1,721,047 research outputs found
Plate motions predictions based on the constraint of toroidal‐poloidal equipartition
No full textUsing a kinematic approach, we study the evolution of a system of plates whose Euler poles and shapes are dictated by the requirement of toroidal‐poloidal equipartition. Though no attempt is made to model the dynamical aspects of the problem and the time‐scales involved, the simulations performed lead to configurations of plates which share with the currently observed one an excess of drift energy with respect to spin energy. The existence of fastly spinning plates seems to be in contradiction with the constraint of toroidal‐poloidal equipartition. Their fate is to quickly disappear to prevent an excess of toroidal energy in the tectonic plates system
Obliquity variations due to climate friction on Mars: Darwin versus layered models
No full textIn response to periodic variations in the obliquity, giant CO2 ice caps may form in the polar regions of Mars. The delayed response of the equatorial bulge of Mars to these periodic loads may introduce a secular term in the time evolution of the obliquity. Up to now, the variations in the flattening of Mars induced by the polar ice caps have been estimated using the Darwin model, which simply consists of a uniform sphere with Newtonian rheology. For Earth-like viscosities of ∼ 3 × 1021 Pa s the Darwin model maximizes the friction and predicts a secular obliquity variation comparable to the currently observed value (∼ 24°). Our main purpose here is to address the role of two well-recognized features of Mars (a thick lithosphere and a core) on the response of its equatorial bulge and thus on the secular changes of obliquity driven by climate friction. This is done using a layered model with Maxwell viscoelastic rheology. We show that for a wide range of admissible mantle viscosities the obliquity variation is sensibly smaller (by a factor of ∼ 2) than the one predicted by the Darwin model. On the basis of this finding, we can rule out the possibility that the present-day obliquity of Mars totally results from climate friction
Effect of subductions and trends in seismically induced Earth rotational variations
No full textIn the past, the influences of large earthquakes on Earth rotation and on changes of the gravity field have been the subject of extensive investigations. Though it is now recognized that earthquakes cannot excite the Chandler wobble and that they only drive a very modest secular drift of the Earth's spin axis, their rotational signatures present some intriguing peculiarities. In fact, they preferentially drive the rotation axis toward a well-defined direction (i.e., ∼ 140°E) and systematically induce negative variations of the Earth's oblateness. In the first part of this work the static dislocation theory is employed to ascertain the origin of these two remarkable trends. This task is accomplished by considering the effects of seismicity on Earth rotation during the last two decades. We find that the above tendencies are operating even on a yearly time scale, and we explain their nature by means of simple geometrical and physical arguments. We also find that the trend of the pole toward 140°E is essentially due to the seismicity which characterizes the western Pacific subduction zones. The seismic excitation function takes for shallow thrust earthquakes a direction roughly opposite to the excitation function associated with mass injection along active subduction zones; this point highlights the potential effects of aseismic mass rearrangement at subduction zones on the secular drift of the pole. Our findings, based on a simple model of steady state subduction, show that in the last two decades this process substantially acts to counterbalance the polar drift due to global seismic activity. We verify that this balance between earthquake-and subduction-induced rotational signatures also characterizes the time variations of the dynamic oblateness, which is intrinsically independent of the inertia changes that drive polar motion. From these results, which may potentially have some impact on the interpretation of long-term polar motion, we conclude that the pole of rotation cannot be significantly perturbed by tectonic motions on a decade time scale
Anomalous Roughness of the Pole Path at the Time of the 1994 Bolivia and Kurile Islands Earthquakes
No full textWe report anomalies in the roughness of the path of the pole of rotation at the time of the Bolivia earthquake of June 9, 1994 (Mω = 8.2) and of the Kurile Islands earthquake of October, 4, 1994 (Mω = 8.3). This finding reinforces the results of previous studies, which have indicated changes in the polar motion curvature at the time of major earthquakes
Chandler wobble excitation by catastrophic flooding of the Black Sea
No full textIt is now widely accepted that during the late Quaternary glaciation the Black Sea formed an isolated inland lake (Ross et al., 1970). New geological data and the recognition of sudden population movements away from the Black Sea coasts suggest that the basin was rapidly flooded through the Bosphorus sill 7150 years bp, causing a sea level rise of ~ 135 m in a few years (Ryan et al., 1997). As shown here, such a catastrophic redistribution of mass has significantly altered the amplitude of the Chandler wobble, the free motion of the pole of rotation around the main inertia axis of the Earth (Lambeck, 1980). We also estimate that during the flooding the pole of rotation was diverted from its secular path and shifted by ~ 30 m, at a rate of several meters per year. These rotational variations are found to be orders of magnitude larger than those produced by other short-term geophysical processes, such as earthquakes seismic moment release (O'Connell and Dziewonski, 1979; Chao et al.,1996), anthropogenic water impoundment (Chao, 1995), and tectonic mass movements (Alfonsi and Spada, 1998). The Black Sea flooding may thus be responsible for the most drastic change in the rotational parameters of the Earth in the recent history of our planet
Did the 1977 Sumba earthquake excite the Chandler wobble?
No full textThe potential role of large earthquakes on the excitation of the Earth's free wobble has been the subject of various investigations. It is nowadays recognized that the coseismic moment released by large earthquakes is not large enough to maintain the wobble, and that they cumulatively only cause a very modest secular motion of the pole. Nevertheless, renewed attention was paid to this subject following the observation of a sharp jump in the Lageos-derived polar motion excitation function at the time of the great 1977 Sumba earthquake. The static dislocation theory proved unable to explain these abrupt variations simply in terms of coseismic moment release, and it was suggested that aseismic movement of blocks of ruptured lithosphere following the main shock could reconcile theoretical predictions with observations. Geological evidence suggests that the 1977 Sumba earthquake was caused by a slab pull force acting along the subducting lithosphere, the main shock was followed by a down-dip motion of the slab. We estimate quantitatively the potential role of this aseismic mass redistribution. We find that the polar motion excitation due to the motions of ruptured lithosphere is not compatible with the Lageos-observed excitation function, being in better agreement with the excitation functions derived by other polar motion data sets. However, we also show that the short-term inertia variations due to the slab movements are considerably larger than those due to the main rupture, thus possibly making earthquakes produced by slab pull forces an efficient mechanism for exciting the Chandler wobble
Anomalous Roughness of the Pole Path at the Time of the 1994 Bolivia and Kurile Islands Earthquakes
No full textWe report anomalies in the roughness of the path of the pole of rotation at the time of the Bolivia earthquake of June 9, 1994 (Mω = 8.2) and of the Kurile Islands earthquake of October, 4, 1994 (Mω = 8.3). This finding reinforces the results of previous studies, which have indicated changes in the polar motion curvature at the time of major earthquakes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Diet for young diabetics: Standard and mediterranean
No full textClose associations have been demonstrated between a number of degenerative diseases (atherosclerosis, cardiovascular disease, hypertension, obesity, etc.) and the unbalanced diet of industrialized countries characterized by excessive amounts of calories, protein, saturated fat, simple carbohydrates, sodium, and other nutrients. Dietary recommendations issued over the last few years are the same for diabetic and nondiabetic individuals. Their goal is to allow normal growth and variable levels of physical activity while at the same time protecting against obesity, dyslipidemia, and other complications due to an unbalanced diet. Thus, young diabetics should eat a normal diet while taking care to keep their blood sugar levels as close as possible to the normal range to prevent the short- and long-term complications of diabetes. The 'Mediterranean diet' in combination with appropriate insulin therapy may be optimal. It consists mainly of fiber-rich complex carbohydrates (grain), vegetables, fruit, yogurt, fish, and olive oil. Explanations on this diet should focus on quality rather than quantity of food stuffs and should be given to each child and family by a multidisciplinary team including an experienced dietician. Prescription of a highly rigid diet has proved ineffective in producing adequate metabolic control, and increases the risk of deviations from the diet
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