81 research outputs found
The complex scaling behavior of non--conserved self--organized critical systems
The Olami--Feder--Christensen earthquake model is often considered the
prototype dissipative self--organized critical model. It is shown that the size
distribution of events in this model results from a complex interplay of
several different phenomena, including limited floating--point precision.
Parallels between the dynamics of synchronized regions and those of a system
with periodic boundary conditions are pointed out, and the asymptotic avalanche
size distribution is conjectured to be dominated by avalanches of size one,
with the weight of larger avalanches converging towards zero as the system size
increases.Comment: 4 pages revtex4, 5 figure
Fractal escapes in Newtonian and relativistic multipole gravitational fields
We study the planar motion of test particles in gravitational fields produced
by an external material halo, of the type found in many astrophysical systems,
such as elliptical galaxies and globular clusters. Both the Newtonian and the
general-relativistic dynamics are examined, and in the relativistic case the
dynamics of both massive and massless particles are investigated. The halo
field is given in general by a multipole expansion; we restrict ourselves to
multipole fields of pure order, whose Newtonian potentials are homogeneous
polynomials in cartesian coordinates. A pure (n)-pole field has (n) different
escapes, one of which is chosen by the particle according to its initial
conditions. We find that the escape has a fractal dependency on the initial
conditions for (n>2) both in the Newtonian and the relativistic cases for
massive test particles, but with important differences between them. The
relativistic motion of massless particles, however, was found to be regular for
all the fields we could study. The box-counting dimension was used in each case
to quantify the sensitivity to initial conditions which arises from the
fractality of the escape route.Comment: 17 pages, 7 figures, uses REVTE
Theory of Suspension Segregation in Partially Filled Horizontal Rotating Cylinders
It is shown that a suspension of particles in a partially-filled, horizontal,
rotating cylinder is linearly unstable towards axial segregation and an
undulation of the free surface at large enough particle concentrations. Relying
on the shear-induced diffusion of particles, concentration-dependent viscosity,
and the existence of a free surface, our theory provides an explanation of the
experiments of Tirumkudulu et al., Phys. Fluids 11, 507-509 (1999); ibid. 12,
1615 (2000).Comment: Accepted for publication in Phys Fluids (Lett) 10 pages, two eps
figure
Effects of clopidogrel in addition to aspirin in patients with acutecoronary syndromes without ST-segment elevation.
Background: Despite current treatments, patients who have acute coronary syndromes without ST-segment elevation have high rates of major vascular events. We evaluated the efficacy and safety of the antiplatelet agent clopidogrel when given with aspirin in such patients. Methods: We randomly assigned 12,562 patients who had presented within 24 hours after the onset of symptoms to receive clopidogrel (300 mg immediately, followed by 75 mg once daily) (6259 patients) or placebo (6303 patients) in addition to aspirin for 3 to 12 months. Results: The first primary outcome -- a composite of death from cardiovascular causes, nonfatal myocardial infarction, or stroke -- occurred in 9.3 percent of the patients in the clopidogrel group and 11.4 percent of the patients in the placebo group (relative risk with clopidogrel as compared with placebo, 0.80; 95 percent confidence interval, 0.72 to 0.90; P<0.001). The second primary outcome -- the first primary outcome or refractory ischemia -- occurred in 16.5 percent of the patients in the clopidogrel group and 18.8 percent of the patients in the placebo group (relative risk, 0.86, P<0.001). The percentages of patients with in-hospital refractory or severe ischemia, heart failure, and revascularization procedures were also significantly lower with clopidogrel. There were significantly more patients with major bleeding in the clopidogrel group than in the placebo group (3.7 percent vs. 2.7 percent; relative risk, 1.38; P=0.001), but there were not significantly more patients with episodes of life-threatening bleeding (2.1 percent vs. 1.8 percent, P=0.13) or hemorrhagic strokes. Conclusions: The antiplatelet agent clopidogrel has beneficial effects in patients with acute coronary syndromes without ST-segment elevation. However, the risk of major bleeding is increased among patients treated with clopidogrel. (N Engl J Med 2001;345:494-502.) Copyright (C) 2001 Massachusetts Medical Society
The Bak-Tang-Wiesenfeld sandpile model around the upper critical dimension
We consider the Bak-Tang-Wiesenfeld sandpile model on square lattices in
different dimensions (D>=6). A finite size scaling analysis of the avalanche
probability distributions yields the values of the distribution exponents, the
dynamical exponent, and the dimension of the avalanches. Above the upper
critical dimension D_u=4 the exponents equal the known mean field values. An
analysis of the area probability distributions indicates that the avalanches
are fractal above the critical dimension.Comment: 7 pages, including 9 figures, accepted for publication in Physical
Review
Crossover phenomenon in self-organized critical sandpile models
We consider a stochastic sandpile where the sand-grains of unstable sites are
randomly distributed to the nearest neighbors. Increasing the value of the
threshold condition the stochastic character of the distribution is lost and a
crossover to the scaling behavior of a different sandpile model takes place
where the sand-grains are equally transferred to the nearest neighbors. The
crossover behavior is numerically analyzed in detail, especially we consider
the exponents which determine the scaling behavior.Comment: 6 pages, 9 figures, accepted for publication in Physical Review
Transitions in non-conserving models of Self-Organized Criticality
We investigate a random--neighbours version of the two dimensional
non-conserving earthquake model of Olami, Feder and Christensen [Phys. Rev.
Lett. {\bf 68}, 1244 (1992)]. We show both analytically and numerically that
criticality can be expected even in the presence of dissipation. As the
critical level of conservation, , is approached, the cut--off of the
avalanche size distribution scales as . The
transition from non-SOC to SOC behaviour is controlled by the average branching
ratio of an avalanche, which can thus be regarded as an order
parameter of the system. The relevance of the results are discussed in
connection to the nearest-neighbours OFC model (in particular we analyse the
relevance of synchronization in the latter).Comment: 8 pages in latex format; 5 figures available upon reques
Eruptive history of the Late Quaternary Ciomadul (Csomád) volcano, East Carpathians, part II: magma output rates
This study, which builds on high-precision unspiked Cassignol-Gillot K-Ar age determinations, presents an advanced DEMbased volumetrical analysis to infer long-term magma output rates for the Late Quaternary Ciomadul (Csomád) dacitic lava dome complex (East Carpathians, Romania). The volcanic field of Ciomadul developed on the erosional surface of Lower Cretaceous flysch and ~ 2 Ma old andesites and experienced an extended eruptive history from ~ 850 to < 30 ka. Predominantly effusive activity took place during the first stage (~ 850 to ~ 440 ka), producing volumetrically minor, isolated, peripheral domes. Subsequently, after a ~ 250 ky repose interval, a voluminous central dome cluster developed in the second stage (~ 200 to < 30 ka). During the youngest phase of evolution (~ 60 to < 30 ka), highly explosive eruptions also occurred, resulting in the formation of two craters (Mohos and St. Ana). The calculated ~ 8.00 ± 0.55 km3 total volume of the lava domes, which includes the related volcaniclastic (1.57 km3 ) as well as erosionally removed (0.18 km3 ) material, is in line with dimensions of other medium-sized dacitic lava domes worldwide. This volume was extruded at an average long-term magma output rate of 9.76 km3 / My (0.0098 km3 /ky). However, most of the domes (7.53 ± 0.51 km3 ) were formed in the 200 to < 30 ka period, implying a significantly increased magma output rate of 37.40 km3 /My (0.0374 km3 /ky), more than 30 times higher than in the first stage. Within these long-term trends, individual lava domes of Ciomadul (e.g. those with volumes between 0.02 and 0.40 km3 ) would have been emplaced at much higher rates over a period of years to tens of years. The active periods, lasting up to hundreds of years, would have been followed by repose periods ~ 30 times longer. The most recent eruption of Ciomadul has been dated here at 27.7 ± 1.4 ka. This age, which is in agreement with radiocarbon dates for the onset of lake sediment accumulation in St. Ana crater, dates fragmented lava blocks which are possibly related to a disrupted dome. This suggests that during the last, typically explosive, phase of Ciomadul, lava dome extrusion was still ongoing. In a global context, the analysis of the volumetric dynamism of Ciomadul’s activity gives insights into the temporal variations in magma output; at lava domes, short-term (dayor week-scale) eruption rates smooth out in long-term (millenia-scale) output rates which are tens of times lower
Random Neighbor Theory of the Olami-Feder-Christensen Earthquake Model
We derive the exact equations of motion for the random neighbor version of
the Olami-Feder-Christensen earthquake model in the infinite-size limit. We
solve them numerically, and compare with simulations of the model for large
numbers of sites. We find perfect agreement. But we do not find any scaling or
phase transitions, except in the conservative limit. This is in contradiction
to claims by Lise & Jensen (Phys. Rev. Lett. 76, 2326 (1996)) based on
approximate solutions of the same model. It indicates again that scaling in the
Olami-Feder-Christensen model is only due to partial synchronization driven by
spatial inhomogeneities. Finally, we point out that our method can be used also
for other SOC models, and treat in detail the random neighbor version of the
Feder-Feder model.Comment: 18 pages, 6 ps-figures included; minor correction in sec.
Scaling in a Nonconservative Earthquake Model of Self-Organised Criticality
We numerically investigate the Olami-Feder-Christensen model for earthquakes
in order to characterise its scaling behaviour. We show that ordinary finite
size scaling in the model is violated due to global, system wide events.
Nevertheless we find that subsystems of linear dimension small compared to the
overall system size obey finite (subsystem) size scaling, with universal
critical coefficients, for the earthquake events localised within the
subsystem. We provide evidence, moreover, that large earthquakes responsible
for breaking finite size scaling are initiated predominantly near the boundary.Comment: 6 pages, 6 figures, to be published in Phys. Rev. E; references
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