293 research outputs found
The role of static stress diffusion in the spatio-temporal organization of aftershocks
We investigate the spatial distribution of aftershocks and we find that
aftershock linear density exhibits a maximum, that depends on the mainshock
magnitude, followed by a power law decay. The exponent controlling the
asymptotic decay and the fractal dimensionality of epicenters clearly indicate
triggering by static stress. The non monotonic behavior of the linear density
and its dependence on the mainshock magnitude can be interpreted in terms of
diffusion of static stress. This is supported by the power law growth with
exponent of the average main-aftershock distance. Implementing
static stress diffusion within a stochastic model for aftershock occurrence we
are able to reproduce aftershock linear density spatial decay, its dependence
on the mainshock magnitude and its evolution in time.Comment: 4 figure
Time-energy correlations in solar flare occurrence
The existence of time-energy correlations in flare occurrence is still an
open and much debated problem. This study addresses the question whether
statistically significant correlations are present between energies of
successive flares as well as energies and waiting times. We analyze the GOES
catalog with a statistical approach based on the comparison of the real catalog
with a reshuffled one where energies are decorrelated. This analysis reduces
the effect of background activity and is able to reveal the role of
obscuration. We show the existence of non-trivial correlations between waiting
times and energies, as well as between energies of subsequent flares. More
precisely, we find that flares close in time tend to have the second event with
large energy. Moreover, after large flares the flaring rate significantly
increases, together with the probability of other large flares. Results suggest
that correlations between energies and waiting times are a physical property
and not an effect of obscuration. These findings could give important
information on the mechanisms for energy storage and release in the solar
corona
Synchronized oscillations and acoustic fluidization in confined granular materials
According to the acoustic fluidization hypothesis, elastic waves at a
characteristic frequency form inside seismic faults even in the absence of an
external perturbation. These waves are able to generate a normal stress which
contrasts the confining pressure and promotes failure. Here, we study the
mechanisms responsible for this wave activation via numerical simulations of a
granular fault model. We observe the particles belonging to the percolating
backbone, which sustains the stress, to perform synchronized oscillations over
ellipticlike trajectories in the fault plane. These oscillations occur at the
characteristic frequency of acoustic fluidization. As the applied shear stress
increases, these oscillations become perpendicular to the fault plane just
before the system fails, opposing the confining pressure, consistently with the
acoustic fluidization scenario. The same change of orientation can be induced
by external perturbations at the acoustic fluidization frequency
Induced and endogenous acoustic oscillations in granular faults
The frictional properties of disordered systems are affected by external
perturbations. These perturbations usually weaken the system by reducing the
macroscopic friction coefficient. This friction reduction is of particular
interest in the case of disordered systems composed of granular particles
confined between two plates, as this is a simple model of seismic fault.
Indeed, in the geophysical context frictional weakening could explain the
unexpected weakness of some faults, as well as earthquake remote triggering. In
this manuscript we review recent results concerning the response of confined
granular systems to external perturbations, considering the different
mechanisms by which the perturbation could weaken a system, the relevance of
the frictional reduction to earthquakes, as well as discussing the intriguing
scenario whereby the weakening is not monotonic in the perturbation frequency,
so that a re-entrant transition is observed, as the system first enters a
fluidized state and then returns to a frictional state.Comment: 15 pages, 12 figure
Generic features of the fluctuation dissipation relation in coarsening systems
The integrated response function in phase-ordering systems with scalar,
vector, conserved and non conserved order parameter is studied at various space
dimensionalities. Assuming scaling of the aging contribution we obtain, by numerical simulations
and analytical arguments, the phenomenological formula describing the
dimensionality dependence of in all cases considered. The primary
result is that vanishes continuously as approaches the lower
critical dimensionality . This implies that i) the existence of a non
trivial fluctuation dissipation relation and ii) the failure of the connection
between statics and dynamics are generic features of phase ordering at .Comment: 6 pages, 5 figure
Memory in Self Organized Criticality
Many natural phenomena exhibit power law behaviour in the distribution of
event size. This scaling is successfully reproduced by Self Organized
Criticality (SOC). On the other hand, temporal occurrence in SOC models has a
Poisson-like statistics, i.e. exponential behaviour in the inter-event time
distribution, in contrast with experimental observations. We present a SOC
model with memory: events are nucleated not only as a consequence of the
instantaneous value of the local field with respect to the firing threshold,
but on the basis of the whole history of the system. The model is able to
reproduce the complex behaviour of inter-event time distribution, in excellent
agreement with experimental seismic data
Estimating the generation interval from the incidence rate, the optimal quarantine duration and the efficiency of fast switching periodic protocols for COVID‑19
The transmissibility of an infectious disease is usually quantified in terms of the reproduction
number Rt representing, at a given time, the average number of secondary cases caused by an
infected individual. Recent studies have enlightened the central role played by w(z), the distribution
of generation times z, namely the time between successive infections in a transmission chain. In
standard approaches this quantity is usually substituted by the distribution of serial intervals, which
is obtained by contact tracing after measuring the time between onset of symptoms in successive
cases. Unfortunately, this substitution can cause important biases in the estimate of Rt . Here we
present a novel method which allows us to simultaneously obtain the optimal functional form of
w(z) together with the daily evolution of Rt , over the course of an epidemic. The method uses, as
unique information, the daily series of incidence rate and thus overcomes biases present in standard
approaches. We apply our method to one year of data from COVID-19 officially reported cases in the
21 Italian regions, since the first confirmed case on February 2020. We find that w(z) has mean value
z ≃ 6 days with a standard deviation a ≃ 1 day, for all Italian regions, and these values are stable
even if one considers only the first 10 days of data recording. This indicates that an estimate of the
most relevant transmission parameters can be already available in the early stage of a pandemic. We
use this information to obtain the optimal quarantine duration and to demonstrate that, in the case
of COVID-19, post-lockdown mitigation policies, such as fast periodic switching and/or alternating
quarantine, can be very efficient
Interface fluctuations, bulk fluctuations and dimensionality in the off-equilibrium response of coarsening systems
The relationship between statics and dynamics proposed by Franz, Mezard,
Parisi and Peliti (FMPP) for slowly relaxing systems [Phys.Rev.Lett. {\bf 81},
1758 (1998)] is investigated in the framework of non disordered coarsening
systems. Separating the bulk from interface response we find that for statics
to be retrievable from dynamics the interface contribution must be
asymptotically negligible. How fast this happens depends on dimensionality.
There exists a critical dimensionality above which the interface response
vanishes like the interface density and below which it vanishes more slowly. At
the interface response does not vanish leading to the violation of the
FMPP scheme. This behavior is explained in terms of the competition between
curvature driven and field driven interface motion.Comment: 11 pages, 3 figures. Significantly improved version of the paper with
new results, new numerical simulations and new figure
Crossover in Growth Law and Violation of Superuniversality in the Random Field Ising Model
We study the nonconserved phase ordering dynamics of the d = 2, 3 random
field Ising model, quenched to below the critical temperature. Motivated by the
puzzling results of previous work in two and three di- mensions, reporting a
crossover from power-law to logarithmic growth, together with superuniversal
behavior of the correlation function, we have undertaken a careful
investigation of both the domain growth law and the autocorrelation function.
Our main results are as follows: We confirm the crossover to asymptotic
logarithmic behavior in the growth law, but, at variance with previous
findings, the exponent in the preasymptotic power law is disorder-dependent,
rather than being the one of the pure system. Furthermore, we find that the
autocorre- lation function does not display superuniversal behavior. This
restores consistency with previous results for the d = 1 system, and fits
nicely into the unifying scaling scheme we have recently proposed in the study
of the random bond Ising model.Comment: To be published in Physical Review
On the influence of time and space correlations on the next earthquake magnitude
A crucial point in the debate on feasibility of earthquake prediction is the
dependence of an earthquake magnitude from past seismicity. Indeed, whilst
clustering in time and space is widely accepted, much more questionable is the
existence of magnitude correlations. The standard approach generally assumes
that magnitudes are independent and therefore in principle unpredictable. Here
we show the existence of clustering in magnitude: earthquakes occur with higher
probability close in time, space and magnitude to previous events. More
precisely, the next earthquake tends to have a magnitude similar but smaller
than the previous one. A dynamical scaling relation between magnitude, time and
space distances reproduces the complex pattern of magnitude, spatial and
temporal correlations observed in experimental seismic catalogs.Comment: 4 Figure
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