and phase with simple quasigeostrophic dynamics.
X‐ray pulses observed above a mesoscale convective system
Authors: Eack Kenneth B. , Beasley William H. , Rust W. David , Marshall Thomas C. , Stolzenburg Maribeth
Published: February 2004

During a balloon flight into and above the stratiform region of a mesoscale convective system, we observed three x‐ray pulses while the balloon was at an altitude of approximately 15 km MSL (atmospheric pressure of 130 mb). These pulses were one to two orders of magnitude above the background x‐ray count rate with peak fluxes between 37 and 270 (cm²‐s‐sr)−1 and durations of about one second. No significant electric field was measured at the time of these pulses.
Changes in CH4 and CO growth rates after the eruption of Mt. Pinatubo and their link with changes in tropical tropospheric UV flux
Authors: Dlugokencky E. J. , Dutton E. G. , Novelli P. C. , Tans P. P. , Masarie K. A. , Lantz K. O. , Madronich S.
Published: February 2004

Trace gas measurements from air samples collected weekly at a globally distributed network of sampling sites revealed sharp increases in the growth rates of CH4 and CO in the tropics and high southern latitudes immediately following the eruption of Mt. Pinatubo on June 15, 1991. The eruption emitted ∼20 Mt SO2 into the lower stratosphere. Calculations made with a radiative transfer model show that UV actinic flux in the wavelength region 290–330 nm was attenuated by ∼12% immediately after the eruption due to direct absorption by SO2, and that it was perturbed for up to 1 year after the eruption due to scattering by sulfate aerosols. We suggest that the decreased UV flux decreased the steady‐state [OH] and led to the observed anomalously large growth rates for CH4 and CO during late‐1991 and early‐1992.
Thermal anisotropies in the solar wind: Evidence of heating by interstellar pickup ions?
Authors: Richardson John D. , Phillips John L. , Smith Charles W. , Gray Perry C.
Published: February 2004

A recent paper by Gray et al. [1996] shows that the Alfvén ion cyclotron instability is generated by newly created pickup ions and heats the thermal solar wind protons in the direction perpendicular to the magnetic field. This instability operates most effectively in regions where the plasma β is low, so this mechanism predicts that the ratio of the temperatures perpendicular and parallel to the magnetic field should be larger in low‐β regions of the solar wind. We look for this effect in ISEE‐3 and Voyager 2 data. The near‐Earth ISEE‐3 data show no evidence for greater thermal anisotropies at low β. Voyager 2 data obtained between 1 and 8 AU show that the ambient proton thermal anisotropy is a function of β, with parallel temperatures generally greater than perpendicular temperatures except when β is small. For Voyager 2 data, the average ratio of perpendicular to parallel temperature is about 0.9, but this ratio is ∼2 when β is less than 0.1 and ∼4 when β is less than 0.01.
Tropical sea surface temperatures and the Earth's orbital eccentricity cycles
Authors: Gupta Shyam M. , Fernandes A. A. , Mohan Raghul
Published: February 2004

The tropical oceanic warm pools are climatologically important regions because their sea surface temperatures (SSTs) are positively related to atmospheric greenhouse effect and the cumulonimbus‐cirrus cloud anvil. Such a warm pool is also present in the central tropical Indian Ocean. Paleo‐SSTs estimated by the radiolarian transfer functions for the last ∼200–1400 kiloyears (ka) in a deep‐sea sediment core from this warm pool fluctuated within 2.75°C (26.14–28.89°C) around the threshold temperature (27 °C) for the super greenhouse effect. It is possible that this SST variation might have led to the corresponding fluctuation in the greenhouse effect and the cirrus cloud anvil in the geological past. These paleo‐SSTs also exhibited cyclicities at ∼100‐ and ∼400‐ka corresponding to the Earth's orbital eccentricity cycles. Results, therefore imply that the tropical Indian Ocean warm pool persisted during the Quaternary and the paleo‐SSTs fluctuating at the orbital eccentricity frequencies might have modulated the intensity of greenhouse effect.
Satellite detection of natural films on the ocean surface
Authors: Espedal Heidi A. , Johannessen Ola M. , Knulst Johan
Published: February 2004

Natural films on the ocean surface influence air‐sea fluxes of momentum, heat and gas. It is therefore essential to be able to map the geographical extent and variability of such films to determine the resulting impact on the earth's climate system. Satellite based Synthetic Aperture Radar (SAR) may prove very useful, and perhaps the only way, to quantify global scale film distribution. In order to investigate the composition of natural film and its effect on radar return, in situ samples of the surface microlayer were for the first time taken simultaneously with ERS‐1 SAR coverage of the sea. Our results show that the film was enriched in organic composition and that the concentration of fatty acids was generally one order of magnitude higher inside than outside natural film areas, resulting in a 6–17 dB decrease in radar backscatter.
The mesopause altitude: Only two distinctive levels worldwide?
Authors: von Zahn U. , Höffner J. , Eska V. , Alpers M.
Published: February 2004

A large number of temperature profiles of the altitude range 80 to 105 km were obtained between 71°S and 54°N latitude from late April to early July 1996. The measurements were performed by a ship‐borne lidar, resolving the Doppler broadening of the K(D1) resonance line. The most notable result of this field campaign is the finding that throughout our observation series the mesopause altitude was located at altitudes of either 100±3 km or 86±3 km. The high “winter” level extended from 71°S to 23°N, the low “summer” level from 24°N until the end of the field observations at 54°N. Our latitudinally distributed observations indicate strongly a worldwide bimodal character of the mesopause altitude. Furthermore, our mesopause temperatures are generally lower than previonsly measured in the northern hemisphere.
A surface‐trapped intrusion of slope water onto the continental shelf in the Mid‐Atlantic Bight
Authors: Gawarkiewicz Glen G. , Linder Christopher A. , Lynch James F. , Newhall Arthur E. , Bisagni James J.
Published: February 2004

A high‐resolution hydrographic transect across the shelfbreak south of Nantucket Shoals in August, 1995, revealed an unusual surface‐trapped intrusion of saline Slope water onto the continental shelf. The intrusion was confined to the upper 25 m of the water column and penetrated 15 km shoreward of the 100‐m isobath, the typical position of the shelfbreak front. The maximum salinity within the intrusion was 35.0. Several strong jets were present within the surface layer with maximum velocities between 0.30 and 0.44 m s−1 in the alongshelf direction. Satellite thermal imagery confirms the presence of a warm‐core ring at the offshore edge of the section and shows a warm feature penetrating shoreward of the 100‐m isobath.
Neural networks, foF2, sunspot number and magnetic activity
Authors: Williscroft Lee‐Anne , Poole Allon W. V.
Published: February 2004

Neural networks (NNs) have been used to solve many problems associated with geophysics. This paper describes a new application of NNs to determine optimum parameters for the prediction of foF2, as well as the first application of NNs to the prediction of foF2. We have trained several NNs to predict the noon value of foF2 at Grahamstown (33°S, 26°E) using season, solar activity and magnetic activity as input data, taken over one sunspot cycle (1973–1983). Using the criterion that the best indices of solar and magnetic activity are the ones that give the lowest rms error between predicted and measured foF2, we have determined optimum averaging lengths for these indices. Our optimum index for solar activity is a two month running mean of daily sunspot number, in contrast to the value of 1 year recommended by the International Reference Ionosphere (IRI). Our determination for magnetic activity is a running mean of ak of two days. A neural net trained with these optimum indices can predict the daily noon value of foF2 in Grahamstown with an rms error of .95 MHz. The rms error of the monthly average is .48 MHz.
Variations of cloudiness, precipitable water, and relative humidity over the United States: 1973–1993
Authors: Elliott William P. , Angell James K.
Published: February 2004

Water vapor variables were calculated from radiosonde stations in each of six U.S. regions for which cloudiness observations were available. Associations among variations of cloudiness, precipitable water and relative humidity during the 1973–93 period over the U.S. as a whole and in the six regions were determined. Trends in the quantities as well as relations among seasonal variations have been examined. There was a general increase in precipitable water in each season and in each region and a suggestion of increases in relative humidity. These were not accompanied by a general increase in cloudiness that was significant. However, there were significant correlations between seasonal anomalies of cloudiness and both relative humidity and precipitable water.
The optical spectrum of aircraft St. Elmo's fire
Authors: Wescott E. M. , Sentman D. D. , Heavner M. J. , Hallinan T. J. , Hampton D. L. , Osborne D. L.
Published: February 2004

On February 26, 1995, during a NASA sponsored mission to Peru to study red sprites and blue jets, the instrumented Westwind 2 jet aircraft encountered spectacular St. Elmo's fire from the wing pods, tail and nose while flying through a cloud at an altitude of 13.83 km (45,376 ft). The phenomenon was captured on low light level monochromatic and color television systems aboard the aircraft, and its spectrum was recorded on a low light level TV spectrograph with response from 395.0 to 750.0 nm. The cameras and spectrograph also recorded scattered intra‐cloud lightning and a possible lightning discharge near, or to, the aircraft. The spectrum of St. Elmo's fire was primarily the second (2nd) positive bands of N2. The data were consistent with a population of electrons having relatively low energy (<18 eV).
Observation of upper tropospheric sulfur dioxide‐ and acetone‐pollution: Potential implications for hydroxyl radicaland aerosol formation
Authors: Arnold F. , Schneider J. , Gollinger K. , Schlager H. , Schulte P. , Hagen D. E. , Whitefield P. D. , van Velthoven P.
Published: February 2004

Aircraft‐based measurements of sulfur dioxide, acetone, carbon dioxide, and condensation nuclei (CN) were made over the north‐eastern Atlantic at upper tropospheric altitudes, around 9000 m. On October 14, 1993, strong SO2‐ and acetone‐pollution (both up to 3 ppbv) were observed, which were accompanied by a CO2‐enhancement of up to 6 ppmv, and large CN‐concentrations of up to about 1500 cm−3 (for radii ≥ 6 nm). CN, excess CO2, and to a lesser degree also acetone, were positively correlated with SO2. Air mass trajectory analyses indicate, that most of the air masses encountered by our aircraft originated from the polluted planetary boundary layer of the North‐Eastern U. S. approximately 4–5 days prior to our measurements, and that polluted boundary layer air experienced fast vertical transport to the upper troposphere as well as horizontal transport across the Atlantic. From our data we conclude, that in the polluted air mass around 9000 m altitude HOx‐formation, photochemical SO2‐conversion to gaseous H2SO4, and eventually also CN‐formation by homogeneous bimolecular (H2SO4‐H2O) nucleation may have taken place with enhanced efficiency.
The CO2 concentration of air trapped in GISP2 ice from the Last Glacial Maximum‐Holocene transition
Authors: Smith H. J. , Wahlen M. , Mastroianni D. , Taylor K. C.
Published: February 2004

Measurements of the CO2 concentration of air trapped in Greenland Ice Sheet Project Two ice show an increase from approximately 190 ppm to 270 ppm during the 8000 years between the end of the last glacial maximum and the beginning of the Holocene. This increase is in agreement with other parallel records from both Greenland and Antarctica, although the GISP2 data exhibit considerable variability over periods as small as 50–100 years and show that CO2 can be affected by processes occurring within the ice after bubble formation. The in‐situ reactions which contribute to the modification of the concentrations of paleoatmospheric CO2 may include both CO2 uptake as well as CO2 production. The existence of two major episodes of CO2 enrichment, in addition to the variability of the data, prevents a precise determination of the progression of the increase in atmospheric CO2, although the 80 ppm rise between 18.0 kyBP and 10.4 kyBP is evident. These results strongly suggest that some intervals of the GISP2 CO2 record must be interpreted in the context of the chemical composition of the ice, and that records of atmospheric CO2 in polar ice can be considered accurate only if the ice is either (essentially) carbonate‐free or contains abundant carbonate.
Lidar observations of polar stratospheric clouds and stratospheric temperature in winter 1995/96 over northern Norway
Authors: Hansen Georg , Hoppe Ulf‐Peter
Published: February 2004

In winter 1995/96, extreme conditions were observed in the lower and middle stratosphere over most of the European Arctic. At the ALOMAR facility in Northern Norway, an ozone DIAL system observed PSCs on every measurement occasion in January and over a 10‐days period in February 1996. On January 23, 1996, the PSC layer reached a maximum vertical extension, ranging from 19 to 27 km. In narrow layers, superimposed on the “background” PSC, the backscatter ratio R reached maximum values of 16 at 353 nm and about 9 on 308 nm. Morphology and colour index indicate that these narrow layers were type‐II PSCs. Just above the PSC layer, temperatures below 190 K were recorded regularly. The temperatures at the edges of the “background” PSC allow this layer to be composed of STS.
The clustering of rising diapirs and plume heads
Authors: Kelly Amanda , Bercovici David
Published: February 2004

Buoyant diapirs or plume heads in the Earth's mantle are typically modelled as a solitary bodies, ascending vertically and uninfluenced by neighboring diapirs. However, laboratory experiments and basic theory suggest that two or more rising diapirs can become attracted to each other, thereby influencing their ascent trajectory such that they cluster and possibly coalesce. Clustering is controlled by the horizontal and vertical separation between diapirs, as well as their relative sizes. This phenomena has implications for the origins of large igneous provinces as well as the Earth's long‐wavelength hotspot distribution.
The major volatile elements of the Earth: Their origin, behavior, and fate
Authors: Javoy M.
Published: February 2004

Stable isotope constraints impose a heterogeneous accretion of the earth's volatiles: the primary, reduced material, 99.8% of the planet, brought 6% of the hydrogen, 48% of the carbon and 70% of the nitrogen of the Upper Earth (upper mantle, crust, ocean‐atmosphere). A late veneer (1.4 per mil of the bulk Earth's mass), of CI composition, supplied 94% of the hydrogen, 52% of the carbon and 30% of the nitrogen.

The present distribution of the major volatiles in the Upper Earth corresponds to a negligible supply from the lower mantle. Measured in units of the superficial reservoirs the upper mantle contains: 0.21 ocean masses of water, ∼10 atmosphere masses of nitrogen, 5.4 crustal carbon masses of carbon.

This corresponds to a very significant trapping of carbon and nitrogen by the mantle during the Earth's history, whereas water repartition stayed about unchanged and strongly favoured the superficial reservoir. There is, however, a very significant difference between carbon and nitrogen, the present outgassing and subducting fluxes of carbon being equilibrated both in masses and isotopically, whereas there is still a gross isotopic disequilibrium for nitrogen, the large, isotopically heavy, subducted flux, still outweighing the small, isotopically light, outgassed flux.

This is due to the fact that, while carbon and water are essentially incompatible components, nitrogen is a compatible element because of the presence of very stable mantle nitrides (probably osbornite).
Trench migration and subduction zone geometry
Authors: Olbertz D. , Wortel M. J. R. , Hansen U.
Published: February 2004

Seismic tomography results show a large variety in upper mantle structure along convergent plate boundaries. We numerically investigate the effect of trench migration on the evolution of a slab (with temperature dependent viscosity) encountering a viscosity interface. We find that subduction zone geometry is sensitive to even small rates (1 cm/yr) of retrograde motion: increase in trench migration rate decreases the dip angle of the slab and its ability to penetrate the lower mantle. Upon including a background mantle flow it turns out that trench migration relative to the upper mantle flow is more decisive than the absolute plate velocities. Subduction zone geometry appears to be strongly time‐dependent. We conclude that individual tectonic setting and time‐dependent slab behaviour can account for many different types of observable subduction zone anomalies. Our model temperature can adequately account for magnitudes and patterns of seismic anomalies as obtained from seismic tomography.
Along‐axis variability in crustal accretion at the Mid‐Atlantic Ridge: Results from the OCEAN study
Authors: Henstock Timothy J. , White Robert S. , McBride John H.
Published: February 2004

The OCEAN experiment is an integrated geophysical study of a region of the Cape Verde abyssal plain that formed at 140 Ma. Deep seismic reflection and ocean bottom hydrophone (OBH) refraction data were acquired along lines parallel and perpendicular to the paleoridge axis trend identified from a detailed magnetic anomaly survey. The igneous basement is overlain by about 1.3 km of sediment which enables improved imaging of intracrustal structure beyond that possible near the Mid‐Atlantic Ridge axis. We describe the results of a 150‐km long profile oriented parallel to magnetic anomalies M15 and M16, along which deep seismic reflection data collected by the British Institutions Reflection Profiling Syndicate are complemented by refraction data constrained by four OBHs. The line spans an entire spreading segment between two fracture zones; the northern of which has an offset of 40 km and the other (central) has an offset of only 10 km. Away from the fracture zones, the mean igneous crustal thickness is 7.2 km; near both fracture zones, thinning of up to 4 km is observed, giving a mean igneous crustal thickness over the whole segment of approximately 6.5 km. Differences are seen between the two fracture zones in their seismic velocity structure, in the associated basement topography, and in the presence of a strong reflection extending into the mantle beneath the northern fracture zone. The boundary between oceanic layers 2 and 3 correlates with variably coherent normal incidence reflections and a change in the character of the reflectivity. A number of planar reflections up to 10 km in length are present within the middle and lower crust, dipping outward from beneath low‐amplitude basement highs at ∼15°; these appear to be present only within layer 3. The Moho has several expressions in the reflection data, including isolated reflection events, a local increase in reflected amplitudes, and a downward decrease in coherent reflections. At the center of the segment there is a zone at the base of the crust within which both high‐ and low‐velocity materials are present. This zone shows an enhanced level of discontinuous normal incidence reflectivity and may represent an initial fractionation event as melt was emplaced at the spreading ridge.
A global, self‐consistent, hierarchical, high‐resolution shoreline database
Authors: Wessel Pål , Smith Walter H. F.
Published: February 2004

We present a high‐resolution shoreline data set amalgamated from two databases in the public domain. The data have undergone extensive processing and are free of internal inconsistencies such as erratic points and crossing segments. The shorelines are constructed entirely from hierarchically arranged closed polygons. The data can be used to simplify data searches and data selections or to study the statistical characteristics of shorelines and landmasses. The data set can be accessed both electronically over Internet and from the National Geophysical Data Center, Boulder, Colorado; it comes with access software and routines to facilitate decimation based on a standard line‐reduction algorithm.
A hybrid wave propagation simulation technique for ocean acoustic problems
Authors: Robertsson Johan O. A. , Levander Alan , Holliger Klaus
Published: February 2004

Intermediate range bottom interacting deep ocean acoustic experiments (up to ∼50 km offset in water depths exceeding 1 km using frequencies in the 100–500 Hz range) encompass a variety of different scales: the water column varies smoothly over many wavelengths, whereas the seafloor is rough at scales from 1 m to several kilometers. We present a general hybrid technique to solve the two‐dimensional acoustic/viscoelastic equations for problems that include these different wave propagation regimes. This approach makes realistic large‐scale computations tractable and assures high accuracy. The technique, which we call Hybrid Adaptive Regime Visco‐Elastic Simulation Technique (HARVEST), consists of three parts. A Gaussian beam method is used to propagate the source wave field through a vertically varying water column to the scattering region near the seafloor. This extrapolated source wave field is inserted into a viscoelastic finite difference grid on which the complex acoustic/anelastic interaction of the wave field with the rough seafloor is computed. The backscattered wave field collected on hypothetical vertical and horizontal lines is then extrapolated by means of the Kirchhoff integral to a receiver array distant from the scattering locale. The fidelity of the method is demonstrated by comparison with solutions from a WKBJ approximation method and full finite difference simulations.
Long‐period variations in gravity field caused by mantle anelasticity
Authors: Zhu Yaozhong , Shum C. K. , Cheng M. K. , Tapley B. D. , Chao Benjamin Fong
Published: February 2004

A numerical perturbation approach is used to assess the effect of mantle anelasticity on the Earth's zonal response coefficient κ, the zonal geopotential coefficient J2, and the orbital node of geodetic satellites. The theoretical predictions are used to compare solutions of elastic ocean tide models and tides observed by geodetic satellites for Mf, Mm, Ssa, and Sa. The discrepancies between the satellite observed tides and the solution from Schwiderski and recent altimetric ocean tide models can be partly explained by the predicted effect of mantle anelasticity at Mf and Mm frequencies. Approximately 80% of the observed semiannual variation in δJ2 by geodetic satellites can be attributed to the tidal deformation from anelasticity and from the classical equilibrium ocean. The predicted κ for the 18.6‐year tide was computed for an Earth with an anelastic mantle and a nonequilibrium ocean. The range of the lower and upper bounds of κ for the 18.6‐year tide is from 0.345 to 0.377 in amplitude, and from −6° to 3° in phase, with preferred amplitude of 0.365 and phase of −0°.8.
Volatile emissions from the crater and flank of Oldoinyo Lengai volcano, Tanzania
Authors: Koepenick K. W. , Brantley S. L. , Thompson J. M. , Rowe G. L. , Nyblade A. A. , Moshy C.
Published: February 2004

As a comparison to airborne infrared (IR) flux measurements, ground‐based sampling of fumarole and soil gases was used to characterize the quiescent degassing of CO2from Oldoinyo Lengai volcano. Aerial and ground‐based measurements are in good agreement: ∼75% of the aerially measured CO2flux at Lengai (0.05–0.06 × 1012mol yr−1or 6000–7200 tonnes CO2d−1) can be attributed to seven large crater vents. In contrast to Etna and Vulcano Island, where 15–50% of the total CO2flux emanates diffusely through the volcanic flanks, diffuse emissions were measured only within 500 m of the crater rim at Lengai, contributing <2% of the total flux. The lack of extensive flank emissions may reflect the dimensions of the magma chamber and/or the lack of a shallow fluid flow system. Thermodynamic restoration of fumarole analyses shows that gases are the most CO2‐rich and H2O‐poor reported for any volcano, containing 64–74% CO2, 24–34% H2O, 0.88–1.0% H2, 0.1–0.4% CO and <0.1% H2S, HCl, HF, and CH4. Volatile emissions of S, Cl, and F at Oldoiyno Lengai are estimated as 4.5, 1.5, and 1.0 × 107mol yr−1, respectively. Accuracy of the airborne technique was also assessed by measuring the C emission rate from a coal‐burning power plant. CO2fluxes were measured within ±10% near the plant; however, poor resolution at increased distances caused an underestimation of the flux by a factor of 2. The relatively large CO2fluxes measured for alkaline volcanoes such as Oldoinyo Lengai or Etna may indicate that midplate volcanoes represent a large, yet relatively unknown, natural source of CO2.
Late Silurian plutons in Yucatan
Authors: Steiner M. B. , Walker J. Douglas
Published: February 2004

U‐Pb measurements of zircons from two composite plutons in the Maya Mountains of the Yucatan Block (Belize) give Late Silurian ages. Zircons from one of the five compositional phases of the Mountain Pine Ridge pluton yield an age of 418±3.6 Ma. A second compositional phase gives a minimum age of 404 Ma, and zircons from a third phase, although plagued with high common Pb, yield ages consistent with the other two. Zircons from one compositional phase of the Hummingbird‐Mullins River pluton indicate an age of about 410–420 Ma. These data demonstrate that two of the three Maya Mountains plutons residing among the strata of the Late Pennsylvanian through Permian Santa Rosa Group are older than that sedimentation. Although the third pluton was not dated, both the similarity of sedimentary facies patterns adjacent to it to those adjacent to one of the plutons dated as Late Silurian and a published single Rb‐Sr age of 428 ± 41 Ma suggest this third pluton also was emergent during Santa Rosa deposition. Thus the new U/Pb dates and other data suggest that all three Maya Mountains plutons pre‐date Late Carboniferous sedimentation and that none intrude the Santa Rosa Group. Although very uniform ages of about 230 Ma amongst all plutons, derived from abundant earlier dating by the K‐Ar system, led to the conclusion that intrusion mostly had occurred in the Late Triassic, the U‐Pb ages (obtained from the same sites as the K‐Ar dates) demonstrate that the K‐Ar ages do not derive from a Late Triassic intrusive episode. The K‐Ar dates probably are a signature of the rifting associated with Pangean breakup and formation of the Gulf of Mexico. In a reconstructed Pangea, the position of the Maya Mountains Late Silurian plutons suggests that the Late Silurian Acadian‐Caledonian orogen of eastern North America extended through the region of the future Gulf of Mexico. Finally, the U‐Pb ages of the Maya Mountains plutons are the same as those of a group of shocked zircons found in the Chicxulub impact structure and its fallout layer. The presence of these ages in both locations suggests that the Maya Mountains exposures may be representative of the basement of the Yucatan Block, hence of the basement impacted by the Chicxulub object.
Time‐dependent uplift of the Kenai Peninsula and adjacent regions of south central Alaska since the 1964 Prince William Sound earthquake
Authors: Cohen Steven C.
Published: February 2004

Leveling, Global Positioning System, very long baseline interferometry, and tide gauge observations indicate that time‐dependent uplift has occurred on the Kenai Peninsula and adjacent regions of southern Alaska that subsided coseismically in the 1964 Prince William Sound earthquake. These observations have been critically assessed and used along with seismological and geological information as constraints in various aseismic slip and viscoelastic flow models of crustal deformation. The postseismic uplift rate in most of the region is significantly greater than the expected interseismic average. Some, but not all, of the data indicate that the uplift rate was quite rapid (more than several centimeters per year) in the years immediately following the earthquake and has slowed to a few centimeters per year or less since then. The modeling shows that the uplift can be explained by aseismic slip at depth but not by viscoelastic flow. The slip at depth since 1964 must have a transient as well as a steady component, since the deduced slip is considerably greater than the roughly 1.5 m expected from North American‐Pacific plate convergence since the 1964 event. The minimum amount of slip required to fit the observations comes from an elastic model and is about 2.75 m. Viscoelastic models require more slip, since the dominant effect of viscoelastic flow is to produce subsidence in the region that is uplifted. A simple parameterization of the transient component using an amplitude and decay time is adequate to explain the observations. While the parameters deduced from the viscoelastic models have some ambiguity, the viscosity of models that best fit the observations cannot be much less than several times 1019 Pa s when the thickness of the North American plate is taken to be about 90 km.
Numerical simulation of elastic wave propagation in fractured rock with the boundary integral equation method
Authors: Gu Boliang , Nihei Kurt T. , Myer Larry R.
Published: February 2004

This paper describes a boundary integral equation method for simulating two‐dimensional elastic wave propagation in a rock mass with nonwelded discontinuities, such as fractures, joints, and faults. The numerical formulation is based on the three‐dimensional boundary integral equations that are reduced to two dimensions by numerical integration along the axis orthogonal to the plane of interest. The numerical technique requires the assembly and solution of the coefficient matrix only for the first time step, resulting in a significant reduction in computational time. Nonwelded discontinuities are each treated as an elastic contact between blocks of a fractured rock mass. Across such an elastic contact, seismic stresses are continuous and particle displacements are discontinuous by an amount which is proportional to the stress on the discontinuity and inversely to the specific stiffness of the discontinuity. Simulations demonstrate that such formulated boundary element method successfully models elastic wave propagation along and across a single fracture generated by a line source.
Provenance and thermal history of the Franciscan accretionary complex: Constraints from zircon fission track thermochronology
Authors: Tagami Takahiro , Dumitru Trevor A.
Published: February 2004

The Franciscan accretionary complex of California contains large belts of sediments that were subducted to depths of ≈10 to 30 km, metamorphosed under high‐pressure and low‐temperature conditions, and later returned to the surface. Reconnaissance zircon fission track (FT) data (21 samples) place constraints on the sedimentary provenance, thermal history, and/or exhumation history of various Franciscan units. In a transect across the northern Coast Ranges through Mendocino Pass, metagraywacke samples from the Valantine Springs Formation of the Franciscan Eastern belt experienced maximum temperatures (Tmax) ranging from ≈250° to >≈310°C. Zircon FT and previously reported apatite FT data indicate that parts of the western Eastern and eastern Central belts cooled at about 100–70 Ma from ≈300°C to ≈100°C. Considering the low paleogeothermal gradient of roughly 9°C/km during Franciscan subduction, these rocks were apparently uplifted and exhumed from ≈30 to ≈10 km depth at this time. One sample from the Eyler Mountain sequence in the northwestern Diablo Range similarly shows Tmax ≥ ≈310°C and major cooling at ≈100–70 Ma. This uplift and exhumation may have reflected accelerated accretion at the base as well as the front of the Franciscan wedge accompanied by extensional thinning at shallower levels, as a result of massive sediment influx into the Franciscan trench from the Sierra Nevada magmatic arc. In the northern Coast Ranges, the western part of the Central belt and the Coastal belt experienced lower Tmax, probably <200°C, and retain source area (detrital) zircon FT ages. Burnt Hills terrane samples in the Diablo Range also experienced Tmax <≈230°C and retain detrital ages. The detrital single‐grain age patterns from these areas are compatible with those expected from the Sierra Nevada magmatic arc and its country rocks. This is consistent with derivation of Franciscan sediments from arc terranes in California and provides no particular suggestion that these parts of the Franciscan are highly allochthonous. Paleogene Coastal belt sandstones also yield detrital ages compatible with derivation from the Sierra Nevada but also include early Tertiary zircon populations compatible with reconstructions suggesting these sandstones were sourced partly from the Idaho Batholith.
Frictional behavior of large displacement experimental faults
Authors: Beeler N. M. , Tullis T. E. , Blanpied M. L. , Weeks J. D.
Published: February 2004

The coefficient of friction and velocity dependence of friction of initially bare surfaces and 1‐mm‐thick simulated fault gouges (<90 μm) of Westerly granite were determined as a function of displacement to >400 mm at 25°C and 25 MPa normal stress. Steady state negative friction velocity dependence and a steady state fault zone microstructure are achieved after ∼18 mm displacement, and an approximately constant strength is reached after a few tens of millimeters of sliding on initially bare surfaces. Simulated fault gouges show a large but systematic variation of friction, velocity dependence of friction, dilatancy, and degree of localization with displacement. At short displacement (<10 mm), simulated gouge is strong, velocity strengthening and changes in sliding velocity are accompanied by relatively large changes in dilatancy rate. With continued displacement, simulated gouges become progressively weaker and less velocity strengthening, the velocity dependence of dilatancy rate decreases, and deformation becomes localized into a narrow basal shear which at its most localized is observed to be velocity weakening. With subsequent displacement, the fault restrengthens, returns to velocity strengthening, or to velocity neutral, the velocity dependence of dilatancy rate becomes larger, and deformation becomes distributed. Correlation of friction, velocity dependence of friction and of dilatancy rate, and degree of localization at all displacements in simulated gouge suggest that all quantities are interrelated. The observations do not distinguish the independent variables but suggest that the degree of localization is controlled by the fault strength, not by the friction velocity dependence. The friction velocity dependence and velocity dependence of dilatancy rate can be used as qualitative measures of the degree of localization in simulated gouge, in agreement with previous studies. Theory equating the friction velocity dependence of simulated gouge to the sum of the friction velocity dependence of bare surfaces and the velocity dependence of dilatancy rate of simulated gouge fails to quantitatively account for the experimental observations.
Can the Okhotsk Plate be discriminated from the North American plate?
Authors: Seno Tetsuzo , Sakurai Taro , Stein Seth
Published: February 2004

The plate geometry in northeast Asia has been a long‐standing question, with a major issue being whether the Sea of Okhotsk and northern Japanese islands are better regarded as part of the North American plate or as a separate Okhotsk plate. This question has been difficult to resolve, because earthquake slip vectors along the Kuril and Japan trenches are consistent with either Pacific‐North America or Pacific‐Okhotsk plate motion. To circumvent this difficulty, we also use slip vectors of earthquakes along Sakhalin Island and the eastern margin of the Japan Sea and compare them to the predicted Eurasia‐Okhotsk and Eurasia‐North America motions. For a model with a separate Okhotsk plate, we invert 10 Eurasia‐Okhotsk and 255 Pacific‐Okhotsk slip vectors with Pacific‐North America and Eurasia‐North America NUVEL‐1 data. Alternatively, for a model without an Okhotsk plate, those Eurasia‐Okhotsk and Pacific‐Okhotsk data are regarded as Eurasia‐North America and Pacific‐North America data, respectively. The model with an Okhotsk plate fits the data better than one in which this region is treated as part of the North American plate. Because the improved fit exceeds that expected purely from the additional plate, the data indicate that the Okhotsk plate can be resolved from the North American plate. The motions on the Okhotsk plate's boundaries predicted by the best fitting Euler vectors are generally consistent with the recent tectonics. The Eurasia‐Okhotsk pole is located at northernmost Sakhalin Island and predicts right‐lateral strike slip motion on the NNE striking fault plane of the May 27, 1995, Neftegorsk earthquake, consistent with the centroid moment tensor focal mechanism and the surface faulting. Along the northern boundary of the Okhotsk plate, the North America‐Okhotsk Euler vector predicts left‐lateral strike slip, consistent with the observed focal mechanisms. On the NW boundary of the Okhotsk plate, the Eurasia‐Okhotsk Euler vector predicts E‐W extension, discordant with the limited focal mechanisms and geological data. This misfit may imply that another plate is necessary west of the Magadan region in southeast Siberia, but this possibility is hard to confirm without further data, such as might be obtained from space‐based geodesy.