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Nettles Meredith, Dziewoski Adam M, . (2008). Radially anisotropic shear velocity structure of the upper mantle globally and beneath North America
. J. Geophys. Res., 113(B2), B02303–.
Keywords: North America, seismic velocity, anisotropy, 7208 Seismology: Mantle, 7218 Seismology: Lithosphere, 7270 Seismology: Tomography, 7255 Seismology: Surface waves and free oscillations,
Programme: 133
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Tocheport, A.; Rivera, L.; Chevrot, S. (2007). A systematic study of source time functions and moment tensors of intermediate and deep earthquakes. J. Geophys. Res., 112.
Abstract: We developed an inversion algorithm to determine the Source Time Function and Moment Tensor of intermediate and deep earthquakes from a set of teleseismic body wave records. The method proceeds in two stages. First, a nonlinear inversion by simulated annealing is performed to simultaneously (a) align the waveforms in time, (b) determine a common source time function, and (c) measure a collection of observed station amplitudes. The station amplitudes are then used, in a second stage, as secondary observables to invert for the moment tensors. We perform three different inversions to estimate the complete moment tensor solution, the pure deviatoric solution, and the double-couple solution. All the calculations are extremely simple and, in particular, it is not necessary to compute synthetic seismograms. The method requires well-isolated phases at different stations, which restricts its application to intermediate and deep events. The algorithm is applied to the FDSN broadband records corresponding to the period 1990–2005 of worldwide intermediate and deep seismicity (depth $\ges$100 km) of magnitude greater than 6.5. The source time functions are compared to those obtained from other studies of intermediate and deep events. The moment tensors are also systematically compared to the Harvard-CMT and USGS solutions. Both show a very good agreement with previous studies.
Keywords: Deep earthquakes; body waves inversion; source parameters; 7203 Seismology: Body waves; 7215 Seismology: Earthquake source observations; 7209 Seismology: Earthquake dynamics; 8120 Tectonophysics: Dynamics of lithosphere and mantle: general
Programme: 133
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Merrer, S.; Cara, M.; Rivera, L.; Ritsema, J. (2007). Upper mantle structure beneath continents: New constraints from multi-mode Rayleigh wave data in western North America and southern Africa. Geophysical research letters, 34.
Abstract: We estimate the averaged 1-D shear-wave velocity of the upper mantle beneath western North America and the Kaapvaal region in southern Africa by inverting dispersion measurements of fundamental and higher Rayleigh modes recorded by ?2000 km aperture broadband arrays. The overtones at periods exceeding 25 s constrain the averaged 1-D shear-wave velocity to 650 km depth across the regional arrays. Our overtone analysis confirms the shear-wave velocity differences observed in global tomographic models with similar horizontal resolution: the western North American mantle features a prominent low velocity zone at depths 50–200 km, while the shear velocity in the upper 180–200 km of the mantle beneath southern Africa is at least 6% higher than in western North America which we interpret as the expression of a cratonic keel. There is no resolvable difference in shear-wave velocity between southern Africa and western North America below a depth of about 300 km.
Keywords: Rayleigh waves; upper mantle; 7208 Seismology: Mantle; 7255 Seismology: Surface waves and free oscillations; 7290 Seismology: Computational seismology
Programme: 133;906
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Ciais, P.; Manning, A.C.; Reichstein, M.; Zaehle, S.; Bopp, L. (2007). Nitrification amplifies the decreasing trends of atmospheric oxygen and implies a larger land carbon uptake. Global biogeochemical cycles, 21.
Abstract: Atmospheric O2 trend measurements are used to partition global oceanic and land biotic carbon sinks on a multiannual basis. The underlying principle is that a terrestrial uptake or release of CO2 is accompanied by an opposite flux of O2. The molar ratio of the CO2 and O2 terrestrial fluxes should be 1, if no other elements are considered. However, reactive nitrogen produced by human activities (e.g., fertilizers, N deposition) is also being incorporated into plant tissues. The various reaction pathways of the terrestrial nitrogen cycle cause fluxes of atmospheric O2. Thus the cycles of nitrogen, carbon, and oxygen must be linked together. We report here on previously unconsidered anthropogenic nitrogen-related mechanisms which impact atmospheric O2 trends and thus the derived global carbon sinks. In particular, we speculate that anthropogenic-driven changes are driving the global nitrogen cycle to a more oxidized state, primarily through nitrification, nitrate fertilizer industrial production, and combustion of fossil fuels and anthropogenic biomass burning. The sum of these nitrogen-related processes acts to additionally decrease atmospheric O2 and slightly increase atmospheric CO2. We have calculated that the effective land biotic O2:CO2 molar ratio ranges between 0.76 and 1.04 rather than 1.10 (moles of O2 produced per mole of CO2 consumed) over the period 1993–2003, depending on which of four contrasting nitrogen oxidation and reduction pathway scenarios is used. Using the scenario in which we have most confidence, this implies a 0.23 PgC yr?1 correction to the global land biotic and oceanic carbon sinks of most recently reported estimates over 1993–2003, with the land biotic sink becoming larger and the oceanic sink smaller. We have attributed large uncertainties of 100% to all nitrogen-related O2 and CO2 fluxes and this corresponds up to ±0.09 PgC yr?1 increase in global carbon sink uncertainties. Thus accounting for anthropogenic nitrogen-related terrestrial fluxes of O2 results in a 45% larger land biotic sink of 0.74 ± 0.78 PgC yr?1 and a slightly smaller oceanic sink of 2.01 ± 0.66 PgC yr?1 for the decade 1993–2003.
Keywords: carbon cycle; nitrogen cycle; oxygen; 0330 Atmospheric Composition and Structure: Geochemical cycles
Programme: 439
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Tomasi, C.; Petkov, B.; Benedetti, E.; Vitale, V.; Pellegrini, A.; Dargaud, G.; De Silvestri, L.; Grigioni, P.; Fossat, E.; Roth, W.L.; Valenziano, L. (2006). Characterization of the atmospheric temperature and moisture conditions above Dome C (Antarctica) during austral summer and fall months. J. Geophys. Res., 111.
Abstract: Two sets of radiosounding measurements were taken at Dome C (Antarctica) in December 2003 and January 2003 and 2004, using RS80-A, RS80-H, and RS90 Vaisala radiosondes, and from March to May 2005, employing the RS92 model. They were examined following accurate correction procedures to remove the main relative humidity dry bias and the temperature and humidity lag errors. The results showed that a strong cooling usually characterizes the thermal conditions of the whole troposphere from December/January to April/May, with an average temperature decrease from 245 to 220 K at the ground, of around 10 K at upper tropospheric levels, and of more than 15 K in the lower stratosphere. The relative humidity data were found to be affected by dry bias of 5–10%, on average, for the RS80-A and RS80-H Humicap sensors and by smaller percentages for the other sensors. The mean monthly vertical profiles of absolute humidity were found to decrease sharply throughout the troposphere, especially within the first 3 km, and to diminish considerably passing from December/January to March/April/May, with average values of precipitable water decreasing from 0.75 to 0.28 mm, median values from 0.69 to 0.25 mm, and first and third quartiles from 0.60 to 0.22 mm and from 0.87 to 0.34 mm, respectively. The results demonstrate that Dome C (where a permanent scientific station has been open for winter operations since austral winter 2005) is a site of comparable quality to the South Pole for both validation of satellite radiance measurements and astronomic observations in the infrared, submillimetric, and millimetric wavelength range, performed with large telescopes that cannot be carried on satellites.
Keywords: radiosounding measurements; precipitable water; Antarctic atmosphere; 3349 Atmospheric Processes: Polar meteorology; 1655 Global Change: Water cycles; 0365 Atmospheric Composition and Structure: Troposphere: composition and chemistry; 7539 Solar Physics, Astrophysics, and Astronomy: Stellar astronomy
Programme: 908
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Rivier, L.; Ciais, P.; Hauglustaine, D.A.; Bakwin, P.; Bousquet, P.; Peylin, P.; Klonecki, A. (2006). Evaluation of SF6, C2Cl4, and CO to approximate fossil fuel CO2 in the Northern Hemisphere using a chemistry transport model. J. Geophys. Res., 111.
Abstract: The distribution of the fossil fuel component in atmospheric CO2 cannot be measured directly at a cheap cost. Could anthropogenic tracers with source patterns similar to fossil fuel CO2 then be used for that purpose? Here we present and evaluate a methodology using surrogate tracers, CO, SF6, and C2Cl4, to deduce fossil fuel CO2. A three-dimensional atmospheric chemistry transport model is used to simulate the relationship between each tracer and fossil fuel CO2. In summertime the regression slopes between fossil fuel CO2 and surrogate tracers show large spatial variations for chemically active tracers (CO and C2Cl4), although C2Cl4 presents less scatter than CO. At two tall tower sites in the United States (WLEF, Wisconsin, and WITN, North Carolina), we found that in summertime the C2Cl4 (CO) versus fossil CO2 slope is on average up to 15% (25%) higher than in winter. We show that for C2Cl4 this seasonal variation is due to OH oxidation. For CO the seasonal variation is due to both chemistry and mixing with nonanthropogenic CO sources. In wintertime the three surrogate tracers SF6, C2Cl4, and CO are about equally as good indicators of the presence of fossil CO2. However, our model strongly underestimates the variability of SF6 at both towers, probably because of unaccounted for emissions. Hence poor knowledge of emission distribution hampers the use of SF6 as a surrogate tracer. From a practical point of view we recommend the use of C2Cl4 as a proxy of fossil CO2. We also recommend the use of tracers to separate fossil CO2. Despite the fact that the uncertainty on the regression slope is on the order of 30%, the tracer approach is likely to have less bias than when letting one model with one inventory emission map calculate the fossil CO2 distribution.
Keywords: fossil fuel proxy; SF 6; C 2 Cl 4; CO; carbon fluxes; emissions; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0368 Atmospheric Composition and Structure: Troposphere: constituent transport and chemistry; 0365 Atmospheric Composition and Structure: Troposphere: composition and chemistry; 0345 Atmospheric Composition and Structure: Pollution: urban and regional
Programme: 439
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Thébault, E.; Mandea, M.; Schott, J.J. (2006). Modeling the lithospheric magnetic field over France by means of revised spherical cap harmonic analysis (R-SCHA). J. Geophys. Res., 111.
Abstract: We have recently proposed the revised spherical cap harmonic analysis (R-SCHA) modeling technique. The new mathematical functions represent faithfully the spatial variations of potential fields in a restricted area. In this paper, we tackle the inverse problem and outline the efficiency of the new basis functions with respect to real magnetic data. Processing simultaneously repeat stations, observatory, aeromagnetic, and CHAMP satellite data provides our first vector lithospheric field model over France, which extends from surface to 500 km of altitude. The magnetic field is represented with a minimum horizontal spatial representation of 40 km at the mean Earth radius. The magnetic lithospheric map consistency is confirmed with a comparison to known geological features. The model variation with altitude also suggests that the major French magnetic anomaly, the Paris basin anomaly, is produced by a deep-rooted geological structure. These results demonstrate the superiority of regional modeling over global modeling for delineating small-scale details in the lithospheric field. In view of forthcoming satellite missions, like Swarm, the revised spherical cap harmonic analysis method will help to accurately represent the lithospheric field for more detailed geological interpretations.
Keywords: regional modeling; France; magnetic field; 1517 Geomagnetism and Paleomagnetism: Magnetic anomalies: modeling and interpretation; 1545 Geomagnetism and Paleomagnetism: Spatial variations: all harmonics and anomalies; 0545 Computational Geophysics: Modeling
Programme: 905;139
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Priestley, K.; Debayle, E.; McKenzie, D.; Pilidou, S. (2006). Upper mantle structure of eastern Asia from multimode surface waveform tomography. J. Geophys. Res., 111.
Abstract: We present a new three-dimensional Sv wave speed and azimuthal anisotropy model for the upper mantle of eastern Asia constrained by the analysis of more than 17,000 vertical component multimode Rayleigh wave seismograms. This data set allows us to build an upper mantle model for Asia with a horizontal resolution of a few hundred kilometers extending to ?400 km depth. At 75–100 km depth, there is approximately ±9% wave speed perturbation from the “smoothed PREM” reference model used in our analysis, and the pattern of azimuthal anisotropy is complex. Both the amplitude of the Sv wave speed heterogeneity and the complexity and amplitude of the azimuthal anisotropy decrease with depth. Above ?200 km depth the upper mantle structure of the model correlates with surface geology and tectonics; below ?200 km depth the structures primarily reflect the advection of material in the upper mantle. Since shear wave speed is principally controlled by temperature rather than by composition, Vs(z) can be used to calculate the temperature T(z), and hence map the lithospheric thickness. We use the relationship of Priestley and McKenzie to produce a contour map of the lithospheric thickness of eastern Asia from the surface wave tomography. This shows an extensive region of thick lithosphere beneath the Siberian Platform and the West Siberian Basin that extends to the European Platform, forming the stable Eurasian craton or core. The eastern portion of the Eurasian craton has controlled the geometry of continental deformation and the distribution of kimberlites in eastern Asia.
Keywords: Asian upper mantle; multimode surface waveform; craton; 7208 Seismology: Mantle; 7218 Seismology: Lithosphere; 7255 Seismology: Surface waves and free oscillations; 7270 Seismology: Tomography
Programme: 133
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Thébault, E.; Schott, J.J.; Mandea, M. (2006). Revised spherical cap harmonic analysis (R-SCHA): Validation and properties. J. Geophys. Res., 111.
Abstract: We recently proposed a technique able to represent the spatial variations of the magnetic field at regional scales. However, we pointed out that these preliminary developments were not suited for the complete representation of the geomagnetic field. In this paper, we propose a complete revision, the revised spherical cap harmonic analysis (R-SCHA), which introduces slight changes in order to rectify the previous shortcomings. In addition, some discussions shed a new light on the former spherical cap harmonic analysis (SCHA) and help us to demonstrate its deficiencies and approximations. We finally show that R-SCHA now fully satisfies the natural properties of potential fields. R-SCHA also yields analytical relationships with the spherical harmonics. Taking advantage of the mathematical equivalence of both representations, we explore the relevance of fundamental concepts like spectrum, minimum wavelength, or internal/external field separation. We conclude that these concepts are misleading and must be handled with care in regional modeling. A prime goal being the ability of R-SCHA to represent real data sets, we also investigate and illustrate the effect of finite series expansions. A norm for the regularization of the inverse problem is proposed as well. The conclusions drawn in this paper allow us to validate the method and to assert that the present proposal is suited for modeling and studying the lithospheric magnetic field from ground to satellite altitudes at regional scales.
Keywords: magnetism; regional modeling; Spherical Cap Harmonic Analysis; 1214 Geodesy and Gravity: Geopotential theory and determination; 1517 Geomagnetism and Paleomagnetism: Magnetic anomalies: modeling and interpretation; 1532 Geomagnetism and Paleomagnetism: Reference fields: regional, global; 1541 Geomagnetism and Paleomagnetism: Satellite magnetics: main field, crustal field, external field; 1545 Geomagnetism and Paleomagnetism: Spatial variations: all harmonics and anomalies
Programme: 139;905
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Geirsson, H.; Árnadóttir, T.; Völksen, C.; Jiang, W.; Sturkell, E.; Villemin, T.; Einarsson, P.; Sigmundsson, F.; Stefánsson, R. (2006). Current plate movements across the Mid-Atlantic Ridge determined from 5 years of continuous GPS measurements in Iceland. J. Geophys. Res., 111.
Abstract: We analyze data spanning up to 5 years from 18 continuous GPS stations in Iceland, computing daily positions of the stations with three different high-level geodetic processing software packages. We observe large-scale crustal deformation due to plate spreading across Iceland. The observed plate divergence between the North American and the Eurasian plates is in general agreement with existing models of plate motion. Spreading is taken up within a ?100–150 km wide plate boundary zone that runs through the island. Of the two parallel branches of the plate boundary in south Iceland, the eastern volcanic zone is currently taking up the majority of the spreading and little is left for the western volcanic zone. The plate boundary deformation field has been locally and temporarily affected in south Iceland by two M w = 6.5 earthquakes in June 2000, inflation at Katla volcano during 2000 to 2004, and an eruption of Hekla volcano in February 2000. All stations with significant vertical velocities are moving up relative to the reference station REYK, with the highest velocity exceeding 20 mm/yr in the center of the island.
Keywords: Iceland crustal deformation; continuous GPS; 1240 Geodesy and Gravity: Satellite geodesy: results; 8150 Tectonophysics: Plate boundary: general; 1242 Geodesy and Gravity: Seismic cycle related deformations; 8158 Tectonophysics: Plate motions: present and recent; 7245 Seismology: Mid-ocean ridges
Programme: 316
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