Search Results - (Author, Cooperation:G. Spada)

2018-01-27

Wiley-Blackwell

0094-8276

1944-8007

Geosciences

Physics

2013-08-31

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1365-246X

Blackwell Publishing Journal Backfiles 1879-2005

Geosciences

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.

Electronic Resource

1365-246X

Blackwell Publishing Journal Backfiles 1879-2005

Geosciences

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.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Studies of long-wavelength spectra of X-ray sources provide information about their distances and about the interstellar ...

Electronic Resource

0012-821X

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Geosciences

Physics

Electronic Resource

0031-9201

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Geosciences

Physics

Electronic Resource

0273-1177

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Electronic Resource

0273-1177

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Electronic Resource

0273-1177

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Electronic Resource

0273-1177

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Electronic Resource

0273-1177

Neutron stars ; Transient X-ray sources ; X-rays: binaries ; X-rays: pulsars ; X-rays: sources

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Electronic Resource

0273-1177

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Electronic Resource

0167-5087

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Energy, Environment Protection, Nuclear Power Engineering

Physics

Electronic Resource

0167-5087

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Energy, Environment Protection, Nuclear Power Engineering

Physics

Electronic Resource

0168-9002

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Physics

Electronic Resource

0029-554X

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Energy, Environment Protection, Nuclear Power Engineering

Physics

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Estimates of mantle viscosity have been based on postglacial rebound1 and geoid anomalies2. A stratified mantle with a lower-mantle viscosity of ~1022 Pa s is favoured by geoid anomalies2, whereas from global sea-level changes3 a lower-mantle viscosity has been obtained that is slightly higher than ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] A search for pulsed X rays from 3U0833-45 has been carried out. The Uhuru error box for this source (∼8′ × 24′) contains the Vela pulsar. We obtain an upper limit of ∼ ⅓ the intensity of 3U0833-45 for periodic pulsing with the 89-ms period of the Vela pulsar. Other ...

Electronic Resource

1420-9071

Calliactis parasitica ; nematocytes ; discharge ; hyposmotic shock ; Gd3+

Springer Online Journal Archives 1860-2000

Biology

Medicine

Abstract On acontia ofCalliactis parasitica it was observed that mechanical stimuli applied by a gelatin probe, a method effective in tentacles of Anthozoa, do not induce the discharge of nematocytes. Hyposmotic shock, performed by treatment with NaCl solution 35% hyposmotic with respect to sea water, induces, in the presence of Ca2+, the discharge that spreads along the acontial filament, as previously observed following treatment with SCN−. The hyposmotic shock-induced discharge is blocked by Gd3+ at a concentration of 1 μM. 10 μM Gd3+ prevents also the SCN−-induced discharge. These results suggest the presence of stretch activated cation channels either in nematocytes and/or in supporting cells as well as a possible effect of SCN− on this class of ion channels.

Electronic Resource