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1

Electronic Resource

Geobal post-seismic deformation (1995)

Piersanti, Antonio ; Spada, Giorgio ; Sabadini, Roberto ; [et al.]

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 120 (1995), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: We quantify the effects of post-seismic deformation on the radial and horizontal components of the displacement, in the near- and far-field of strike- and dip-slip point dislocations; these sources are embedded in the elastic top layer of a spherical, self-gravitating, stratified viscoelastic earth. Within the scheme of the normal mode technique, we derive the explicit analytical expression of the fundamental matrix for the toroidal component of the field equations; this component is propagated, together with its spheroidal counterpart, from the core-mantle boundary to the earth's surface. Viscosity stratification at 670km depth influences the radial and horizontal deformation accompanying viscoelastic relaxation in the mantle over time-scales of 103-104 yr, both in the near-field, ranging from 100 to 500 km and in the far-field, from 103 to 5 X 103 km. If the upper mantle is differentiated into a low-viscosity zone beneath the lithosphere and a normal upper mantle, faster relaxation is obtained. For an asthenospheric viscosity of 1020 Pa s we obtain, for a strike-slip dislocation and a seismic moment of 1022 N m characteristic of an average large earthquake, horizontal rates of 1-4 mm yr-1 in the near-field and 0.05-0.4 mm yr-1 in the far-field; these values are maintained over time-scales of 10-103 yr. Larger rates, with shorter duration, are obtained if the viscosity is reduced in the low-viscosity channel. As expected, strike-slip dislocations are the most effective in driving horizontal deformation in the far-field in comparison with dip-slip ones. It is noteworthy that horizontal velocities are maintained longer in the far-field in comparison with radial ones, which is not surprising since momentum is propagated in far regions essentially in the horizontal direction; radial deformation is generally lower in the far-field. VLBI techniques, with a precision of a few parts per billion over distances of 103 km, can detect global post-seismic deformation induced by large earthquakes. Our results affect the interpretation of the transfer of stress and seismic activity among different plate boundaries.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb01838.x

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2

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Polar wandering of a dynamic earth (1993)

Ricard, Yanick ; Spada, Giorgio ; Sabadini, Roberto

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 113 (1993), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The rotational behaviour of a stratified visco-elastic planet submitted to changes in its inertia tensor is studied in a viscous quasi-fluid approximation. This approximation allows for large displacements of the Earth rotation axis with respect to the entire mantle but is only valid for mass redistribution within the planet occurring on the time scale of a few million years. Such a motion, called true polar wander (TPW), is detected by palaeomagneticiens assuming that the Earth's magnetic field remains on average aligned with the spin axis. Our model shows that a downgoing cold slab induces a TPW which quickly brings this slab to the pole for a mantle of uniform viscosity. The same slab is slowly moved toward the equator when a large viscosity increase with depth takes place in the mantle. Our model is also suitable to investigate the effects of a non-steady-state convection on the Earth's rotation. We discuss these effects using a simple mass redistribution model inspired by the pioneering paper of Goldreich & Toomre (1969). It consists of studying the TPW induced by a random distribution of slabs sinking into the mantle. For such a mass redistribution, only a strongly stratified mantle can reduce the Earth's pole velocity below 1d̀ Ma-1, which is the upper bound value observed by palaeomagnetic investigations for the last 200 Ma. Our model also shows that when corrected for the hydrostatic flattening, the Earth's polar inertia generally corresponds to the maximum inertia, as it is presently observed. However, this may not be the case during some short time periods. We also discuss the amount of excess polar flattening that can be related to tidal deceleration. This frozen component is found to be negligible.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb00888.x

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3

Electronic Resource

On A Particular Solution of the Non-Linear Liouville Equations (1993)

Spada, Giorgio ; Sabadini, Roberto ; Ricard, Yanick

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 114 (1993), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The rotational behaviour of a stratified viscoelastic planet is analysed by means of a quasi-analytical method. Our approach is particularly appropriate to study the long-term polar wander induced by internal loads, and in particular to study the effects due to time-dependent mantle convection. We focus on a simple explicit solution of the Liouville non-linear equations, in order to establish the relationships between internal rheological constitution of the planet and polar motion. Both the rate and the direction of polar wander are found to be extremely sensitive to the mantle stratification and in particular to the nature of the 670km depth seismic discontinuity.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb03927.x

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4

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Effects on post-glacial rebound from the hard rheology in the transition zone (1992)

Spada, Giorgio ; Sabadini, Roberto ; Yuen, David A. ; [et al.]

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 109 (1992), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: We analyse the influences of a viscosity increase in the transition zone between 420 and 670 km on the geophysical signatures induced by post-glacial rebound, ranging from the perturbations in the Earth's rotation to the short wavelength features associated with the migration of the peripheral bulge. A self-gravitating model is adopted, consisting of an elastic lithosphere, a three-layer viscoelastic mantle and an inviscid core.The horizontal displacements and velocities and the stress pattern are extremely sensitive to the viscosity increase and to the chemical stratification of the transition zone. The hardening of the upper and the chemical density jumps in mantle below the 420 discontinuity induces a channel effect which contaminates the horizontal deformation both in the near-field and in the far-field from the ice-sheets. These findings indicate that intraplate geodetic data can be used to put bounds on the viscosity increase in the transition zone and on the amount of chemical stratification in the mantle.The stress field induced in the lithosphere by the Pleistocenic ice-sheet disintegration is a very sensitive function of mantle viscosity stratification. The existence of seismic activity along passive continental margins of previously glaciated areas requires a substantial viscosity increase in the mantle, with the viscosity of the transition zone acting as a controlling parameter. A viscously stratified mantle is responsible for a delayed upward migration of stress in the lithosphere which can account for the seismicity today.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00125.x

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5

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Relative Sea Level Variations Caused by Subduction (1997)

Piromallo, Claudia ; Spada, Giorgio ; Sabadini, Roberto ; [et al.]

Springer

Surveys in geophysics 18 (1997), S. 225-238

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ISSN: 1573-0956

Keywords: Dynamic topography ; geoid ; sea level ; subduction ; viscosity profile

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract By means of a stratified Earth model with viscoelastic rheology, we have studied the long-term global fluctuations of Relative Sea Level (RSL) induced by subducting slabs. We have computed RSL variations for both a single subduction and a realistic distribution of slabs by a numerical simulation based on a simplified model of the subduction process. RSL is determined by the offset between the geoid and the dynamic topography; our analysis demonstrates that the latter provides the prevailing contribution. We have studied, in addition, the effects of rheological stratification upon the amplitude and time-evolution of these two quantities and, consequently, of RSL fluctuations. According to our results, an upper bound for the rate of RSL associated with subduction is of the order of 0.1 mm/yr, in agreement with previous studies. This rate of sea level variation is comparable with that attributed to changes in the tectonic regime on a large scale. This preliminary result corroborates the suggestion by other authors to include subduction in the list of geophysical mechanisms which contribute to long-term RSL fluctuations.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1006540126947

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6

Electronic Resource

Excitation of true polar wander by subduction (1992)

Spada, Giorgio ; Ricard, Yanick ; Sabadini, Roberto

[s.l.] : Nature Publishing Group

Nature 360 (1992), S. 452-454

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] The 1969 paper of Goldreich and Toomre11 renewed the interest of the geophysical community12 in the role of polar wandering in Earth's dynamics. Their example of a colony of beetles crawling on the surface of a quasi-rigid Earth suggests that large excursions of the axis of rotation with respect to ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/360452a0

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7

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Time-dependent density anomalies in a stratified, viscoelastic mantle: Implications for the geoid, Earth's rotation and sea-level fluctuations (1993)

Sabadini, Roberto ; Spada, Giorgio ; Ricard, Yanick

Springer

Surveys in geophysics 14 (1993), S. 537-553

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ISSN: 1573-0956

Keywords: Rheology ; viscosity and density jumps ; geoid ; rotation ; sea levels

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract The effects on the ℓ = 2 geoid component and Earth's rotation due to internal mass anomalies are analyzed for a stratified viscoelastic mantle described by a Maxwell rheology. Our approach is appropriate for a simplified modeling of subduction. Sea-level fluctuations induced by long-term rotational instabilities are also considered. The displacement of the Earth's axis of rotation, called true polar wander (TPW) and the induced eustatic sea-level fluctuations, are extremely sensitive to viscosity and density stratification at the 670 km seismic discontinuity. Phase-change models for the transition zone generally allow for huge amount of TPW, except for large viscosity increases; the dominant contribution in Liouville equations comes from a secular term that reflects the viscous behaviour of the mantle. In chemically stratified models, TPW is drastically reduced due to dynamic compensation of the mass anomalies at the upper-lower mantle interface. When the source is embedded in the upper mantle close to the chemical density jump, transient rotational modes are the leading terms in the linear Liouville equations. Long-term rotation instabilities are valuable contributors to the third order cycles in the eustatic sea-level curves. Rates of sea-level fluctuations of the order of 0.05–0.1 mm/yr are induced by displacements of the Earth's axis of rotation compatible with paleomagnetic data.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00690576

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8

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Evolution of the number of communicative civilizations in the Galaxy: implications on Fermi paradox (2020)

Spada, Giorgio ; Melini, Daniele

Cambridge University Press

In: International Journal of Astrobiology. 2020; 19(4): 314-319. Published 2020 Apr 23. doi: 10.1017/s1473550420000063.

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Publication Date: 2020-04-23

Description: It has been recently proposed DeVito [(2019) On the meaning of Fermi's paradox. Futures, 389–414] that a minimal number of contacts with alien radio-communicative civilizations could be justified by their logarithmically slow rate of growth in the Galaxy. Here we further develop this approach to the Fermi paradox, with the purpose of expanding the ensemble of the possible styles of growth that are consistent with the hypothesis of a minimal number of contacts. Generalizing the approach in DeVito (2019), we show that a logarithmic style of growth is still found. We also find that a style of growth following a power law would be admissible, however characterized by an exponent less than one, hence describing a sublinear increase in the number of communicative civilizations, still qualitatively in agreement with DeVito (2019). No solutions are found indicating a superlinear increase in the number of communicative civilizations, following for example an exponentially diverging law, which would cause, in the long run, an unsustainable proliferation. Although largely speculative, our findings corroborate the idea that a sublinear rate of increase in the number of communicative civilizations in the Galaxy could constitute a further resolution of Fermi paradox, implying a constant and minimal – but not zero – number of contacts.

Print ISSN: 1473-5504

Electronic ISSN: 1475-3006

Topics: Biology , Geosciences