bluuebunny/crossref_metadata_2025 huggingface dataset:
Energetic particle dropouts observed in the morning sector by the geostationary satellite GEOS‐2
Authors: Kopányi Vilmos , Korth Axel
Published: February 2004

Energetic particle dropouts were observed in the local morning sector with the particle spectrometer S321 onboard the European geostationary satellite GEOS‐2. The dropouts usually last 10 to 30 minutes. They differ from those commonly observed in the midnight sector during growth phases of substorms and also from magnetopause crossings. They occur during the recovery phase and are well inside the magnetosphere. During the dropouts the total magnetic field increases. Ground magnetic recordings and AE indices show that the dropouts are related to the substorm recovery phase during multiple substorm events. Ground magnetic recordings also reveal that in the majority of the cases a new substorm activation is seen in the midnight sector simultaneously with the recovery phase signatures in the morning sector. The dropouts are most likely associated with locally thinning of the morning side plasma sheet.
Hydrous modified spinel, Mg1.75SiH0.5O4: A new water reservoir in the mantle transition region
Authors: Inoue Toru , Yurimoto Hisayoshi , Kudoh Yasuhiro
Published: February 2004

The water content in modified spinel, synthesized at 15.5 GPa under hydrous conditions, has been measured by means of secondary ion mass spectrometry (SIMSrpar;. We found that the modified spinel crystals contain up to 3.1±0.4 wt % H2O, which is consistent with the amounts estimated from the deficits in the oxide totals of the microprobe analysis. X‐ray diffraction analyses for a single crystal showed that the sample containing 2.5±0.3 wt % H2O is of the modified spinel structure with the lattice parameters a=5.663(1), b=11.546(2), c=8.247(4)Å and V=539.2(5)Å³. The present results suggest that a considerable amount of H2O may exist as hydrous modified spinel in the mantle transition zone, which could have important implications for the constitution and dynamics of the mantle.
Site response modelling by non‐linear waveform inversion
Authors: Riepl Judith , Rietbrock Andreas , Scherbaum Frank
Published: February 2004

Microearthquake waveforms and spectra are well known to be strongly influenced by the shallow subsurface conditions at the recording site. We make use of this fact to determine optimum site models from SH‐wave microearthquake seismograms using a non‐linear waveform inversion technique based on simulated annealing. Site models are optimized in terms of thicknesses, S‐wave velocities, and densities of the uppermost layers (<100 m) for 1D plane layer models. In addition, the source parameters in terms of Brune model source time functions are optimized simultaneously. The synthetic seismograms are calculated using a modification of Haskell's plane SH‐wave algorithm. Using aftershock recordings of the Loma Prieta earthquake, a 1D plane layer model has been obtained for a selected site in Central California which has been occupied during the aftershock experiment. For this dataset, cross‐correlation coefficients between optimized synthetics and observed records reach values above 0.8. The site model at the selected station for varying sources is fairly stable, especially for the uppermost layers. We observe systematic variations of layer parameters with back‐azimuth which would be in agreement with slightly dipping layer interfaces.
Increased solar ultraviolet‐B associated with record low ozone over Texas
Authors: Mims Forrest M. , Ladd Judson W. , Blaha Robert A.
Published: February 2004

A report (Kerr and McElroy, 1993) about record high solar UV‐B irradiance at 300 nm at Toronto in 1993 suggests that global ozone decline may have caused increased UV‐B elsewhere. This suggestion is consistent with observations of increased UV‐B associated with record low ozone near and around Seguin, Texas, during the summer of 1993. A sharp drop in ozone occurred during late June, and on 23 June total ozone fell to 230 Dobson units, which is 25.7% below the Nimbus‐7/TOMS observed mean for June 20–24 from 1979 to 1991 (310 DU). Direct UV‐B at 300 nm was the highest since ground observations were begun in February 1990. During July and August 1993, the mean total ozone was, respectively, 13% and 10.7% lower than during July and August 1990. When the Sun was not obscured by clouds or haze during July and August 1993, the mean direct irradiance at 300 nm was, respectively, 18.1% and 13.4% higher than during July and August 1990.
A method for estimating vertical distribution of the SAGE II opaque cloud frequency
Authors: Wang Pi‐Huan , McCormick M. Patrick , Minnis Patrick , Kent Geoffrey S. , Yue Glenn K. , Skeens Kristi M.
Published: February 2004

A method is developed to infer the vertical distribution of the occurrence frequency of clouds that are opaque to the Stratospheric Aerosol and Gas Experiment (SAGE) II instrument. An application of the method to the 1986 SAGE II observations is included in this paper. The 1986 SAGE II results are compared with the 1952–1981 cloud climatology of Warren et al. [1986, 1988].
The large‐scale summer circulation of the California Current
Authors: Strub P. Ted , James Corinne
Published: February 2004

Satellite data from the Geosat altimeter and the Advanced Very High Resolution Radiometer (AVHRR) are used to show the large‐scale structure of the surface circulation of the California Current System in summer. These data show the connection between an equatorward jet and temperature front off Oregon that lies within 100 km of the coast, similar to that first observed in the 1960's and 1970's, and a jet that meanders along the convoluted offshore edge of a temperature front off California, as repeatedly observed in the 1980's.
Anomalous meridional thermospheric neutral winds in the AE‐E NATE data: Effects of the equatorial nighttime pressure bulge
Authors: Goembel L. , Herrero F. A.
Published: February 2004

The work described here makes it possible to identify anomalous wind behavior such as the nighttime meridional wind abatements that occur at F‐region heights. A new analysis technique uses a simple empirical wind model to simulate measurements of “normal” winds (as measured by the Neutral Atmosphere and Temperature Experiment (NATE) that flew on the Atmosphere Explorer‐E (AE‐E)) to highlight anomalous wind measurements made by the satellite while in circular orbits at 270–290 km altitude. Our approach is based on the recognition that the “in orbit” wind variation must show the combined effects of the diurnal wind variation as seen from the ground with the latitude variation of the satellite orbit. For the data period 77250–78035 examined thus far, the wind abatement always occurred with a corresponding pressure or temperature maximum, and was detected on 12 out of the 36 nights with data. This study has revealed that the wind abatements occur only during or shortly after increases in solar EUV flux, as indicated by daily radio flux measurements. In the past, nighttime wind reversals at mid‐latitudes have been associated with increased geomagnetic activity. This study indicates that intensified solar EUV heating may be responsible for anomalous thermospheric nighttime winds at mid‐latitudes.
Interaction of HCl with crystalline and amorphous ice: Implications for the mechanisms of ice‐catalyzed reactions
Authors: Graham James D. , Roberts Jeffrey T.
Published: February 2004

The interaction of HCl with ice under ultrahigh vacuum has been studied using temperature programmed desorption and single reflection Fourier transform infrared spectroscopy. Both amorphous and crystalline ice were investigated. At 120 K, HCl is initially absorbed by ice to form a stoichiometric hydrate phase identified as HCl · 6H2O. Upon completion of the bulk phase, HCl is adsorbed to the surface. This initial HCl adsorption probability on crystalline ice is ≈60% of that on amorphous ice. Furthermore, the adsorption probability on crystalline ice decreases more rapidly with increasing HCl uptake. As a result, HCl · 6H2O is more rapidly formed in amorphous ice. The difference in adsorption probabilities is tentatively attributed to a lower coverage of dangling OH groups at the crystalline ice surface.
Winds and vertical wavelengths deduced from the ground‐based measurement of the Doppler shifts of the O2(b ¹∑g+ ‐ X ³∑g−), OI(¹D2 ‐ ¹S0), and the X ²Π OH (6–2) Band P1(2)c,d line emissions in the midlatitude upper middle atmosphere
Authors: Hernandez G. , Smith R. W.
Published: February 2004

Simultaneous ground‐based measurements of the upper middle atmosphere motions, using the OI(¹D2 ‐¹S0) emission and the O2 (b¹∑g+ ‐ X³∑g−) atmospheric band emission as tracers, indicate that the height separation between the emission layers is small, in comparison with the vertical wavelength oscillations present in that atmospheric region, and in agreement with earlier results obtained with rocket measurements. The finding of a long vertical wavelength is corroborated by simultaneous wind motion measurements obtained with the emissions of O[I] and OH (6–2) as tracers, where a vertical wavelength of about 90 km has been determined. This new optical, ground‐based, capability to determine motions and vertical wavelengths in the upper middle atmosphere from simultaneous measurements of multiple emission layers provides a new optical tool to investigate the oscillations present in this region of the upper atmosphere.
Measurement of anomalous cosmic ray oxygen at heliolatitudes ∼25° to ∼64°
Authors: Lanzerotti L. J. , Maclennan C. G. , Gold R. E. , Armstrong T. P. , Roelof E. C. , Krimigis S. M. , Simnett G. M. , Sarris E. T. , Anderson K. A. , Pick M. , Lin R. P.
Published: February 2004

We report measurements of the oxygen component (0.5 ‐ 22 MeV/nucl) of the interplanetary cosmic ray flux as a function of heliolatitude. The measurements reported here were made with the Wart telescope of the HI‐SCALE low energy particle instrument on the Ulysses spacecraft as the spacecraft climbed from ∼24° to ∼64° south solar heliolatitude during 1993 and early 1994. As a function of heliolatitude, the O abundance at 2–2.8 MeV/nucl drops sharply at latitudes above the heliospheric current sheet. The oxygen spectrum obtained above the current sheet has a broad peak centered at an energy of ∼2.5 MeV/nucl that is the anomalous O component at these latitudes. There is little evidence for a latitude dependence in the anomalous O fluxes as measured above the current sheet. Within the heliospheric current sheet, the O measurements are composed of both solar and anomalous origin particles.
VLBI baseline variations from the Ice‐4G Model of postglacial rebound
Authors: Peltier W. R.
Published: February 2004

An analysis is presented of selected VLBI baseline variations expected as a consequence of the continuing process of glacial isostatic adjustment. Although these baseline variations are for the most part dominated by the radial component of the motion, the tangential component can also be an important contributor to the net effect and is in fact dominant for short baselines. The new calculations have been performed with the recently derived ICE‐4G model of the last deglaciation event of the current ice age (Peltier, 1994), and the results demonstrate that those obtained with the previous ICE‐3G model are not reliable.
Observational evidence of tidal/gravity wave interactions using the UWO 2 MHz radar
Authors: Thayaparan T. , Hocking W. K. , MacDougall J.
Published: February 2004

The degree of interaction between atmospheric gravity waves and tides is an issue which is of great importance in middle atmosphere studies. It is especially important with regard to tidal variability which is frequently observed in the mesosphere and lower thermosphere, but as yet has not been subject to adequate study. In this paper, we present some strong observational evidence of such interactions made with the UWO 2 MHz radar near London, Canada. Our observations show significant correlation between gravity waves and tides (diurnal) at certain times of the year, and the nature of the correlation is intermittent and varies with time of year. It is possible that differences between tidal amplitudes at sites with mesoscale separations may be explained in terms of such phenomena. We also propose a simple model which is found to be consistent with the major features of the observed data.
Impulse‐response functions and anthropogenic CO2
Authors: Tubiello Francesco N. , Oppenheimer Michael
Published: February 2004

Non‐linearities in the carbon cycle make the response to atmospheric CO2 perturbations dependent on emission history. We show that even when linear representations of the carbon cycle are used, the calculation of time scales characterizing the removal of excess CO2 depends on past emissions.
Geoelectric power spectra over oceanic distances
Authors: Fujii Ikuko , Lanzerotti L. J. , Utada H. , Kinoshita H. , Kasahara J. , Medford L. V. , Maclennan C. G.
Published: February 2004

We present initial results of simultaneous measurements of the geoelectric potential as measured across two long cable routes in the Pacific (Guam to Philippines and California to Hawaii) separated by ∼6h in local time and ∼30° in latitude. Over the frequency range ∼10−5 to 1.67 × 10−3Hz the power levels tend to be enhanced near local noon along the two routes, especially during geomagnetic quiet intervals. During the seven days analyzed, geoelectric power levels up to 104 (mV/km)²/km were measured on a geomagnetically disturbed day. The power levels at the longest periods, ∼17m to ∼150m, are generally found to be largest along the route at lower geomagnetic latitudes (Guam to Philippines) when comparisons are made at equivalent local times. In contrast, the power levels for periods in the hydromagnetic wave region (periods ≲5m) tend to be larger at all local times for the higher geomagnetic latitude route. The nightside geoelectric power levels along the higher latitude route tend to increase with increasing geomagnetic activity.
Carbonyl sulfide (COS) measurements in the Arctic Polar Vortex
Authors: Kourtidis K. A. , Borchers R. , Fabian P. , Harnisch J.
Published: February 2004

One stratospheric carbonyl sulfide (COS) vertical profile in the Arctic polar vortex has been retrieved from air samples collected by the MPAE balloon‐borne cryogenic sampler on January 18, 1992, at Kiruna, Sweden. The measurements were made in the altitude range 7.5–28.5 km. The upper tropospheric volume mixing ratios of COS were 355±60 pptv. The stratospheric profile shows that COS is subject to considerable subsidence this time of the year. The lowest VMRs were encountered at around 20 km altitude, while above that height a COS increase was observed. A possible explanation could be reversible uptake of COS by frozen sulfuric acid aerosols.
Direct evidence for the undulation of the 660‐km discontinuity beneath Tonga: Comparison of Japan and California array data
Authors: Niu Fenglin , Kawakatsu Hitoshi
Published: February 2004

Short‐period seismograms of Tonga deep earthquakes recorded by Japanese and Californian seismic networks are stacked to identify the S‐P converted wave associated with the 660‐km discontinuity. The travel‐time difference between this S‐P converted wave and the direct P wave is used to constrain the depth of the 660‐km discontinuity. Analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the Tonga subduction zone. Two events which exhibit clear S‐P conversions in both Japan and California data are selected to show directly the depth variations of the 660‐km discontinuity adjacent to the subducting slab. The S‐P conversion points on the ray paths to Japan are observed to be approximately 10 to 30 km deeper than the conversion points on those to California, which represents direct evidence for a slab‐induced depression of the 660‐km discontinuity.
Thermal instability in the inner coma of a comet
Authors: Milikh G. M. , Sharma A. S.
Published: February 2004

The spacecraft and ground based observations of comet Halley inner coma showed a localized ion density depletion region whose origin is not well understood. Although it has been linked to a thermal instability associated with negative ions, the photodetachement lifetime of negative ions (∼ 1 sec) is too short compared to the electron attachment time scale (∼ 100 sec) for this process to have a significant effect. A mechanism for the ion density depletion based on the thermal instability of the cometary plasma due to the excitation of rotational and vibrational levels of water molecules is proposed. The electron energy losses due to these processes peak near 4000 K (0.36 eV) and at temperatures higher than this value a localized cooling leads to further cooling (thermal instability) due to the increased radiation loss. The resulting increase in recombination leads to an ion density depletion and the estimates for this depletion at comet Halley agree with the observations.
Photochemical production of dissolved inorganic carbon from terrestrial organic matter: Significance to the oceanic organic carbon cycle
Authors: Miller William L. , Zepp Richard G.
Published: February 2004

Water collected from riverine, near coastal, and salt marsh sources in the Southeastern United States was evaluated for its ability to produce both carbon monoxide (CO) and dissolved inorganic carbon (DIC) by photochemical oxidation of natural dissolved organic carbon (DOC). Irradiation of whole water samples using simulated sunlight produced CO at rates similar to those measured previously. Production rates for DIC were more than an order of magnitude higher than those observed for CO. Based on observed DIC formation rates, photo‐oxidation of DOC by sunlight should be considered a dominant removal mechanism of organic carbon from the ocean.
The structure of the distant geomagnetic tail during long periods of northward IMF
Authors: Raeder J. , Walker R. J. , Ashour‐Abdalla M.
Published: February 2004

We have used a newly developed, parallelized, global MHD magnetosphere‐ionosphere simulation model with a 400 RE long tail to study the evolution, structure, and dynamics of the distant magnetotail during extended periods of northward IMF. We find that the tail evolves to a nearly time stationary structure about one solar wind transit time after the IMF turns northward. Four regions of different magnetic topology can be distinguished which extend at least to the end of the simulation box at 400 RE. Besides lobe field lines and open solar wind field lines tailward of an X‐line, there is a broad boundary layer of closed field lines which we call the tail flank boundary layer (TFBL). Just inside the TFBL there is a region of closed field loops. Besides the X‐line we find two O‐lines which are enclosed by the closed field loops and are roughly aligned with the tail axis. Together they form a U shaped separator between the northward and the southward plasma sheet fields.
Earthquake recurrence on the southern San Andreas modulated by fault‐normal stress
Authors: Palmer Randy , Weldon Ray , Humphreys Eugene , Saucier Francois
Published: February 2004

Earthquake recurrence data from the Pallett Creek and Wrightwood paleoseismic sites on the San Andreas fault appear to show temporal variations in repeat interval. These sites are located near Cajon Pass, southern California, where detailed mapping has revealed geomorphically and structurally expressed domains of alternating extension and contraction respectively associated with releasing and restraining bends of the San Andreas fault. We investigate the interaction between strike‐slip faults and auxiliary reverse and normal faults as a physical mechanism capable of producing such variations. Under the assumption that fault strength is a function of fault‐normal stress (e.g. Byerlee's Law), failure of an auxiliary fault modifies the strength of the strike‐slip fault, thereby modulating the recurrence interval for earthquakes. In our finite element model, auxiliary faults are driven by stress accumulation near restraining and releasing bends of a strike‐slip fault. Earthquakes occur when fault strength is exceeded and are incorporated as a stress drop which is dependent on fault‐normal stress. The model is driven by a velocity boundary condition over many earthquake cycles. Resulting synthetic strike‐slip earthquake recurrence data display temporal variations similar to observed paleoseismic data within time windows surrounding auxiliary fault failures. Although observed recurrence data for the two paleoseismic sites are too short to be definitive about the temporal variations or the physical mechanism responsible for it, our simple model supports the idea that interaction between a strike‐slip fault and auxiliary reverse or normal faults can modulate the recurrence interval of events on the strike‐slip fault, possibly producing short term variations in earthquake recurrence interval.
Giant sulfur‐dominant particles in remote marine boundary layer
Authors: Mouri Hideaki , Okada Kikuo , Takahashi Shunji
Published: February 2004

This letter reports the finding of giant aerosol particles (with radii greater than 1 µm) composed mainly of sulfur over the equatorial Pacific Ocean. The sample also contains many modified sea‐salt particles. Since the air mass had been over the open sea for more than 10 days, these giant sulfur‐dominant particles may have been formed in the marine boundary layer or transported from the free troposphere.
Hillslope and channel evolution in a marine terraced landscape, Santa Cruz, California
Authors: Rosenbloom Nan A. , Anderson Robert S.
Published: February 2004

A flight of marine terraces along the central California coastline provides a unique setting for the study of topographic evolution. Wavecut platforms mantled by 2–6 m of marine terrace cover deposits are separated by 10–50 m tall decaying sea cliffs. Paleocliff edges become more rounded with age, yet the details of the profiles and frequent bedrock exposure on the upper slopes imply weathering‐limited transport. Five bedrock stream channels etched through the marine terrace sequence display one to three distinct convexities in their longitudinal profiles. Detailed hand level surveys of the hillslopes and of the stream channel longitudinal profiles constrain hillslope evolution and channel incision components of a numerical model of landscape evolution. We account for regolith production as a function of regolith depth. In accord with the field observation that hillslope processes are presently dominated by the activities of burrowing rodents, the transport process is taken to be diffusive. Stream incision is assumed to be controlled by stream power, for which we use the surrogate of local drainage area‐slope product. Best fits of the numerical model to field data imply: hillslope diffusivity is 10 m2 kyr−1; regolith production rate on bare bedrock is 0.3 m kyr−1, and falls off rapidly with regolith cover, and the constant controlling the efficiency of stream incision is 5 to 7×10−7m−1 kyr1.
Thermomechanical erosion of the lithosphere by mantle plumes
Authors: Davies Geoffrey F.
Published: February 2004

A simplified semianalytic theory is presented that describes the process of lithosphere thinning through thermomechanical erosion by a hot mantle upwelling or plume. The theory compares well with published numerical models but predicts that the rate of thinning is very dependent on the lateral scale of the upwelling and on the viscosity through the lithosphere‐asthenosphere transition. A plume tail is likely to cause rapid local thinning, reducing the lithosphere thickness substantially within a few million years. Plume heads or broadly spreading plume tail material are unlikely to cause significant thinning by this mechanism alone. These results are consistent with geophysical evidence for localized thinning under the Hawaiian volcanic chain but not under the broader Hawaiian swell. They are also qualitatively consistent with the distribution of basaltic volcanism in oceanic and continental hotspot tracks. The theory predicts that thin continental lithosphere may be further thinned sufficiently by a plume tail to cause crustal melting within about 20 m.y. Thus this mechanism may account for some delayed silicic magmatism on some slow continental plates. Higher thinning rates would have occurred in the Archean.
Systematic non‐double‐couple components of earthquake mechanisms: The role of fault zone irregularity
Authors: Kuge Keiko , Lay Thorne
Published: February 2004

Several recent studies have revealed statistical correlations between earthquake mechanism type and associated non‐double‐couple components in catalogs of seismic moment tensors. This systematic behavior may result either from biases in the solutions due to Earth structure effects in different tectonic regimes, or from source radiation effects. For certain large non‐double‐couple events, detailed analyses have shown that multiple subevents with different fault orientations produce the non‐double‐couple radiation. We generalize this idea and simulate non‐double‐couple components (NDCC) resulting from subfaults with variable geometry, showing that the statistical behavior of the NDCC in moment tensor catalogs can be generally accounted for by such source complexity. Assuming that an earthquake fault consists of many subfaults with random fluctuations about some mean geometry, the total moment tensor for failure of the system under a regional stress state can be represented by the sum of moment tensors of the subfaults. The sign of the NDCC in the composite moment tensor directly reflects the stress state applied to the fault system, and the NDCC amplitude is expected to be systematic over a wide range of cumulative seismic moment (Mo ) given the basic fractal nature of fault systems. These simulation results are consistent with the global behavior of the NDCC reported in the Harvard centroid moment tensor catalog for 1977–1991. Parameters such as the applied stress state, parameterization and randomness of the subfault geometry, and seismic moment distribution among the subfaults affect the predicted NDCC amplitude. Change in the parameters as a function of subfault seismic moment controls the shape of the NDCC ‐ Mo relationship. Regional variations of the NDCC ‐ Mo relationship may thus reflect regional variations in the fault zone parameters, raising the possibility that the regional NDCC behavior may be used to infer stress state and subfault distribution in various source regions.
Paleomagnetic evidence for high‐temperature emplacement of the 1883 subaqueous pyroclastic flows from Krakatau Volcano, Indonesia
Authors: Mandeville Charles W. , Carey Steven , Sigurdsson Haraldur , King John
Published: February 2004

The paroxysmal 1883 eruption of Krakatau volcano in Indonesia discharged at least 6.5 km3 (dense rock equivalent) of pyroclastic material into the shallow waters of the Sunda Straits within a 15‐km radius of the volcano. Progressive thermal demagnetization studies of individually oriented pumice clasts from a core sample of the submarine pyroclastic deposits show that 41 out of 47 clasts exhibit single‐component remanence with mean inclination of −2.4°. The partial thermoremanent magnetization components of both pumice and lithic clasts are well grouped in orientation, indicating that substantial cooling of clasts must have occurred following deposition. Estimated subaqueous emplacement temperature for such clasts is greater than 500°C. Rare two‐component lithic fragments exhibit inflection points on vector endpoint diagrams that mark the temperature below which the fragments acquired magnetization of similar orientation. These inflection points range from 350° to 550°C, indicating a minimum subaqueous emplacement temperature of 350°C. Paleomagnetic evidence for high‐emplacement temperature supports the hypothesis that proximal 1883 submarine pyroclastic deposits resulted from entrance of hot, subaerially generated pyroclastic flows into the sea. Similar deposits have been interpreted from the geologic record, but this is the first documented example of submarine pyroclastic flows from a historic eruption. The Krakatau deposits thus serve as an important modern analog for the study of pyroclastic flow/seawater interactions.
Influence of the Sierra Leone mantle plume on the equatorial Mid‐Atlantic Ridge: A Nd‐Sr‐Pb isotopic study
Authors: Schilling J.‐G. , Hanan B. B. , McCully B. , Kingsley R. H. , Fontignie D.
Published: February 2004

We report on a Pb‐Nd‐Sr isotope and rare earth study of Mid‐Atlantic Ridge (MAR) basalt glasses collected across the equatorial fracture zones from 7°S to 5°N (65 stations). The 1600‐km‐long profile reveals two mixing zones in the mantle that are isotopically distinct but cover the same range of (La/Sm)n ratios (0.3–2), with a gradational boundary between the Romanche and the Chain fracture zones. The potential mantle temperature profile inferred from Na2O content is also quite distinct. The north zone is dominated by a major, La/Sm and HIMU type Pb isotope anomaly centered at 1.7°N±300 km, which is flanked by two zones mildly radiogenic in Pb but depleted in light REE. A kinematic and evolutionary model describing the dispersion and interaction of the Sierra Leone plume with the asthenosphere and the MAR in the last 75 m.y. is proposed for this zone, which includes St. Paul and St. Peter's Rocks. In contrast, over the south zone the isotope/geochemical profiles are well correlated at all length scales and opposite in sign from the inferred potential mantle temperature profile and mean percent fusion. Broad negative gradients are observed between the Romanche and the Charcot fracture zones, superimposed by spikelike anomalies at the intersection with the eastern part of the Romanche and Chain transform faults, where cold plate edge effects prevail. The heterogeneous mantle model of Sleep [1984] and Langmuir and Bender [1984] is applicable to this zone, that is the volatile and radiogenic Pb‐rich lumps are preferentially melted during mantle decompression and passively sampled. The lumps may reflect the early dispersion of the St. Helena or Ascension mantle plumes under a thick lithosphere, followed by redistribution due to intense shearing, continental lithosphere delamination, and secondary mantle convection. The presence of a depleted asthenosphere unpolluted by plumes along the 400‐km‐long MAR segment between the Charcot and Ascension fracture zones is also apparent in the data.
Using in situ produced cosmogenic isotopes to estimate rates of landscape evolution: A review from the geomorphic perspective
Authors: Bierman Paul R.
Published: February 2004

The application of in‐situ produced cosmogenic isotopes to problems in geomorphology has increased rapidly over the past decade. At least 57 papers and numerous abstracts have been published since the mid‐1980s when the first mass‐spectrometric measurements of terrestrially produced cosmogenic isotopes were made. Taken at face value, these studies provide quantitative information about rates of landscape evolution and landform age; however, the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret isotopic data, the validity of assumptions inherent to these models, and the geologic surroundings in which the samples were collected. This paper attempts to place cosmogenic isotope studies in a geomorphic context by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted. At present, the establishment of high‐precision, cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic surfaces, the erosion of rock from sampled boulders, the potential for isotope inheritance from previous exposure, and the uncertainty of isotopic measurements. Uncertainties in isotope production rates and the observed variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated reliably with those obtained by other techniques. Estimation of erosion rates at single points on the landscape gives useful small‐scale information. Extrapolation of these rates over longer time and larger spatial scales is less sure and most likely biased toward lower erosion rates by die inadvertent selection of resistant sample sites. However, because erosion rates are so poorly constrained at present, even estimates to within a factor of 2 may be of significant value to geomorphologists and tectonicists.
The influence of regional stress and magmatic input on styles of monogenetic and polygenetic volcanism
Authors: Takada Akira
Published: February 2004

A new model is proposed to relate the development of monogenetic and polygenetic volcanoes to magmatic input and regional stress in the lithosphere. Output‐stress diagrams relate magmatic output rate and crustal deformation rate (or strain rate). The magmatic output rates of polygenetic volcanoes and monogenetic volcano fields, including lava fields and central (axial) volcanoes on the mid‐oceanic ridge systems, are estimated. Crustal deformation rates obtained from the literature are used as indicators of differential stress (Δσ). These data are normalized over time periods of 104 years, and over areas of 103 km2. The ratio of output rate to input rate, inferred from ophiolite sections, and from crustal deformation around volcanoes, is about 0.1 to 0.3. Based on the average ratio, an input‐stress diagram may be obtained at some volcanoes. Polygenetic volcanoes are plotted in the region of both larger output rate and smaller Δσ. Monogenetic volcano fields are plotted on a rift trend, or in the region of a small output (≤1 km3/104 yr/103 km2). In some regions, polygenetic volcanoes and monogenetic volcanoes coexist, for example, during 0.1 m.y. periods, and within the area of 30 km × 30 km. The output‐stress diagram indicates that coexistence of these volcano types results from variation in differential stress, or variation in production rate of magma in the mantle at different levels over periods of time less than 0.1 m.y. This is supported using extrusive volumes and crustal deformation data from the eastern volcanic zone, Iceland, Taupo Volcanic Zone, New Zealand, TransMexican Volcanic Belt, Mexico, and the volcanic field on and around the Izu peninsula, Japan. The observed relationship between deformation rate, magmatic output rate, and volcano type supports the crack interaction theory of magma‐filled cracks. The output‐stress diagram also indicates that the balance between local stress induced by magma accumulation and regional stress, and stress relaxation, govern the structure of a volcano.
Long‐term probabilistic forecasting of earthquakes
Authors: Kagan Y. Y. , Jackson D. D.
Published: February 2004

We estimate long‐term worldwide earthquake probabilities by extrapolating catalogs of seismic moment solutions. We base the forecast on correlations of seismic moment tensor solutions. The forecast is expressed as a map showing predicted rate densities for earthquake occurrence and for focal mechanism orientation. Focal mechanisms are used first to smooth seismicity maps to obtain expected hazard maps and then to forecast mechanisms for future earthquakes. Several types of smoothing kernels are used: in space domain we use the 1/distance kernel for the distribution of seismicity around any epicenter. The kernel is parameterized using two adjustable parameters: maximum distance and directivity (distribution of seismicity around an epicenter with regard to the focal mechanism of an earthquake). For temporal prediction we use the Poisson hypothesis of earthquake temporal behavior. We test these forecasts: we use the first half of a catalog to smooth seismicity level, and the second half of the catalog is used to validate and optimize the prediction. To illustrate the technique we use available data in the Harvard catalog of seismic moment solutions to evaluate seismicity maps for several seismic regions. The method can be used with similar catalogs. The technique is completely formal and does not require human operator intervention, hence the prediction results can be objectively tested. Moreover, the maps can be used as the Poisson null hypothesis for testing by the likelihood method against any other prediction model which shares the same sample space (the same zones, time window, and acceptance criteria).
Thermodynamic properties of ferromagnesium silicate perovskites from vibrational spectroscopy
Authors: Lu Ren , Hofmeister Anne M. , Wang Yanbin
Published: February 2004

Infrared (IR) reflectance and absorbance spectra obtained from polycrystalline and single‐crystal perovskites of MgSiO3 and (Mg0.9Fe0.1)SiO3 compositions yielded virtually all the expected fundamental modes. Near‐IR absorbance spectra indicate that the polycrystals were anhydrous, whereas the single crystal contained about 0.06 wt % H2O. A Kieffer‐type model was used to calculate heat capacity, entropy, and Debye temperature for the orthorhombic perovskite phase. The calculated values at 298 K are Cp = 82 ± 2 J/mol K and S = 56 ± 2 J/mol K. Our calculated Cp agrees with calorimetric data within 5%. Above 500 K, the Debye temperature is nearly constant and equals 960±30 K. The bulk modulus is calculated from vibrational frequencies to be 265±13 GPa which lies within 2% of recent experiments. The Clapeyron slope for the phase transition between ilmenite and orthorhombic perovskite is calculated to be 1.110.6 MPa/K which is close to the experimental determination.
Cluster growth modeling of plateau erosion
Authors: Stark Colin P.
Published: February 2004

The pattern of erosion of a plateau along an escarpment may be modeled using cluster growth techniques, recently popularized in models of drainage network evolution. If erosion on the scarp takes place in discrete events at rates subject to local substrate strength, the whole range of behavior is described by a combination of three cluster growth mechanisms: invasion percolation, Eden growth and diffusion‐limited aggregation (DLA). These model the relative importance of preexisting substrate strength, background weathering, and seepage weathering and erosion respectively. The rate of seepage processes is determined by the efflux of groundwater at the plateau margin, which in turn is determined by the pressure field in the plateau aquifer. If this process acted alone, it would produce erosion patterns in the form of Laplacian fractals, with groundwater recharge from a distant source, or Poissonian fractals, with groundwater recharge uniform over the plateau. DLA is used to mimic the Laplacian or Poissonian potential field and the corresponding seepage growth process. The scaling structure of clusters grown by pure DLA, invasion percolation, or Eden growth is well known; this study presents a model which combines all three growth mechanisms for the first time. Mixed growth processes create clusters with different scaling properties and morphologies over distinct length scale ranges, and this is demonstrable in natural examples of plateau erosion.
Three‐dimensional critical wedges: Tectonics and topography in oblique collisional orogens
Authors: Koons Peter O.
Published: February 2004

In orogens deforming in oblique compression, both the long wavelength (mean) topography of averaged slopes and the higher frequency topography (first harmonic) of valleys and ridges are related to the mechanics of deformation in three dimensions and cannot be adequately modeled in two dimensions. A method of analysis of the principal stresses, failure planes, and topography is presented for three‐dimensional critical wedges. In a critical Coulomb orogen which is at failure throughout, the components of the stress tensor are interrelated through the failure condition defined in terms of the principal stresses. Variation of any of the Cartesian components must be accommodated by adjustment of one or more of the other components so that the orogen remains at failure. This coupling of Cartesian components to maintain the wedge at failure permits exploration of the behavior of a three‐dimensional, critical orogen in which shear stresses arise outside of the plane of convergence. Internal adjustment of shear stresses to externally imposed stress conditions results in the formation of characteristic orogen shapes. For an oblique collision zone such as the Southern Alps of New Zealand, the resultant mountain belt is one of an obliquely deforming outboard fold‐thrust belt with subdued relief rising abruptly into a steep backslope to the main divide which falls steeply to the indentor, forming the inboard slope. The outboard topographic slope is reduced in relation to that of the two‐dimensional orogen. The rigid indentor has a fixed strike and therefore imposes a failure orientation adjacent to the indentor that differs from that of the ideal three‐dimensional critical wedge solution. This produces several different slip orientations and minor partitioning of strain near the indentor. First harmonic topography of valleys and ridges arises due to nonlinear, heterogeneous behavior of both erosional and mechanical processes and is characterized by uplift‐controlled ridges in the outboard and incision‐controlled valleys in the inboard.
The denudation history of the onshore continental margin of SE Brazil inferred from apatite fission track data
Authors: Gallagher K. , Hawkesworth C. J. , Mantovani M. S. M.
Published: February 2004

A suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast Brazil are presented. Some of the results reflect resetting related to the eruption of the Paraná continental flood basalt (∼130 Ma), and four others are directly related to Late Cretaceous magmatism (∼80 Ma). The majority of the results were obtained from Precambrian basement rocks and sediments from the Paraná Basin. The fission track ages broadly increase inland from 60–90 Ma on the coastal plain to >300 Ma in the continental hinterland. The track length distributions show a characteristic trend, being negatively skewed at the coast and showing increasing bimodality inland, except for the oldest samples which are more unimodal. These trends do not reflect rift‐related heating and resetting but are the result of protracted denudation since the opening of the South Atlantic. The results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland. The topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology. For the region further south, model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long‐wavelength topography, unless the flexural rigidity is more than 1025 N m. In this case the initial elevation of the rift margin would need to be >2.5 km in order to accommodate the amount of denudation inferred from the fission track data. However, it is possible that the effective flexural rigidity varies across the margin, becoming greater in the continental interior. Alternatively, the broad scale wavelength topography may be the result of magmatic underplating related to the Paraná continental flood basalt event. If so, estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5.
Crack nucleation mechanism in saline ice
Authors: Picu R. C. , Gupta V. , Frost H. J.
Published: February 2004

A mechanism for crack nucleation in saline ice is presented, by considering a planar array of hexagonal grains containing the brine pockets as a model of poly crystalline saline ice. It is shown through a thermodynamic analysis that important local stresses arise associated with the internal pressure which builds up inside a brine pocket due to a drop in the temperature. As the temperature drops, the water inside the brine freezes, and because of the variation in the water density on freezing, this results in a buildup of pressure inside the pocket. For typical field conditions, assuming elastic behavior for the matrix, pressures as high as 7 MPa are estimated. Next, using a finite element method, the stress concentration at a grain triple junction is determined under the influence of the stress field associated with a nearby brine pocket. The resulting stress state is used to determine the condition for crack nucleation. The analysis is restricted to only elastic deformation regimes with isotropic grains, albeit with elastic constants corresponding to extreme values in a single freshwater ice crystal. The mechanism discussed here provides an explanation for the widely observed brine channels in sea ice. In addition, the effect of the externally applied stress is also considered, and the resulting stress singularities at the grain triple junctions analyzed by an asymptotic method as well as by a numerical solution. Both the strength and an approximate energy criteria suggest crack nucleation from the brine pocket surface towards the grain triple junction. The results are shown to be consistent with the experimental observations.
Evolution of the Santa Cruz Mountains, California, through tectonic growth and geomorphic decay
Authors: Anderson Robert S.
Published: February 2004

I describe a strategy for modeling a specific large scale topographic feature that recognizes the discrepancy between tectonic and geomorphic scales and allows for important feedbacks between these processes. In the geomorphic model, the largest channel in each 4×4 km cell is explicitly treated, the channel incision being driven by a stream power rule. Hillslopes internal to each cell respond rapidly to incision of the local channel, prompting their treatment in the model as steady state forms. I present analytic expressions for the local relief, which is controlled by the local channel incision rate, and rate constants associated with the dominant hillslope process, either diffusive or landsliding. Initial conditions include any preexisting topography, and a lightly etched a channel network. The model is illustrated using the Santa Cruz Mountains, California. I show that an earlier hypothesis (that the Santa Cruz Mountains are due to advection of topography past an uplift source related to a restraining bend in the San Andreas Fault) remains viable when more specific geomorphic processes are considered. The local tectonics are represented by crustal thickening necessitated by conservation of mass in the bend, right‐lateral translation of crust with respect to the bend, and flexural response to the distributed load. The model produces reasonable geographic and statistical distributions of topography using rates of tectonic and geomorphic processes that are within the range of those measured locally. This argues that this restraining bend is a long‐lived feature of the San Andreas Fault. The slightly higher crest and shorter length of the southern Santa Cruz Mountains may potentially be explained by slightly faster slip of the crust west of the fault with respect to the bend than of that to the east. Deflection is minimal beneath these relatively small mountains, disallowing the interesting feedback of erosionally driven uplift. The predicted pattern of exhumation could be used to guide the search for fission track sites that could aid in the testing of the model.