1,721,588 research outputs found
Long-term rotation and mantle dynamics of the Earth, Mars, and Venus
No full textIn response to internal and surface tectonic processes the terrestrial planets have the ability to displace the axis of rotation with respect to the mantle. This behavior is quantified by means of a nondimensional number, defined here as the rotational number Ro, that allows classification of the planets into two categories, the first containing Mars and the Earth, where true polar wander is a feasible mechanism, and the second, to which Venus belongs, where rotation equilibrium is attained by means of mega-wobbles driven by internal mass anomalies. The number Ro is related to the timescale characterizing the readjustment of the equatorial bulge during long-term polar motion and to the length of the sidereal day. If these two timescales are well separated, the planet experiences true polar wander. Nonlinear Liouville equations for stratified viscoelastic bodies with linear Maxwell rheology are solved in order to assess the relevance of surface and mantle processes in driving long-term rotation instabilities in the terrestrial planets. The amount of true polar wander estimated for the Earth and Mars is reproduced correctly by our modeling with mantle viscosities and lithospheric thickness consistent with other studies. The major difference between the Earth and Mars is the driving mechanism, subduction for the Earth and lithospheric loading for Mars. When mantle viscosities similar to those of the Earth are considered for Venus, the most updated estimate for the offset of about 0.5° between the rotation axis and the axis of maximum inertia is well reproduced during the mega-wobbles induced by internal mass redistribution. We show that the degree 2 topography of these three planets can be affected by their rotation, which is responsible for the dominance of the sectorial component on the Earth and Mars and for the zonal component on Venus
True polar wander affects the Earth dynamic topography and favours a highly viscous lower mantle
No full textAbstract. True Polar Wander (TPW), the global motion of the Earth's mantle relative to the
axis of rotation, is considered for its impact on the degree 2 surface dynamic topography
induced by mass anomalies in a viscously stratified mantle. These sources, responsible for a
geoid and topographic signal at the Earth's surface, mimic the effects of subductions and
other thermal instabilities on long-term Earth's rotation. A rotating, viscous planet has the
ability to displace the axis of rotation with respect to internal mass anomalies in order to ..
The spin and inertia of Venus
No full textOne of the most remarkable rotational features of Venus is the presence of a relatively large offset between spin and maximum inertia axis. According to the most recent gravity field models, this offset may amount to ∼0.5°, considerably larger than the one which characterizes the Earth's Chandler wobble (∼0.3′′). At present, it is not clear whether the offset of Venus is the result of the limited resolution of the gravity models or, alternatively, it may be regarded as an evidence in support to the existence of a wobble. Purpose of this note is to demonstrate on quantitative grounds that an offset between spin axis and maximum inertia axis is to be expected on the time‐scales of mantle convection, due to the peculiar rotational characteristics of Venus. We also show that the inertia perturbation associated to the rising of hot thermal plumes from the core‐mantle boundary to the surface is large enough to maintain offsets ranging between 0.1 and 1°. Of course, this does not preclude that other processes, such as Venusquakes or periodic atmospheric variabilities, may contribute significantly to the observed offset on shorter time‐scales
Postmodernist rereadings of Virginia Woolf's to the lighthouse
Full text linkThis thesis deals with those contemporary novels, known as biofictions, which not only rewrite the lives of historical, canonical authors in a fictional way, but also engage in a dialogue with their precursors’ texts. Biofictions have extended the tradition of life writing and, through the practice of rewriting, have made a significant contribution to reading the past in relation to the present.
Since, in recent years, Virginia Woolf has been the protagonist of many biofictions and several of her novels and themes have been reworked in a variety of different ways, I chose to investigate the reason for her appeal to contemporary tastes. Thus, I focused on her most autobiographical novel, To the Lighthouse, in which Virginia Woolf openly drew inspiration from her own life experience, her memories and feelings, and transformed biographical facts into fiction, so much so that it is certainly a novel about her family, childhood and her struggles to become an artist.
My choice was guided by the awareness that life writing has been reconfigured from a postmodernist perspective, and, since Woolf’s life and work are continuously being rewritten, I wanted to examine whether To the Lighthouse, a personal real life history rewritten as fiction, could be read as an antecedent of contemporary biofictions. Virginia Woolf herself, in fact, engaged with the question of life writing, extended its range and explored the relationship between auto/biography and fiction, a tradition that Postmodernism has further developed. To the Lighthouse uses auto/biography, but extends its limits and turns it into something between biography and fiction. Virginia Woolf borrows elements and events from her own life and “recycles” them to offer her own vision of the world, to the extent that To the Lighthouse can be read both as pure fiction and as fictional autobiography.
The effects of Woolf’s experiments in life writing and of her blurring the rigid borders between fact and fiction are central to those postmodernist novels, which deal with the complex relationship between life and fiction. Her novel is definitely a work of fiction, but I argue that being so full of both life (bio) and personal history, it allows us to draw a connection between her form of life writing and contemporary biofictions.
I hope to contribute to this field by discussing two postmodernist biofictions: Maggie Gee’s Virginia Woolf in Manhattan and Susan Sellers’ Vanessa and Virginia, which I read not only as an evident rewriting of Woolf’s life, but also as a dialogue, more or less obvious, with To the Lighthouse. In doing so, I adopt an intertextual approach, which places these biofictions in relation not only to Woolf’s life, but also to her novel.
I follow two main routes of exploration: the first is to see how in To the Lighthouse, Virginia Woolf mixes real facts from her life with events and thoughts she only imagines, thus creating a work of fiction. The second is to see how the two postmodernist novels, the object of this thesis, bestride two fields: the bio-fictional, which engages with Woolf’s life, mixes real and imaginary experiences and recreates her thoughts, and the intertextual, which engages significantly with Woolf’s work, namely with To the Lighthouse.
Virginia Woolf in Manhattan and Vanessa and Virginia, with their many references to To the Lighthouse illuminate Woolf’s continuous interest in life writing, which she revealed in many essays and her significant experiment in “using” life in her novel. Thus, they make a contribution to the refashioning of To the Lighthouse: both novels centre around such themes as family ties, personal losses, the effort that artistic creation requires and the value of fame, which are pivotal in To the Lighthouse and adopt Woolf’s pioneering technique of exploring the inner life of her characters. Their books are thought-provoking and raise serious questions about our relationship with Virginia Woolf and, more specifically, with To the Lighthouse
A thermomagnetoelastic model of the earthquake source mechanism
No full textAn explicit expression is derived for the body force, magnetic moment and heat source to be applied in the absence of a dislocation to produce a radiation pattern identical to that of a dislocation. These quantities depend only on the fault mechanism and the material properties of the medium in the immediate vicinity of the fault. The theory is developed for dislocations in an anisotropic inhomogeneous thermo-elastic medium that exhibits pyromagnetic and piezomagnetic properties. The magnetic moment equivalent, that completely describes the magnetical effects of the dislocation, is found to be dependent on the discontinuities of the disllacement prescribed across the fault, through the piezomagnetic coefficients. Discontinuities in the thermal field and its derivatives do not contribute to the magnetic moment equivalent, thus leading to the conclusion that pyromagnetism cannot produce magnetic effects
Orientations of seismic ruptures from P polarities and P pulse widths inversions: the Sellano (Umbria-Marche, Italy) case study
No full textAn EGF technique to infer the rupture velocity history of a small earthquake: a possible solution to the tradeoff among Q and source parameters
No full text
Some effects of elastic pre-stress within the Earth
No full textA dynamical theory, regarding the superposition of small deformations upon large ones, is applied to the study of the prestress effects in an isotropic medium. The primary configuration is in a dynamic state and no assumption is made regarding the smallness of time derivatives of the displacement field. In this framework, a reciprocity theorem is derived and the representation theorem for the displacement field is obtained in terms of the Green's function. Body force equivalents are derived in terms of the discontinuity of the displacement across the fault surface. The ray theory is briefly reviewed and applied to the study of seismic-wave propagation in homogeneous isotropic media. The prestressed configuration in proximity of the fault surface is treated as a perturbation on seismic waves and its effects are found to be of first order in the perturbation at the origin. The wave front equation and the nodal-surface equation for compressional waves are derived and both are found to suffer significant changes due to the perturbation
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