52 research outputs found
Crossover from quasi-static to dense flow regime in compressed frictional granular media
We investigate the evolution of multi-scale mechanical properties towards the
macroscopic mechanical instability in frictional granular media under
multiaxial compressive loading. Spatial correlations of shear stress
redistribution following nucleating contact sliding events and shear strain
localization are investigated. We report growing correlation lengths associated
to both shear stress and shear strain fields that diverge simultaneously as
approaching the transition to a dense flow regime. This shows that the
transition from quasi static to dense flow regime can be interpreted as a
critical phase transition. Our results suggest that no shear band with a
characteristic thickness has formed at the onset of instability
A physical model for seismic noise generation by turbulent flow in rivers
Previous studies suggest that the seismic noise induced by rivers may be used to infer river transport properties, and previous theoretical work showed that bedload sediment flux can be inverted from seismic data. However, the lack of a theoretical framework relating water flow to seismic noise prevents these studies from providing accurate bedload fluxes and quantitative information on flow processes. Here we propose a forward model of seismic noise caused by turbulent flow. In agreement with previous observations, modeled turbulent flow-induced noise operates at lower frequencies than bedload-induced noise. Moreover, the differences in the spectral signatures of turbulent flow-induced and bedload-induced forces at the riverbed are significant enough that these two processes can be characterized independently using seismic records acquired at various distances from the river. In cases with isolated turbulent flow noise, we suggest that riverbed stress can be inverted. Finally, we validate our model by comparing predictions to previously reported observations. We show that our model captures the spectral peak located around 6–7 Hz and previously attributed to water flow at Hance Rapids in the Colorado River (United States); we also show that turbulent flow causes a significant part of the seismic noise recorded at the Trisuli River in Nepal, which reveals that the hysteresis curve previously reported there does not solely include bedload, but is also largely influenced by turbulent flow-induced noise. We expect the framework presented here to be useful to invert realistic bedload fluxes by enabling the removal of the turbulent flow contribution from seismic data
Scaling analysis of deformation field within granular materials: application to strain localization
Discrete element method (DEM) simulations using periodic boundary conditions and molecular dynamics are conducted on a frictional granular media. Two dimensional strain controlled biaxial tests are carried out on an assembly of circular particles interacting via elastic contacts and Coulomb friction. The spatial correlations that take place within the deformation field along the loading path are tracked by a scaling analysis of the continuous strain rate field. This method allows us to discuss the degree of strain localization occurring throughout the test. The analysis of the correlation length in the early stages of macroscopic deformation
leads to the identification of two distinct behaviors. First, a divergence of the correlation length on the first deformation invariant, i.e. the divergence, is reported at the onset of macroscopic dilation. This suggests an interpretation of the contraction peak as a critical point. Secondly, an increase of the correlation length on the second deformation invariant, i.e. the shear, is also observed before the peak load. However, saturation remains on the scaling law. We argue that this second behavior is associated to macroscopic shear banding: our analysis accurately gives its outbreak on the stress versus strain curve. Finally, a dependence of the correlation length as a function of the deformation window considered is reported. This shows that scaling
properties within the deformation field emerge from long range interactions within an assembly of rigid frictional particles
Predicting short-period, wind-wave-generated seismic noise in coastal regions
Substantial effort has recently been made to predict seismic energy caused by ocean waves in the 4–10 s period range. However, little work has been devoted to predict shorter period seismic waves recorded in coastal regions. Here we present an analytical framework that relates the signature of seismic noise recorded at 0.6–2 s periods (0.5–1.5 Hz frequencies) in coastal regions with deep-ocean wave properties. Constraints on key model parameters such as seismic attenuation and ocean wave directionality are provided by jointly analyzing ocean-floor acoustic noise and seismic noise measurements. We show that 0.6–2 s seismic noise can be consistently predicted over the entire year. The seismic noise recorded in this period range is mostly caused by local wind-waves, i.e. by wind-waves occurring within about 2000 km of the seismic station. Our analysis also shows that the fraction of ocean waves traveling in nearly opposite directions is orders of magnitude smaller than previously suggested for wind-waves, does not depend strongly on wind speed as previously proposed, and instead may depend weakly on the heterogeneity of the wind field. This study suggests that wind-wave conditions can be studied in detail from seismic observations, including under specific conditions such as in the presence of sea ice
(Finite) statistical size effects on compressive strength
The larger structures are, the lower their mechanical strength. Already discussed by Leonardo da Vinci and Edmé Mariotte several centuries ago, size effects on strength remain of crucial importance in modern engineering for the elaboration of safety regulations in structural design or the extrapolation of laboratory results to geophysical field scales. Under tensile loading, statistical size effects are traditionally modeled with a weakest-link approach. One of its prominent results is a prediction of vanishing strength at large scales that can be quantified in the framework of extreme value statistics. Despite a frequent use outside its range of validity, this approach remains the dominant tool in the field of statistical size effects. Here we focus on compressive failure, which concerns a wide range of geophysical and geotechnical situations. We show on historical and recent experimental data that weakest-link predictions are not obeyed. In particular, the mechanical strength saturates at a nonzero value toward large scales. Accounting explicitly for the elastic interactions between defects during the damage process, we build a formal analogy of compressive failure with the depinning transition of an elastic manifold. This critical transition interpretation naturally entails finite-size scaling laws for the mean strength and its associated variability. Theoretical predictions are in remarkable agreement with measurements reported for various materials such as rocks, ice, coal, or concrete. This formalism, which can also be extended to the flowing instability of granular media under multiaxial compression, has important practical consequences for future design rules
Sea ice inertial oscillations in the Arctic Basin
International audienceAn original method to quantify the amplitude of inertial motion of oceanic and ice drifters, through the introduction of a non-dimensional parameter M defined from a spectral analysis, is presented. A strong seasonal dependence of the magnitude of sea ice inertial oscillations is revealed, in agreement with the corresponding annual cycles of sea ice extent, concentration, thickness, advection velocity, and deformation rates. The spatial pattern of the magnitude of the sea ice inertial oscillations over the Arctic Basin is also in agreement with the sea ice thickness and concentration patterns. This argues for a strong interaction between the magnitude of inertial motion on one hand, the dissipation of energy through mechanical processes, and the cohesiveness of the cover on the other hand. Finally, a significant multi-annual evolution towards greater magnitudes of inertial oscillations in recent years, in both summer and winter, is reported, thus concomitant with reduced sea ice thickness, concentration and spatial extent
Research Letter
Water that pressurizes the base of glaciers and ice sheets enhances glacier velocities and modulates glacial erosion. Predicting ice flow and erosion therefore requires knowledge of subglacial channel evolution, which remains observationally limited.Water that pressurizes the base of glaciers and ice sheets enhances glacier velocities and
modulates glacial erosion. Predicting ice flow and erosion therefore requires knowledge of subglacial
channel evolution, which remains observationally limited. Here we demonstrate that detailed analysis of
seismic ground motion caused by subglacial water flow at Mendenhall Glacier (Alaska) allows for continuous
measurement of daily to subseasonal changes in basal water pressure gradient, channel size, and sediment
transport. We observe intermittent subglacial water pressure gradient changes during the melt season, at
odds with common assumptions of slowly varying, low-pressure channels. These observations indicate
that changes in channel size do not keep pace with changes in discharge. This behavior strongly affects
glacier dynamics and subglacial channel erosion at Mendenhall Glacier, where episodic periods of high water
pressure gradients enhance glacier surface velocity and channel sediment transport by up to 30% and
50%, respectively. We expect the application of this framework to future seismic observations acquired at
glaciers worldwide to improve our understanding of subglacial processes.This study was funded by NSF grant EAR-1453263. We thank Flavien Beaud and an anonymous reviewer for thorough reviews that improved the manuscript. We also thank Michael Lamb, Olivier Gagliardini, Jean-Philippe Avouac, Gael Durand and Adrien Gilbert for fruitful discussions.Ye
Subseasonal changes observed in subglacial channel pressure, size, and sediment transport
Water that pressurizes the base of glaciers and ice sheets enhances glacier velocities and modulates glacial erosion. Predicting ice flow and erosion therefore requires knowledge of subglacial channel evolution, which remains observationally limited. Here we demonstrate that detailed analysis of seismic ground motion caused by subglacial water flow at Mendenhall Glacier (Alaska) allows for continuous measurement of daily to subseasonal changes in basal water pressure gradient, channel size, and sediment transport. We observe intermittent subglacial water pressure gradient changes during the melt season, at odds with common assumptions of slowly varying, low-pressure channels. These observations indicate that changes in channel size do not keep pace with changes in discharge. This behavior strongly affects glacier dynamics and subglacial channel erosion at Mendenhall Glacier, where episodic periods of high water pressure gradients enhance glacier surface velocity and channel sediment transport by up to 30% and 50%, respectively. We expect the application of this framework to future seismic observations acquired at glaciers worldwide to improve our understanding of subglacial processes
Seismic Mapping of Subglacial Hydrology Reveals Previously Undetected Pressurization Event
Understanding the dynamic response of glaciers to climate change is vital for assessing water
resources and hazards, and subglacial hydrology is a key player in glacier systems. Traditional observations
of subglacial hydrology are spatially and temporally limited, but recent seismic deployments on and around
glaciers show the potential for comprehensive observation of glacial hydrologic systems. We present results
from a high-density seismic deployment spanning the surface of Lemon Creek Glacier, Alaska. Our study
coincided with a marginal lake drainage event, which served as a natural experiment for seismic detection of
changes in subglacial hydrology. We observed glaciohydraulic tremor across the surface of the glacier that
was generated by the subglacial hydrologic system. During the lake drainage, the relative changes in seismic
tremor power and water flux are consistent with pressurization of the subglacial system of only the upper
part of the glacier. This event was not accompanied by a significant increase in glacier velocity; either some
threshold necessary for rapid basal motion was not attained, or, plausibly, the geometry of Lemon Creek Glacier
inhibited speedup. This pressurization event would have likely gone undetected without seismic observations,
demonstrating the power of cryoseismology in testing assumptions about and mapping the spatial extent of
subglacial pressurization.This work was made possible in part by
hard work in the field by Margot Vore,
Daniel Bowden, Galen Kaip, and the
students and staff of the 2017 Juneau
Icefield Research Program. We especially
thank Matt Beedle for provision of the
photogrammetrically-produced DEM
of Lake Linda, following lake drainage.
This work was also aided by the advice
of Mike Gurnis and Rob Clayton. We
thank Paul Winberry and two anonymous
reviewers for their helpful feedback,
which improved this paper greatly. This
material is based upon work supported by
the National Science Foundation Graduate Research Fellowship under Grant
No. DGE-1745301. This work was made
possible in part by a University of Idaho
seed grant, #FY18-01. DEM provided
by the Polar Geospatial Center under
NSF-OPP awards 1043681, 1559691, and
1542736.Ye
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