108 research outputs found
Complexity in dislocation dynamics: model
We propose a numerical model to study the viscoplastic deformation of ice
single crystals. We consider long-range elastic interactions among
dislocations, the possibility of mutual annihilation, and a multiplication
mechanism representing the activation of Frank-Read sources due to dislocation
pinning. The overdamped equations of motion for a collection of dislocations
are integrated numerically using different externally applied stresses. Using
this approach we analyze the avalanche-like rearrangements of dislocations
during the dynamic evolution. We observe a power law distribution of avalanche
sizes which we compare with acoustic emission experiments in ice single
crystals under creep deformation. We emphasize the connections of our model
with non-equilibrium phase transitions and critical phenomena
Statistical analysis of rockfall volume distributions: implications for rockfall dynamics.
International audienceWe analyze the volume distribution of natural rockfalls on different geological settings (i.e., calcareous cliffs in the French Alps, Grenoble area, and granite Yosemite cliffs, California Sierra) and different volume ranges (i.e., regional and worldwide catalogs). Contrary to previous studies that included several types of landslides, we restrict our analysis to rockfall sources which originated on subvertical cliffs. For the three data sets, we find that the rockfall volumes follow a power law distribution with a similar exponent value, within error bars. This power law distribution was also proposed for rockfall volumes that occurred along road cuts. All these results argue for a recurrent power law distribution of rockfall volumes on subvertical cliffs, for a large range of rockfall sizes (102–1010 m3), regardless of the geological settings and of the preexisting geometry of fracture patterns that are drastically different on the three studied areas. The power law distribution for rockfall volumes could emerge from two types of processes. First, the observed power law distribution of rockfall volumes is similar to the one reported for both fragmentation experiments and fragmentation models. This argues for the geometry of rock mass fragment sizes to possibly control the rockfall volumes. This way neither cascade nor avalanche processes would influence the rockfall volume distribution. Second, without any requirement of scale-invariant quenched heterogeneity patterns, the rock mass dynamics can arise from avalanche processes driven by fluctuations of the rock mass properties, e.g., cohesion or friction angle. This model may also explain the power law distribution reported for landslides involving unconsolidated materials. We find that the exponent values of rockfall volume on subvertical cliffs, 0.5 ± 0.2, is significantly smaller than the 1.2 ± 0.3 value reported for mixed landslide types. This change of exponents can be driven by the material strength, which controls the in situ topographic slope values, as simulated in numerical models of landslides [Densmore et al., 1998; Champel et al., 2002]. INDEX TERMS: 5104 Physical Properties of Rocks: Fracture and flow; 1815 Hydrology: Erosion and sedimentation; 8122 Tectonophysics: Dynamics, gravity and tectonics
Line creep in paper peeling
The dynamics of a "peeling front" or an elastic line is studied under creep
(constant load) conditions. Our experiments show an exponential dependence of
the creep velocity on the inverse force (mass) applied. In particular, the
dynamical correlations of the avalanche activity are discussed here. We compare
various avalanche statistics to those of a line depinning model with non-local
elasticity, and study various measures of the experimental avalanche-avalanche
and temporal correlations such as the autocorrelation function of the released
energy and aftershock activity. From all these we conclude, that internal
avalanche dynamics seems to follow "line depinning" -like behavior, in rough
agreement with the depinning model. Meanwhile, the correlations reveal subtle
complications not implied by depinning theory. Moreover, we also show how these
results can be understood from a geophysical point of view.Comment: 22 pages, 14 fig
Evolution of seismic signals and slip patterns along subduction zones: insights from a friction lab scale experiment.
International audienceContinuous GPS and broadband seismic monitoring have revealed a variety of disparate slip patterns especially in shallow dipping subduction zones, among which regular earthquakes, slow slip events and silent quakes1,2. Slow slip events are sometimes accompanied by Non Volcanic Tremors (NVT), which origin remains unclear3, either related to fluid migration or to friction. The present understanding of the whole menagerie of slip patterns is based upon numerical simulations imposing ad hoc values of the rate and state parameters a and b4-6 derived from the temperature dependence of a and b of a wet granite gouge7. Here we investigate the influence of the cumulative slip on the frictional and acoustic patterns of a lab scale subduction zone. Shallow loud earthquakes (stick-slip events), medium depth slow, deeper silent quakes (smooth sliding oscillations) and deepest steady-state creep (continuous sliding) are reproduced by the ageing of contact interface with cumulative displacement8. The Acoustic Emission evolves with cumulative displacement and interface ageing, following a trend from strong impulsive events, similar to earthquake seismic signals, to a collection of smaller amplitude and longer duration signals, similar to Non Volcanic Tremors. NVT emerge as the recollection of the local unstable behaviour of the contact interface globally evolving towards the stable sliding regime
Predictability of Volcano Eruption: lessons from a basaltic effusive volcano
Volcano eruption forecast remains a challenging and controversial problem
despite the fact that data from volcano monitoring significantly increased in
quantity and quality during the last decades.This study uses pattern
recognition techniques to quantify the predictability of the 15 Piton de la
Fournaise (PdlF) eruptions in the 1988-2001 period using increase of the daily
seismicity rate as a precursor. Lead time of this prediction is a few days to
weeks. Using the daily seismicity rate, we formulate a simple prediction rule,
use it for retrospective prediction of the 15 eruptions,and test the prediction
quality with error diagrams. The best prediction performance corresponds to
averaging the daily seismicity rate over 5 days and issuing a prediction alarm
for 5 days. 65% of the eruptions are predicted for an alarm duration less than
20% of the time considered. Even though this result is concomitant of a large
number of false alarms, it is obtained with a crude counting of daily events
that are available from most volcano observatoriesComment: 4 pages, 4 figure
A Two-Threshold Model for Scaling Laws of Non-Interacting Snow Avalanches
The sizes of snow slab failure that trigger snow avalanches are power-law
distributed. Such a power-law probability distribution function has also been
proposed to characterize different landslide types. In order to understand this
scaling for gravity driven systems, we introduce a two-threshold 2-d cellular
automaton, in which failure occurs irreversibly. Taking snow slab avalanches as
a model system, we find that the sizes of the largest avalanches just
preceeding the lattice system breakdown are power law distributed. By tuning
the maximum value of the ratio of the two failure thresholds our model
reproduces the range of power law exponents observed for land-, rock- or snow
avalanches. We suggest this control parameter represents the material cohesion
anisotropy.Comment: accepted PR
Intermittent dislocation flow in viscoplastic deformation
The viscoplastic deformation (creep) of crystalline materials under constant
stress involves the motion of a large number of interacting dislocations.
Analytical methods and sophisticated `dislocation-dynamics' simulations have
proved very effective in the study of dislocation patterning, and have led to
macroscopic constitutive laws of plastic deformation. Yet, a statistical
analysis of the dynamics of an assembly of interacting dislocations has not
hitherto been performed. Here we report acoustic emission measurements on
stressed ice single crystals, the results of which indicate that dislocations
move in a scale-free intermittent fashion. This result is confirmed by
numerical simulations of a model of interacting dislocations that successfully
reproduces the main features of the experiment. We find that dislocations
generate a slowly evolving configuration landscape which coexists with rapid
collective rearrangements. These rearrangements involve a comparatively small
fraction of the dislocations and lead to an intermittent behavior of the net
plastic response. This basic dynamical picture appears to be a generic feature
in the deformation of many other materials. Moreover, it should provide a
framework for discussing fundamental aspects of plasticity, that goes beyond
standard mean-field approaches that see plastic deformation as a smooth laminar
flow
Short term forecasting of explosions at Ubinas volcano, PerĂş
Most seismic eruption forerunners are described using Volcano-Tectonic earthquakes, seismic energy release, deformation rates or seismic noise analyses. Using the seismic data recorded at Ubinas volcano (Peru) between 2006 and 2008, we explore the time evolution of the Long Period (LP) seismicity rate prior to 143 explosions. We resolve an average acceleration of the LP rate above the background level during the 2-3 hours preceding the explosion onset. Such an average pattern, which emerges when stacking over LP time series, is robust and stable over all the 2006-2008 period, for which data is available. This accelerating pattern is also recovered when conditioning the LP rate on the occurrence of an other LP event, rather than on the explosion time. It supports a common mechanism for the generation of explosions and LP events, the magma conduit pressure increase being the most probable candidate. The average LP rate acceleration toward an explosion is highly significant prior to the higher energy explosions, supposedly the ones associated with the larger pressure increases. The dramatic decay of the LP activity following explosions, still reinforce the strong relationship between these two processes. We test and we quantify the retrospective forecasting power of these LP rate patterns to predict Ubinas explosions. The prediction quality of the forecasts (e.g. for 17% of alarm time, we predict 63% of Ubinas explosions, with 58% of false alarms) is evaluated using error diagrams. The prediction results are stable and the prediction algorithm validated, i.e. its performance is better than the random guess
Long Term Friction: from Stick-Slip to Stable Sliding
We have devised an original laboratory experiment where we investigate the
frictional behaviour of a single crystal salt slider over a large number of
deformation cycles. Because of its physical properties, salt, a surrogate for
natural faults, allows for friction and plastic deformation and pressure
solution creep to be efficient on the same timescale. During the same
experiment, we observe a continuous change of the frictional behaviour of the
slider under constant conditions of stiffness, temperature and loading
velocity. The stick-slip regime is progressively vanishing, eventually reaching
the stable sliding regime. Concomitantly, the contact interface, observed under
the microscope, develops a striated morphology with contact asperities increase
in length and width, arguing for an increase in the critical slip distance dc.
Complementary experiments including velocity jumps show that the frictional
parameters of the rate and state friction law, a and b, progressively vanish
with the cumulative slip. In our experimental conditions, the ultimate stage of
friction is therefore rate and state independent.Comment: 10 pages; 4 figures; 1 Tabl
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