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  • 1
    Digitale Medien
    Digitale Medien
    Mountain building and earth rotation (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 117 (1994), S. 0 
    Details
    ISSN: 1365-246X
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Geologie und Paläontologie
    Notizen: Whereas the present-day true polar wander and the secular non-tidal acceleration of the Earth have usually been attributed to postglacial rebound, it has recently been suggested that non-glacially induced vertical tectonic movements taking place under non-isostatic conditions can also be effective in changing the Earth's rotation. We present a case study in which we analyse the effects of some simple uplift histories of the Himalayas and the Tibetan Plateau on the rotational axis and on the second-degree zonal harmonic of the geoid, for time-scales of up to a few million years. We first assume a permanent amount of overcompensation, which is consistent with observed geoid anomalies over the Himalayas, and then we model by means of the normal-mode techniques, the viscous relaxation in the mantle, with the elastic lithosphere supporting elastically 2 per cent of isostatic disequilibrium. In our normal-mode analysis, the Earth is divided into five layers: an effectively elastic lithosphere, a viscoelastic shallow upper mantle, transition zone and lower mantle characterized by the Maxwell rheology and an inviscid core. The readjustment of the equatorial bulge due to viscous flow in the mantle is taken into account in our studies by solving the linearized Liouville equations for the conservation of angular momentum, via the Love numbers formalism.Polar wander is sensitive to the rate of relaxation of the modes M1 and M2 due to the discontinuities between the three mantle layers, positioned at 420 and 670 kilometres depth. The rate of readjustment is sensitive to the viscosity of the transition zone whenever the lower mantle/shallow upper mantle viscosity ratio is small. The highest present-day velocity of polar wander due to Himalayan and Tibetan Plateau uplift is estimated to be 1° Myr−1 for an isoviscous mantle that has the same magnitude of the observed value, reduced to 0.1° Myr−1 for a factor 50 viscosity increase in the lower mantle. These numbers are about the same as those found from postglacial rebound that occurs on the short time-scale of a thousand years instead of the million years of our analysis, but represent upper bounds for mountain building, obtained only in the case in which a permanent deviation from isostasy of at least 2 per cent is assumed. In general, the proposed mechanism is less efficient in driving long-term rotation instabilities than deep-seated processes characterized by the same time-scale of a million years such as subduction; polar-wander velocity is extremely sensitive to the depth of the uncompensated anomalous root of the topography for the models in which full mantle relaxation is allowed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1111/j.1365-246X.1994.tb02457.x
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
    Digitale Medien
    Digitale Medien
    Compressible rotational deformation (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 126 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Geologie und Paläontologie
    Notizen: In recent years a number of studies have investigated the influence of compressibility on geophysical observables such as postglacial rebound deformation rates and the geoid. Some of these studies indicate that long-term signatures such as the geoid might be sensitive to compressibility. As both load relaxation and tidal-effective relaxation of the equatorial bulge are operative in a dependent way, polar wander can potentially be more sensitive to compressible rheologies if the interference between the two relaxation mechanisms is constructive. This has motivated us to study the influence of compressibility on true polar wander by means of spherical, laterally homogeneous, self-gravitating analytical earth models. As we wish to study both short-term rotational changes and polar wander on geological time-scales, we employ a Maxwell viscoelastic model instead of a Newtonian viscous model. The latter is commonly used in geoid modelling. The purpose of this paper is to concentrate on the basic physical aspects of the differences between compressible and incompressible rotational deformation, rather than applying the procedures to fine-graded multi-layered PREM models with realistic forcing functions. An important issue of our method concerns the analytical instead of numerical way of solving the differential equations by the propagator matrix method. Compressible viscoelastic relaxation has usually been treated numerically until now.The results show that homogeneous earth models do not have significant differences on long time-scales between compressible and the corresponding incompressible cases. Compressibility introduces a denumerably infinite set of short-time relaxation modes. The relaxation times of these dilatation modes can be approximated analytically. Two-layer core-mantle models show relatively large differences between incompressible and compressible Maxwell rheologies. Simplified models of true polar wander triggered by Heaviside loads show that differences of several tens of per cent between incompressible and compressible Maxwell rheologies are possible. True polar wander is decreased in the compressible case on both short and long time-scales, which means that smaller viscosities are required to explain polar-wander measurements than in the incompressible case.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb04700.x
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  • 3
    facet.materialart.
    Unbekannt
    Tide gauge observations in Antarctica (1958–2014) and recent ice loss (2017)
    Cambridge University Press
    Details
    Publikationsdatum: 2017-02-06
    Beschreibung: Several historical sea level time series from Antarctic tide gauges, available from the Permanent Service for Mean Sea Level, are analysed. Two sea level curves, obtained by averaging data from the Antarctic Peninsula and West Antarctica, for 1958–2014, show trends of (2.0±0.1) and (1.8±0.2) mm yr-1, respectively. By empirical mode decomposition, cyclic and non-cyclic components of sea level change were separated. A periodicity of 4–5 years was confirmed and attributed to the effects of the Antarctic Circumpolar Wave. The non-cyclic components were found to show a ‘levelling off’ of ≈ 1 mm yr-1 since c. 2000, which cannot be attributed to the isostatic response to Holocene ice melting. Using assessed mass balance data from the West Antarctic Ice Sheet and the Antarctic Peninsula, we studied the response to current ice loss in the region and found that the levelling off could be partly explained by accelerated melting during the last approximately two decades. This may represent the first evidence of sea level fingerprints of glacial melting in Antarctica.
    Print ISSN: 0954-1020
    Digitale ISSN: 1365-2079
    Thema: Biologie , Geographie , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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  • 4
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    Ice melting and earthquake suppression in Greenland (2014)
    Elsevier Science Limited
    Details
    Publikationsdatum: 2017-04-04
    Beschreibung: It has been suggested that the Greenland ice sheet is the cause of earthquake suppression in the region. With few exceptions, the observed seismicity extends only along the continental margins of Greenland, which almost coincide with the ice sheet margin. This pattern has been put forward as further validation of the earthquake suppression hypothesis. In this review, new evidence in terms of ice melting, post-glacial rebound and earthquake occurrence is gathered and discussed to re-evaluate the connection between ice mass unloading and earthquake suppression. In Greenland, the spatiotemporal distribution of earthquakes indicates that seismicity is mainly con- fined to regions where the thick layer of ice is absent and where significant ice melting is presently occurring. A clear correlation between seismic activity and ice melting in Greenland is not found. However, earthquake locations and corresponding depth distributions suggest two distinct governing mechanisms: post-glacial rebound promotes moderate-size crustal earthquakes at Greenland’s regional scale, while current ice melting promotes shallow low magnitude seismicity locally
    Beschreibung: Published
    Beschreibung: 94–106
    Beschreibung: 1T. Geodinamica e interno della Terra
    Beschreibung: JCR Journal
    Beschreibung: open
    Schlagwort(e): Greenland ; Earthquakes ; Ice sheet melting ; 02. Cryosphere::02.02. Glaciers::02.02.05. Ice dynamics
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
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  • 5
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    Intermittent Sea Level Acceleration (2013)
    Elsevier Science Limited
    Details
    Publikationsdatum: 2017-04-04
    Beschreibung: Using instrumental observations from the Permanent Service for Mean Sea Level (PSMSL), we provide a new assessment of the global sea{level acceleration for the last 2 centuries (1820-2010). Our results, obtained by a stack of tide gauge time series, con firm the existence of a global sea level acceleration (GSLA) and, coherently with independent assessments so far, they point to a value close to 0:01 mm/yr2. However, di fferently from previous studies, we discuss how change points or abrupt inflections in individual sea level time series have contributed to the GSLA. Our analysis, based on methods borrowed from econometrics, suggests the existence of two distinct driving mechanisms for the GSLA, both involving a minority of tide gauges globally. The first effectively implies a gradual increase in the rate of sea level rise at individual tide gauges, while the second is manifest through a sequence of catastrophic variations of the sea level trend. These occurred intermittently since the end of the 19th century and became more frequent during the last four decades.
    Beschreibung: Published
    Beschreibung: 64-72
    Beschreibung: JCR Journal
    Beschreibung: open
    Schlagwort(e): tide gauge ; sea level rise ; sea level acceleration ; 03. Hydrosphere::03.01. General::03.01.04. Ocean data assimilation and reanalysis
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
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  • 6
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    Geomagnetic South Atlantic Anomaly and global sea level rise: A direct Connection? (2012)
    Elsevier Science Limited
    Details
    Publikationsdatum: 2017-04-04
    Beschreibung: We highlight the existence of an intriguing and to date unreported relationship between the surface area of the South Atlantic Anomaly (SAA) of the geomagnetic field and the current trend in global sea level rise. These two geophysical variables have been growing coherently during the last three centuries, thus strongly suggesting a causal relationship supported by some statistical tests. The monotonic increase of the SAA surface area since 1600 may have been associated with an increased inflow of radiation energy through the inner Van Allen belt with a consequent warming of the Earth’s atmosphere and finally global sea level rise. An alternative suggestive and original explanation is also offered, in which pressure changes at the core–mantle boundary cause surface deformations and relative sea level variations. Although we cannot establish a clear connection between SAA dynamics and global warming, the strong correlation between the former and global sea level supports the idea that global warming may be at least partly controlled by deep Earth processes triggering geomagnetic phenomena, such as the South Atlantic Anomaly, on a century time scale.
    Beschreibung: Published
    Beschreibung: 129-135
    Beschreibung: 3.4. Geomagnetismo
    Beschreibung: JCR Journal
    Beschreibung: restricted
    Schlagwort(e): Earth magnetic field ; South Atlantic Anomaly ; Global Sea Level ; 04. Solid Earth::04.05. Geomagnetism::04.05.05. Main geomagnetic field
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
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  • 7
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    Unbekannt
    A study of the longest tide gauge sea-level record in Greenland (Nuuk/Godthab, 1958-2002) (2014)
    Elsevier Science Limited
    Details
    Publikationsdatum: 2017-04-04
    Beschreibung: We study the longest tide gauge record available from Greenland, that is the Nuuk/Godthab site in southwest Greenland, for the time period 1958–2002. Standard regression methods and the application of the Ensemble Empirical Mode Decomposition technique reveal a rate of sea-level rise of ≈ 2 mm yr− 1, two complete cycles of the 18.6-years lunar nodal tide, and a negligible acceleration. Using previous assessments for the globally averaged sea-level rise during that period, glacial isostatic adjustment modeling and sea-level “fingerprinting” of the mass loss of continental ice sources, terrestrial water sources and oceanic steric effects, we evaluate the various contributions to local sea-level rise at the tide gauge location. The misfit between the observed and the modeled sea-level trend is unlikely to reflect tectonic deformations but, more intriguingly, may indicate that the mass balance of the Greenland ice sheets was, during the second half of the last century, somehow closer to balance than suggested by previous investigations.
    Beschreibung: Published
    Beschreibung: 42-51
    Beschreibung: 4A. Clima e Oceani
    Beschreibung: JCR Journal
    Beschreibung: restricted
    Schlagwort(e): Sea-level change ; Tide gauge observations ; Greenland ice sheet ; 02. Cryosphere::02.02. Glaciers::02.02.06. Mass balance
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
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