bluuebunny/crossref_metadata_2025 huggingface dataset:
P‐wave image of the upper mantle structure of central California and southern Nevada
Authors: Biasi Glenn P. , Humphreys Eugene D.
Published: February 2008

We image the upper mantle velocity structure of central California and southern Nevada by inverting teleseismic P‐wave residuals. Beneath the southern Great Valley and adjacent Sierran foothills 4% high velocities extend to a depth of 200–240 km. The magnitude and the depth extent of this anomaly suggest that it is sinking lithosphere. Possible sources of lithospheric material include fragments of slab from Laramide or post‐Laramide time or perhaps North American mantle lithosphere removed from neighboring regions. A west to east contrast from high to low velocities in the shallow mantle beneath the Sierra Nevada correlates with crustal contrasts from low to high heat flow and low to high seismicity, suggesting that the high velocities beneath the western foothills correspond to mantle lithosphere relatively chilled and strengthened by sub‐horizontal Cenozoic subduction. In southern Nevada, low velocity mantle is imaged along the SW extension of the St. George Volcanic lineament, and high velocity mantle is found to the NW beneath most of the Miocene SW Nevada Volcanic Field. Variations in partial melt content most simply explain upper mantle velocity contrasts observed there.
Extremely high proportions of soot particles in the upper troposphere over Japan
Authors: Okada Kikuo , Ikegami Miwako , Uchino Osamu , Nikaidou Yoshinobu , Zaizen Yuji , Tsutsumi Yukitomo , Makino Yukio
Published: February 2008

Observation on tropospheric aerosol particles was carried out on 27 April 1991 over Tsukuba, Japan. Aerosol particles collected in the upper troposphere at 7.5 km altitude were studied by electron microscopy. Soot‐containing particles were collected in a large number fraction (0.53) of the submicron particles. Such a high proportion of soot particles in the upper troposphere has not been reported in previous research. From the X‐ray analysis, vanadium was also detected in some of the particles, indicating the origin of oil combustion. The oil‐well fires in Kuwait are likely to be the origin of these soot particles. Calculated trajectories of the air suggested that these particles over Japan had circled the globe.
Mechanisms of some tropical intraseasonal oscillations
Authors: Goswami B. N. , Goswami P.
Published: February 2008

A underlying mechanism for two westward propagating tropical intraseasonal oscillations is discovered. Destabilization of equatorial normal modes by moist feedbacks, in particular, by evaporation‐wind feedback, is responsible for both of them. The westward propagating 4–5 day oscillation and the quasi‐biweekly oscillation result from destabilization of the mixed Rossby gravity (MRG) wave by the evaporation‐wind feedback. The frequency and scale‐selection of the the unstable MRG wave depend critically on the background mean wind. In mean easterlies (as in the Pacific and the Atlantic), it results in the 4–5 day oscillation while in mean westerlies (as in the Indian Ocean during northern summer) it results in the quasi‐biweekly mode.
3‐D radiation pattern of the drilling bit source in finely stratified media
Authors: Carcione Jose M. , Carrion Philip M.
Published: February 2008

Seismic exploration industry is largely based on the concept of an impulsive point source whose radiation pattern in isotropic media is well‐known. It is spherically isotropic in acoustic unbounded homogeneous media and directional in elastic media. A point source excitation in a simplified isotropic medium lend no credence what may happen in more realistic situations. In this paper, we present an analysis of the the radiation patterns for a vertical and a rotational loads distributed over a disk‐shape surface placed in a finely stratified medium with a random distribution of elastic parameters. This complex source simulates the rotary drill tool used in boreholes. It is shown that the radiation patterns in these cases significantly differ from those computed for a point force in isotropic homogeneous media.
Scale invariance of basaltic lava flows and their fractal dimensions
Authors: Gaonac'h Hélène , Lovejoy Shaun , Stix John
Published: February 2008

The perimeter and area of basaltic lava flows and flow fields from Piton de la Fournaise (Indian Ocean), Mt Etna (Italy) and Puu Oo (Hawaii) obtained from geological maps have been demonstrated to be scaling (power law) functions of the resolution of geological maps. This fractal property shows that structures of all sizes are important. The fact that distinct fractal dimensions exist for different flows is consistent with the idea that, like the topography, the lava dynamics are multifractal.
The climate induced variation of the continental biosphere: A model simulation of the Last Glacial Maximum
Authors: Friedlingstein P. , Delire C. , Müller J. F. , Gérard J. C.
Published: February 2008

A simplified three‐dimensional global climate model was used to simulate the surface temperature and precipitation distributions for the Last Glacial Maximum (LGM), 18 000 years ago. These fields were applied to a bioclimatic scheme wich parameterizes the distribution of eight vegetation types as a function of biotemperature and annual precipitation. The model predicts a decrease, for LGM compared to present, in forested area balanced by an increase in desert and tundra extent, in agreement with a reconstruction of the distribution of vegetation based on paleodata. However, the estimated biospheric carbon content (phytomass and soil carbon) at LGM is less reduced than in the reconstructed one. Possible reasons for this discrepancy are discussed.
Hydrothermal 3He and manganese plumes at 19°29′S on the Central Indian Ridge
Authors: Jean‐Baptiste Philippe , Mantisi Frédérick , Pauwells Hélène , Grimaud Daniel , Patriat Philippe
Published: February 2008

We report the discovery of a hydrothermal helium‐3 plume north of the Indian Triple Junction, on the Central Indian Ridge at 19°29′S. Water column δ3He anomalies show a spike of 33.8% at 2460 meter depth, i.e. the highest δ3He anomaly recorded so far in the Indian ocean.

The helium‐3 data are correlated with manganese, giving a Mn/3He ratio of 700 mol/cm3 (3.8×104 g/cm3STP). The CH4/3He ratio deduced from available CH4/Mn data in the same area is around 7×106. These values are close to what is observed on water column plumes on the East Pacific Rise.

On a regional scale, the 3He distribution over the Indian Triple Junction exhibits a strong δ3He increase at the Ridge crest, which appears to spread eastwards through the deep Indian ocean.
Remote‐sensing observations of F‐region ion drift velocities using Dynamics Explorer‐2 Doppler measurements of the O+(2P) λ732.0 nm emission
Authors: Carr S. S. , Killeen T. L. , Coley W. Robin
Published: February 2008

Limb‐scan observations of Doppler line profiles from the O+(2P) λ732.0 nm emission at F‐region altitudes, made with the Fabry‐Perot interferometer (FPI) on the Dynamics Explorer‐2 (DE‐2) spacecraft, have been analyzed to provide measurements of the meridional component of the ion convection velocity along the instrument line‐of‐sight. The initial DE‐2 results presented here demonstrate the first spaceborne use of the remote‐sensing Doppler technique for measurements of ionospheric convection. The FPI meridional ion drift measurements have been compared with nearly simultaneous in‐situ ion drift measurements from the Retarding Potential Analyzer (RPA) on DE 2. Once allowance is made for the temporal lag between the in‐situ and remote measurements, the results from the two techniques are in good agreement. The results of a simulation study demonstrate that the spaceborne interferometric technique has future utility for 2‐D imaging of the quasi‐instantaneous ion convection pattern.
Unusually distant bow shock encounters at Venus
Authors: Russell C. T. , Zhang T.‐L.
Published: February 2008

When the magnetosonic Mach number is close to unity, the Venus bow shock appears to become very dynamic and is encountered far upstream of its expected location. This behavior suggests caution be used in interpreting distant bow shock locations as evidence for planetary magnetic fields when the Mach number of those distant crossings is unknown.
On the thermal structure of Triton's thermosphere
Authors: Stevens Michael H. , Strobel Darrell F. , Summers Michael E. , Yelle Roger V.
Published: February 2008

The analysis of the Voyager 2 Ultraviolet Spectrometer (UVS) solar occultation data obtained at Triton is consistent with a spherically symmetric, isothermal thermosphere above 400 km at T∞. = 96 K. A detailed calculation of energy loss processes in a pure N2 atmosphere, heating and cooling rates, and resultant thermal structure associated with solar UV irradiance and magnetospheric electron precipitation indicates that solar heating, with calculated T∞ = 70 K, is insufficient to account for the inferred T∞ = 96 K. The magnetosphere must deposit twice as much power as the sun (λ < 800 Å) to heat the thermosphere to 96 K and generate the observed N2 tangential column densities above 450 km. The thermal escape of H and N atoms and the downward diffusion of N atoms to recombine below 130 km results in local ionospheric heating efficiency of 24%. An upper limit on the tropopause CO mixing ratio of 2 × 10−4 is inferred in the absence of aerosol heating to balance its efficient cooling by LTE rotational line emission.
Katabatic wind intensity and direction over Antarctica derived from scatterometer data
Authors: Remy F. , Ledroit M. , Minster J. F.
Published: February 2008

Although satellite radar scatterometers are initially designed to measure ocean surface wind speed and direction, they also provide important observations over continental ice sheets. Data from the Seasat scatterometer analysed over a sector of Antarctica show that the signal is strongly dependent on the incidence angle of observation, but that it also depends on azimuth [Ledroit et al., 1992]. The minimum values of the backscatter coefficient (the ratio of backscattered versus incident power) are always observed in the direction of katabatic winds. A theoretical analysis shows that the backscatter coefficient must be very sensitive to the sastrugi slopes and orientations, which are streamlined features formed on the snow surface in the wind direction [Parish and Bromwich, 1987]. Satellite scatterometers can thus map the characteristics of sastrugis, of importance for the interpretation of microwave measurements above ice sheets; this indirectly provides the intensity and direction of these very persistent and strong winds, which play a important role in the behaviour of the atmosphere and ocean in high southern latitudes.
A replacement for the Coulomb‐Mohr Fracture Criterion
Authors: Wei Kaihong , De Bremaecker Jean‐Claude
Published: February 2008

It has long been known that the Coulomb‐Mohr criterion does not satisfactorily account for the direction of faults and fractures in the Earth. Modern numerical methods now permit the use of a better criterion, i.e., that fracture initiates and propagates in the direction in which the strain energy release is maximum for a given crack length. This criterion is thermodynamically correct, can be used for heterogeneous and elasto‐plastic bodies, and yields results in good agreement with laboratory data, but it is somewhat computer intensive.
Spatial and temporal seismic energy release in the Efate Region (Central New Hebrides Island Arc): Evidence for buckling?
Authors: Chatelain Jean ‐ Luc , Grasso Jean ‐ Robert
Published: February 2008

A study of the spatial and temporal distributions of the seismicity in the central New Hebrides island arc shows that the Efate region (including the interplate boundary and the upper plate) can be considered as a single block under stress. Repeated activation of very specific zones within this block as indicated by seismic clusters is temporally correlated with larger than average energy release rates. These relative high rates are associated with main shocks with magnitude (Ms, mb) > 5.0 in the upper part of the interplate boundary and indicate that slip at the plate boundary is not steady. No correlation is found with main shocks located in the lower part of the interplate boundary, where slip occurs more freely. We propose that these observations are consistent with a buckling process with a wavelength of about 170 km within the overriding plate. Buckling or bending of the upper plate may be due to the impingement of a seamount on the subduction system, elevating the stress level of the Efate region, which behaves like a single mechanical entity.
High heat flow anomalies over salt structures on the Texas Continental Slope, Gulf of Mexico
Authors: Nagihara Seiichi , Sclater John G. , Beckley Lila M. , Behrens E. William , Lawver Lawrence A.
Published: February 2008

We made 74 closely spaced (< 2 km apart) heat flow measurements around and over two salt structures on the Texas continental slope, Gulf of Mexico. The values outlined the shape of the heat flow anomalies over both structures. Based on a preceding high resolution seismic survey, we interpreted these structures to be a cylindrical plug and a salt tongue extending from the crest of a wall‐shaped feeder. The heat flow observations clearly reflect differences between the two features and are consistent with the prior structural interpretation. The values over the salt plug are nearly all greater than 70 mW/m². The measurements over the salt tongue have a sharp heat flow peak of 90 mW/m² associated with the presumed feeder and rather uniform values around 60 mW/m² over the remainder. The variation of heat flow over both structures is smooth and shows no apparent scatter. Heat flow values off these features are uniformly low, around 30 mW/m². Thermal effects from bottom water temperature fluctuation, slope sedimentation, diapiric movement of the salt body, and pore fluid migration appear unable to provide a satisfactory explanation for the observations. However, thickness variations of a highly conductive salt body can easily account for the heat flow anomalies. We suggest that modeling of the conductive anomaly should provide substantial constraints on the bottom geometry of the salt.
On the relative intercalibration of solar wind instruments on IMP‐8 and ISEE‐3
Authors: Russell C. T. , Petrinec S. M.
Published: February 2008

Simultaneous measurements of density on IMP‐8 and ISEE‐3 are intercompared as a function of velocity and ion temperature. Systematic differences are found of a magnitude sufficiently large to be important in studies of the solar wind interaction with the magnetosphere.
Electron acceleration by low frequency electric field fluctuations: Electron conics
Authors: André Mats , Eliasson Lars
Published: February 2008

Electron conics can be generated by fluctuations at roughly one Hz in a parallel electric field accelerating auroral electrons at altitudes of several thousand kilometers. The results from our simulations including such time‐varying electric fields are in good agreement with observed electron distributions. The simulation parameters are consistent with the ion beams and broadband waves associated with electron conics. Our calculations are used to predict for example oscillations in the low altitude electron fluxes. These results can be used to compare different possible mechanisms for the generation of electron conics.
A global model of the substorm current system
Authors: Kan J. R. , Sun W. , Akasofu S.‐I. , Iijima T. , Potemra T. A.
Published: February 2008

Based on the ionospheric currents determined from ground‐based magnetometer data, we propose a global substorm current system. The proposed substorm current system is composed of (a) the region‐1 and −2 currents, (b) the growth‐phase current loops, (c) the substorm current wedge. The growth‐phase current loops are initiated during the growth phase due to reconfiguration of the magnetosphere driven by enhanced magnetospheric convection. The substorm current wedge can be initiated by the expansion onset due to the ionospheric response to the enhanced magneto spheric convection. During the growth phase, the current system consists of currents in (a) and (b), During the expansion phase, the substorm current system consists of currents in (a), (b) and (c). Energy dissipated during the substorm expansion phase is supplied by both the directly‐driven process and the unloading process. This is supported by the observations that depolarization occurs inside the current wedge, while thinning proceeds on the dawn and dusk sides of the current wedge. The ratio of the unloading current to the directly‐driven current is ∼2 during the expansion phase of a substorm.
Low yields of SO2 from dimethyl sulfide oxidation in the marine boundary layer
Authors: Bandy A. R. , Scott D. L. , Blomquist B. W. , Chen S. M. , Thornton D. C.
Published: February 2008

Sulfur dioxide and dimethyl sulfide were determined in the marine boundary layer of the northeast Pacific Ocean west of Seattle, Washington. The mean DMS and SO2 concentrations were 75 and 28 pptv, respectively. During periods of high DMS levels (180 pptv) we found that SO2 levels remained low (25 pptv) and statistically the same as periods of low DMS. Sulfur dioxide showed no observable diurnal variation indicating that nonphotochemically driven losses to aerosol and other surfaces cannot explain the low SO2 levels observed. We conclude that a low efficiency of conversion of DMS to SO2 is the most likely explanation for the low SO2 levels. Implications of the low yield of SO2 in terms of the kinetics of oxidation of DMS are discussed.
Stability of (Mg,Fe)SiO3 perovskite and the structure of the lowermost mantle
Authors: Stixrude Lars , Bukowinski M. S. T.
Published: February 2008

Thermodynamic analysis shows that (Mg,Fe)SiO3 perovskite is stable throughout the likely pressure, temperature and compositional regime of the Earth's mantle. The breakdown of perovskite to its constituent oxides appears unlikely, even under the extreme conditions of the core‐mantle boundary. This reaction had been proposed to reconcile estimates of silicate melting with seismic observation and proposed geotherms.
Conventional spherical harmonic analysis for regional modelling of the geomagnetic field
Authors: De Santis Angelo
Published: February 2008

Spherical Harmonic Analysis (SHA) is normally used to model the three‐dimensional global geomagnetic field. To address the same problem in regional modelling, Haines (1985) proposed Spherical Cap Harmonic Analysis (SCHA). This regional technique involves the computation of more complex Legendre functions with real (generally non‐integer) harmonic degree. Here a new more practical technique is described; it is called Adjusted Spherical Harmonic Analysis (ASHA) because it is based on the expansion of conventional spherical harmonics after the colatitude interval is adjusted to that of a hemisphere. This kind of analysis can also be applied to modelling general two‐dimensional functions.
Density and field structure of a FTE observed in the magnetosphere
Authors: Hubert D. , Lacombe C. , Harvey C. C. , Mangeney A. , Russell C. T.
Published: February 2008

ISEE1 and 2 high resolution electron density and magnetic field measurements have been used to study characteristic structures, identified as FTEs, observed inside the magnetosphere close to the magnetopause. We investigate new properties: a volcano‐like signature in the magnetic field strength, a thin sheet structure at the leading edge, and a wake. The plasma compressibility is analysed through the structure. Auto and cross spectral analysis of the data reveal the signature of compressive modes in the wake of the FTE.
Spontaneous processes for nucleation, dynamic propagation, and stop of earthquake rupture
Authors: Shibazaki Bunichiro , Matsu'ura Mitsuhiro
Published: February 2008

We developed a model which can naturally explain the entire process of nucleation, dynamic propagation, and stop of earthquake rupture. In this model we represent frictional interaction between fault surfaces by a slip‐dependent constitutive relation, and use it as the fundamental law governing the entire process of earthquake rupture. We considered a broad weak zone with a locally strong part (asperity) on a fault plane and examined the entire rupture process proceeding with the increase of external shear stress through numerical simulations. The nucleation process first proceeds quasi‐statically at the weakest portion with the increase of external stress and brings about high stress concentration at the asperity. Then dynamic rupture starts at the asperity, but this dynamic rupture is arrested soon. After the arrest of the local dynamic rupture, the quasi‐static nucleation process proceeds again and brings about high stress concentration at both ends of the weak zone. When the stress concentration reaches a critical level, dynamic rupture starts at the endzones and propagates outward. The rupture propagation is gradually accelerated toS‐wave velocity until it reaches much stronger parts (barriers). The dynamic rupture is decelerated and finally arrested, with it propagating into the barrier. Our model successfully describes the transition process from nucleation to dynamic rupture propagation observed in laboratory experiments of stick‐slip.
First observations of super fast auroral waves
Authors: Hough Gavin , Kosch Michael J. , Scourfield Malcolm J. W.
Published: February 2008

Super fast auroral waves, propagating equatorward with speeds of up to 1200 km/s over distances exceeding 1400 km, have been observed at Sanae (L=4) Antarctica by an all sky low light level TV system. Such speeds greatly exceed those previously reported for fast auroral waves, or of any other auroral form, hence the description superfast. The altitude and speed of the waves have been determined using a novel technique that requires only one point of observation. Baseline triangulation from multiple ground based observations can be avoided because the earth is not flat.
Ionic mechanisms for heterogeneous stratospheric reactions and ultraviolet photoabsorption cross sections for NO2+, HNO3, AND NO3− in sulfuric acid
Authors: Burley Joel D. , Johnston H. S.
Published: February 2008

We present room temperature photo‐absorption cross sections between 180 and 340 nm for potassium nitrate dissolved in sulfuric acid‐water solutions of 0, 80, and 96% sulfuric acid by mass. Other investigators have obtained ultraviolet absorption spectra for similar solutions above 220 nm, and there is a large literature on the spectra, species, and nitration reactions in sulfuric acid solutions. The predominant nitrogen‐containing species are the nitrate anion (NO3−) in pure water or dilute sulfuric acid solutions, molecular nitric acid (HNO3) in 80% sulfuric acid, and the nitronium ion (NO2+) in 96% sulfuric acid. Upon reviewing the literature involving these species, we consider elementary ionic and molecular reactions in mechanisms of stratospheric heterogeneous catalysis. We formulate a general mechanism for acid catalyzed reactions, with examples that convert inactive HCl and ClNO3 into photochemically active ClNO2, ClNO, Cl2, HOCl, and ClO, and we show that reactions forming all of these products are thermodynamically allowed.
Determination of the relative ozone and PAN deposition velocities at night
Authors: Shepson P. B. , Bottenheim J. W. , Hastie D. R. , Venkatram A.
Published: February 2008

A series of measurements of PAN and ozone was conducted during summer at three rural sites in Canada: Egbert and Dorset, Ontario, and Kejimkujik, Nova Scotia. For nights when a stable surface inversion layer forms, ozone and PAN at the surface are found to undergo first order decay, assumed to be due only to dry deposition. Analysis of the measurement data leads to determination of the relative dry deposition velocities. For all three sites, we find that Vd(O3)/Vd(PAN) = 0.42±0.19, at night. This ratio is roughly a factor of 5–6 times smaller than previously assumed. This smaller relative deposition velocity ratio can have a significant impact on model estimations of PAN concentrations near the surface. We estimate that for these sites, the PAN deposition velocity is at least 0.5 cm/s, and may be greater during daytime. This can have a significant impact on the tropospheric lifetime of PAN.
Variations in surface ozone trends over Europe
Authors: Low P. S. , Kelly P. M. , Davies T. D.
Published: February 2008

Surface ozone data from 20 European stations of differing character (remote, rural, suburban and urban) were analysed for linear trends over a common period of 1978–1988. Pronounced annual and seasonal variations are apparent in the trends in different areas. Relatively few of the trends are statistically significant and there is no dominant region‐wide trend. However, there does appear to be some commonality of behaviour on a subregional scale as positive or negative trends tend to be clustered together in some areas. Stations on the maritime fringe of the network generally exhibit negative trends in summer average concentrations, whilst those located further into the continental interior exhibit positive trends. Most of the stations in the network exhibit positive trends in the winter averages. The short‐term variability evident in this study reflects the complex controls affecting surface ozone concentration and suggests that it may be misleading to draw conclusions concerning long‐term trends from records of limited duration.
Moment tensor inversion of induced microseisnmic events: Evidence of non‐shear failures in the −4 < M < −2 moment magnitude range
Authors: Feignier Bruno , Young R. Paul
Published: February 2008

To assess objectively the source mechanism of mining‐induced seismic events, we developed an algorithm to invert for the moment tensor components. From this inversion, the ratio of isotropic/deviatoric component in the source is obtained. We are applying this method to microseismic events (−4<M<−2) recorded at the Underground Research Laboratory (URL) in Pinawa, Manitoba, where a tunnel is being excavated at 420 m depth. To monitor the seismic activity induced by this opening an array of 16 triaxial accelerometers was installed around the planned tunnel. Results from the moment tensor inversion on a fist data set provide evidence of non‐shear failure mechanisms. Furthermore, the location of events displaying a tensile component corresponds to a breakout observed underground in the roof of the tunnel.
Shear‐wave polarization alignments recorded above the Kaoiki Fault Zone, Hawaii
Authors: Booth David C. , Wyss Max , Gillard Dominique
Published: February 2008

Eighty‐four shear wave records from the seismometer station AIN at Kaoiki, Hawaii, covering the years 1979–1983, were examined for the presence of shear wave splitting and possible precursory change associated with a mainshock in 1983. Time‐delayed arrivals with different polarizations were detected, suggestive of shear‐wave splitting, and the polarizations of the first‐arriving shear waves were aligned in the direction ESE‐WNW. This direction is consistent with the anisotropy being caused by cracks aligned approximately perpendicular to the direction of least principal stress, which is uniform over the ray path in an otherwise heterogeneous stress field. No time‐dependent variation of the polarization of the leading split shear‐wave was observed. Investigation of temporal variation of time delays between split shear‐waves was not possible due to the presence of scattered arrivals.
Degrees 2, 4, 6 inferred from seismic tomography
Authors: Montagner Jean‐Paul , Romanowicz Barbara
Published: February 2008

Lateral heterogneities of geophysical fields (gravity, magnetic, seismic velocities...) are usually expended into spherical harmonics. Free oscillation and geoid data show that heterogeneity in the deep Earth's mantle is dominated by degree 2. However, its geographical pattern and its location at depth is still questionable. Recent tomographic models of seismic velocity, anisotropy and anelasticity obtained from GEOSCOPE and GDSN data, make it possible to gain more insight on this problem. They show that at depth larger than 400km ± 100km, the degree 2 and t a less extent, the degree 6 arise as the most important features. These new models also display a large degree 4 for radial anisotropy at large depths. A simple flow pattern can explain the predominance of these different degrees 2, 4, 6 and it is shown that degrees 2, 4, 6 in the transition zone are not independent. They are compared to the corresponding degrees of the hotspot distribution and geoid. A possible relation between the degree 2 of velocity and the degree 4 of radial anisotropy is also displayed. However, the different location of degree 2 in the upper mantle and the lower mantle is still puzzling.
Constraints on mantle viscosity from relative sea level variations in Hudson Bay
Authors: Mitrovica J. X. , Peltier W. R.
Published: February 2008

A number of previous analyses have suggested that the relative sea level record in Hudson Bay constrains the viscosity of the lower mantle to be near 1021 Pa s. These conclusions have been based on a limited number of forward calculations which have assumed an isoviscous lower mantle region. We compute Frechet kernels for the RSL data in Hudson Bay to determine the detailed depth‐dependent sensitivity of the data to variations in a viscosity profile which is consistent with the past inferences. We find that the RSL data provide a robust constraint on the average viscosity in the top half of the lower mantle only (the average must be near 1021 Pa s). The data are less sensitive to variations in the viscosity below approximately 1800 km depth and in the upper mantle. Forward analyses included herein show that the data admit models whose average viscosity in the deep mantle (below 1800 km depth) and in the upper mantle can differ significantly from the value (near 1021 Pa s) estimated for the top half of the lower mantle. The total variation in the viscosity from the surface to the CMB can exceed an order of magnitude or more and still satisfy the constraint provided by the Hudson Bay RSL data set. The necessity of invoking an isoviscous mantle model in previous studies is thus seen to be a consequence of the limited class of viscosity model solutions employed in those studies.
Geoelectric field measurements on a planetary scale: Oceanographic and geophysical applications
Authors: Chave A. D. , Luther D. S. , Lanzerotti L. J. , Medford L. V.
Published: February 2008

The characteristics of an initial year of electric field measurements obtained from a long (≈3900 km), unpowered submarine cable extending from Point Arena, California, to Hanauma Bay, Hawaii are reported. The power spectrum estimated from the cable time series is similar to that from seafloor point electric field sensors at frequencies above 1 cpd, but differs at lower frequencies by decreasing monotonically down to at least 0.02 cpd. The multiple squared coherence between the cable data and three component geomagnetic fluctuations measured at Fresno, Ca., is high (γ²≈0.9) from 1.0‐0.09 cpd, then declines slowly with decreasing frequency. Maps of the squared coherence between the cable voltage and the surface air pressure, wind stress, and wind stress curl derived from the Fleet Numerical Oceanography Center product over the entire eastern North Pacific show no significant relationships at frequencies above ≈0.05 cpd, but strong nonlocal coherence with the wind stress curl forcing obtains at lower frequencies. Taken together, these observations support a transition from high frequency dominance by external sources to low frequency dominance by oceanic motional sources at 0.05‐0.1 cpd, a considerably smaller value than has previously been observed with point electric field sensors on the ocean bottom. The difference is caused by horizontal averaging of the short spatial scale motional component over the long submarine cable.
Source phase and amplitude anomalies of long‐period surface waves
Authors: Um Junho , Dahlen F. A.
Published: February 2008

In addition to the Fermat phase anomaly and the amplitude anomaly produced by geometric focusing and defocusing along the propagation path, long‐period surface waves on a laterally heterogeneous earth exhibit phase and amplitude variations due to the deviation in takeoff azimuth of the ray at the source. The resulting perturbation in source phase is generally small enough to be ignored; however, the source amplitude anomaly may be comparable to that produced by focusing and defocusing. Adding the source anomaly significantly reduces the variance of amplitude measurements performed on a large suite of synthetic vertical‐component coupled‐mode seismograms. Future tomographic analyses that incorporate amplitude measurements should account for this effect.
Precursory and coseismic anomalies in well water levels observed for the February 2, 1992 Tokyo Bay Earthquake
Authors: Igarashi George , Wakita Hiroshi , Sato Tsutomu
Published: February 2008

Possible precursor water level changes of a moderate earthquake were detected by the long‐term groundwater observation sites operated over 5 years in Japan. Three observation wells showed anomalous changes: rises and a fall in water levels of 3–10 cm which began simultaneously 1–1.5 days prior to the February 2, 1992 Tokyo Bay earthquake (M5.9). The distance from the hypocenter to the wells were 90–110 km. These possible precursors were followed by coseimic water level changes of 1–20 cm. Some portions of rises may be attributed to effects of concurrent precipitation, however, the water level fall of 10 cm observed at one well indicates that the change is anomalous and is highly likely to be an earthquake precursor.
An investigation into the causes of stratospheric ozone loss in the southern Australasian Region
Authors: Lehmann P. , Karoly D. J. , Newman P. A. , Clarkson T. S. , Matthews W. A.
Published: February 2008

Measurements of total ozone at Macquarie Island (55°S, 159°E) reveal statistically significant reductions of approximately twelve percent during July to September when comparing the mean levels for 1987‐90 with those in the seventies. In order to investigate the possibility that these ozone changes may not be a result of dynamic variability of the stratosphere, a simple linear model of ozone was created from statistical analysis of tropopause height and isentropic transient eddy heat flux, which were assumed representative of the dominant dynamic influences. Comparison of measured and modelled ozone indicates that the recent downward trend in ozone at Macquarie Island is not related to stratospheric dynamic variability and therefore suggests another mechanism, possibly changes in photochemical destruction of ozone.
Propagation of a large Forbush decrease in cosmic‐ray intensity past the Earth, Pioneer 11 at 34 AU, and Pioneer 10 at 53 AU
Authors: Van Allen James A. , Fillius R. Walker
Published: February 2008

During 1991, large and distinctive Forbush decreases in cosmic‐ray intensity were observed at Pioneer 11 at 34 AU on 21 August and at Pioneer 10 at 53 AU on 30 September. An apparently related shock passed Voyager 1 at 46 AU on 14 September. These three events are plausibly associated with each other and with a large Forbush decrease at the Earth on 12 June. The apparent radial speed of propagation was about 820 km s−1, independent of radial distance; and the decrease of cosmic‐ray intensity was of similar magnitude (≈ 20 percent) at Earth, Pioneer 11, and Pioneer 10, i.e., over a radial range 1 to 53 AU and over a range of heliocentric ecliptic longitude at least as great as 173°. For over a month following passage of the interplanetary shock by Pioneer 11, detectable intensities of locally accelerated protons of Ep ≈ 1 MeV were present. The causative solar flare for the entire family of apparently related events was identified with reasonable certainty as a 3B/X 12.0 one in active region 6659 on 11 June at W17 N31. These new observations together with comparable ones reported previously provide the basis for a new quantitative model of the propagation of Forbush decreases in the outer heliosphere, described by RWF in a paper submitted to the Journal of Geophysical Research.
Missing pressure in the dayside ionosphere of Venus
Authors: Cloutier P. A. , Stewart B. K. , Taylor H. A.
Published: February 2008

Data obtained by various instruments on the Pioneer‐Venus spacecraft were used to study the conservation of momentum flux from the solar wind through the dayside ionopause into the thermal Venus ionosphere. A consistent pressure deficit was found below the ionopause, with a strong dependence on solar wind pressure. Independent of solar wind pressure, the pressure deficit was found to decrease with decreasing altitude below the ionopause. Measurements of this pressure deficit (missing pressure) are presented as a function of altitude for various solar wind conditions. The identity of the missing pressure component and the correlation with solar wind pressure are discussed.
PcP and ScP beneath the western Pacific detected by a deep ocean‐Bottom hydrophone array
Authors: Nagumo Shozaburo , McCreery Charles S.
Published: February 2008

A deep ocean‐bottom hydrophone array near Wake Island in the western Pacific has detected PcP and ScP from several deep‐focus earthquakes that occurred beneath the Sea of Okhotsk and beneath the Fiji Basin. Two representative events have been analyzed. The PcP of the Okhotsk event is very strong and sharp, with a predominant frequency of 2 Hz, whereas the PcP of the Fiji event is weak, with a predominant frequency closer to 1 Hz. The strong contrast between PcP from these two deep‐focus earthquakes demonstrates differences in reflectivity at the core‐mantle boundary as well as heterogeneities in seismic velocity and attenuation in the lower mantle. Travel times and spectral analyses show that seismic velocity, attenuation factor Q, and reflectivity are all high beneath the Northwest Pacific Basin and normal beneath the equatorial Pacific.
Electromagnetic images of regional structure in the southern Canadian Cordillera
Authors: Jones Alan G. , Gough D. Ian , Kurtz Ron D. , DeLaurier Jon M. , Boerner David E. , Craven James A. , Ellis Rob G. , McNeice Gary W.
Published: February 2008

As part of Lithoprobe's Southern Cordilleran transect investigations, magnetotelluric (MT) soundings were made at 160 sites providing unprecedented coverage from the Rockies to the west coast. Striking lateral variation, which spatially correlates with the morphogeological belt boundaries, is apparent at periods sensing the lower crust (≈10 s). For the Rockies, MT phases are around 35°, indicative of a moderately resistive (100's – 1000's Ω·m) North American Basement. Foreland belt phases are transitional and increase from 60° in the east to 70° in the west. Omineca and Coast belt phases are high (75°), implying a conductive (10–30 Ω·m) lower crust, whereas Intermontane belt phases are more than 10° lower (equivalent to ≈150 Ω·m). The regional variation in conductivity correlates to first order with surface heat flow changes along the profile and is also correlative with coincident seismic reflection sections in some aspects.
Magnetic susceptibility cyclicity and magnetic dissolution in Cretaceous limestones of the southern Alps (Italy)
Authors: Tarduno John A.
Published: February 2008

Sampling of a reversed polarity interval previously reported from limestones of the “Albian” Valle del Mis section (Southern Alps, Italy) indicates that it represents a recording of chron M3 of Barremian age rather than a brief reversed polarity ‘event’ in the Cretaceous Normal Polarity Superchron (K‐N). Hysteresis measurements suggest that magnetic susceptibility in these limestones is dominated by the paramagnetic clay fraction. Bulk magnetic susceptibility values exhibit a cyclicity which is correlated with the 100‐kyr Milankovitch eccentricity cycle. The stable record of reversed polarity is interrupted at six stratigraphically narrow intervals (< 1 m) by the apparent lack of any mineral having a stable remanent magnetization. I suggest these “paleomagnetic barren” intervals mark selective dissolution of magnetite related to increases in organic carbon. Such dissolution may explain some patterns of magnetization and re‐magnetization observed in world‐wide carbonates.
Precise determination of Earth's center of mass using measurements from the global positioning system
Authors: Vigue Yvonne , Lichten Stephen M. , Blewitt Geoffrey , Heflin Michael B. , Malla Rajendra P.
Published: February 2008

Global Positioning System (GPS) data from a worldwide geodetic experiment were collected during a 3 week period early in 1991. We estimated geocentric station coordinates using the GPS data, thus defining a dynamically determined reference frame origin which should coincide with the Earth center of mass, or geocenter. The 3‐week GPS average geocenter estimates agree to 7–13 cm with geocenter estimates determined from satellite laser ranging, a well‐established technique. The RMS of daily GPS geocenter estimates were 4 cm for x and y, and 30 cm for z.
Climatic variability of temperature and humidity over the tropical western Pacific
Authors: Gutzler David S.
Published: February 2008

The vertical structures of recent decadal‐scale trends of temperature and humidity over the tropical western Pacific Ocean are documented using daily data at 16 pressure levels (from the surface to 300 mbar) from four radiosonde stations. Both temperature and specific humidity have increased throughout the troposphere since the mid‐1970 's. The specific humidity increase is sufficiently large that relative humidity also shows positive trends at all tropospheric levels above the surface despite the warming trend. The specific humidity trend is greatest near the surface and decreases with height, whereas the temperature trend increases with height above the boundary layer. Specific humidity in the tropics is shown to be a sensitive indicator of long‐term climate change, because the vertical structure of the specific humidity trend (largest near the surface) is distinctly different from the vertical structure of the seasonal cycle or interannual variability of specific humidity variability, which exhibit peak amplitudes in mid‐troposphere. Thus short‐term climate variability of humidity is a poor proxy for long‐term climate change. Reproduction of the difference in vertical structures of long‐term and short‐term specific humidity fluctuations would provide a good test of the fidelity of model projections of climate change.
Temperature anisotropies in a magnetospheric FTE
Authors: Smith M. F. , Owen C. J.
Published: February 2008

We present AMPTE UKS data from a well‐studied magnetospheric flux transfer event, showing detailed ion phase‐space distributions for each region of this layered event. We show that the perpendicular temperature anisotropy maximizes at the center of the event. This is inconsistent with recent suggestions that FTE signatures may result from the spacecraft moving into and then out of the magnetosheath via the plasma depletion layer. We present an explanation for the temperature anisotropy structure in terms of a reconnection model.
Magma chamber deflation recorded by the global positioning system: The Hekla 1991 Eruption
Authors: Sigmundsson Freysteinn , Einarsson Páll , Bilham Roger
Published: February 2008

Between January 17 and March 11, 1991, 0.15 km3 of lava erupted initially from several radial fissures and subsequently from a single fissure on the SE flank of Hekla volcano, Iceland. Hekla is surrounded by an array of control points measured in 1989 using GPS geodesy and re‐measured after the eruption. These measurements indicate that the eruption was associated with a surface deflation volume of

centered on Hekla

. The depth to the magma reservoir is
km, poorly constrained due to the absence of GPS control points close to the volcano.
Uplift in the Nepal Himalaya revealed by Spirit leveling
Authors: Jackson Michael , Barrientos Sergio , Bilham Roger , Kyestha D. , Shrestha Buddhi
Published: February 2008

A 250‐km‐long first‐order leveling line linking the geodetic surveys of Tibet and India through the Kingdom of Nepal has been measured twice in the past 15 years. Changes in relative bench mark elevation projected along a 200 km line orthogonal to the mean structural trend in the Himalaya indicate a pattern of uplift with wavelengths of 30–50 km not related to earthquake occurrence. Relative vertical velocities of bench marks of up to 4 mm/year show no evident correlation with level line slope. An absence of microearthquake data prevents localized uplift from being attributed to slip on mapped or buried faults, although circumstantial evidence suggests this is likely. In the North, in line with the high Himalayan peaks, relative uplift rates of 4 ± 1 mm/yr and a wavelength of 50 km are coincident with a steepening in the Main Boundary Thrust and the Moho proposed by Lyon‐Caen and Molnar [1985]. In the South, uplift rates of 2 ± 0.3 mm/yr with a wavelength of 30 km correspond to active folding of an anticline on the southern flank of the Lesser Himalaya. Observed tilt rates of 0.06 μrad/year are similar to the long term tilt rates needed to produce the 40–70° flanks of the anticline. A range of detachment geometries and slip velocities can generate the observed deformation field, but geologically realistic models fall into a group of horizontal to shallow dipping, sub‐horizontal faults between 12 and 6 km in depth. No unique solution exists, although the wavelength of the observed deformation field permits models with significant slip closer than 4 km to the surface to be excluded.
A gravity current model for the May 18, 1980 Mount St. Helens plume
Authors: Bursik M. I. , Carey S. N. , Sparks R. S. J.
Published: February 2008

Observations of the stratospheric plume from the May 18, 1980, Mount St. Helens eruption suggest that it spread in the crosswind direction as an intrusive gravity current, as it was transported downwind. Grain size analyses of the plinian tephra are consistent with this model, suggesting that to distances of many hundreds of kilometers, turbulent atmospheric diffusion played a secondary role in plume spreading and tephra dispersal.
Droughts in river basins of the western United States
Authors: Loaiciga Hugo A. , Michaelsen Joel , Garver Sara , Haston Laura , Leipnik Roy B.
Published: February 2008

Tree‐ring reconstructions of centuries‐long streamflows in four river basins of western United States were used to assess the severity, frequency and distribution of duration of droughts in the basins, and the probability of extreme drought in well‐populated areas of California and Arizona. Our results show (i) greater long‐drought threat than that inferred from historical streamflow records; (ii) weak statistical dependence of streamflows across the river basins; and (iii) significant probability of droughts in individual river basins.
z−3/2 powerlaw decay of Laplacian fields induced by disorder: Consequences for the inverse problem
Authors: Sornette Didier
Published: February 2008

We consider a Laplacian field (▽2V=0) in a semi‐infinite medium bounded by a frontier ∂B, with imposed boundary values or sources at ∂B. When the boundary conditions are periodically distributed, it is well‐known that the Laplacian field V decays exponentially due to screening of all multipoles. It is demonstrated in this paper that disorder is a singular perturbation in the sense that small random fluctuations around a periodic modulation, which are always present in nature, lead to long‐range powerlaw decreasing fluctuations of the Laplacian field V. Powerlaw decay is due to the appearance of Fourier components of arbitrary large wavelengths in the boundary conditions. This result calls for a reexamination of various geophysical inverse problems (such as temperature, electrical, gravity and magnetic anomalies), in which any possible small disorder on the sources is traditionally neglected, thereby filtering out the very low wavenumbers. This singular effect of disorder must apply to more general partial differential operators as long as the Green function is long‐range.