471 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
Viscosity critical behaviour at the gel point in a 3d lattice model
Within a recently introduced model based on the bond-fluctuation dynamics we
study the viscoelastic behaviour of a polymer solution at the gelation
threshold. We here present the results of the numerical simulation of the model
on a cubic lattice: the percolation transition, the diffusion properties and
the time autocorrelation functions have been studied. From both the diffusion
coefficients and the relaxation times critical behaviour a critical exponent k
for the viscosity coefficient has been extracted: the two results are
comparable within the errors and are in close agreement with the Rouse model
prediction and with some experimental results. In the critical region below the
transition threshold the time autocorrelation functions show a long time tail
which is well fitted by a stretched exponential decay.Comment: 14 pag., RevTex, 9 figure
Critical neural networks with short and long term plasticity
In recent years self organised critical neuronal models have provided
insights regarding the origin of the experimentally observed avalanching
behaviour of neuronal systems. It has been shown that dynamical synapses, as a
form of short-term plasticity, can cause critical neuronal dynamics. Whereas
long-term plasticity, such as hebbian or activity dependent plasticity, have a
crucial role in shaping the network structure and endowing neural systems with
learning abilities. In this work we provide a model which combines both
plasticity mechanisms, acting on two different time-scales. The measured
avalanche statistics are compatible with experimental results for both the
avalanche size and duration distribution with biologically observed percentages
of inhibitory neurons. The time-series of neuronal activity exhibits temporal
bursts leading to 1/f decay in the power spectrum. The presence of long-term
plasticity gives the system the ability to learn binary rules such as XOR,
providing the foundation of future research on more complicated tasks such as
pattern recognition.Comment: 8 pages, 7 figure
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
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
Fracture in Three-Dimensional Fuse Networks
We report on large scale numerical simulations of fracture surfaces using
random fuse networks for two very different disorders. There are some
properties and exponents that are different for the two distributions, but
others, notably the roughness exponents, seem universal. For the universal
roughness exponent we found a value of zeta = 0.62 +/- 0.05. In contrast to
what is observed in two dimensions, this value is lower than that reported in
experimental studies of brittle fractures, and rules out the minimal energy
surface exponent, 0.41 +/- 0.01.Comment: 4 pages, RevTeX, 5 figures, Postscrip
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