Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A53: Geophysical Applications of Granular FlowsInvited
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Sponsoring Units: GSNP Chair: Corey O'Hern, Yale Univ Room: BCEC 253C |
Monday, March 4, 2019 8:00AM - 8:36AM |
A53.00001: Geophysical applications of granular flows: probing earthquakes applying machine learning Invited Speaker: Paul Johnson The influence of granular flows is widespread in geophysics, from the physics of earthquakes, avalanche and landslide, to debris flow to the effects of strong ground motion due to earthquakes. In this presentation I focus on earthquakes and the influence of the granular core of the fault—the fault gouge. Earthquakes take place when two juxtaposed fault blocks are stressed sufficiently to overcome the frictional force holding them in place and they abruptly slip relative to each other. Earthquake faults exhibit a continuum of behaviors ranging from stick slip associated with strong shaking, to slow slip which is primarily aseismic, to very slow slip that is essentially aseismic and can take place over hours to months. The different slip behaviors appear to be controlled by the combined elasticity of the fault gouge the adjacent fault blocks. As faults slip, the fault core emits elastic waves. We analyze these waves applying machine learning continuous acoustic data streams in the laboratory and continuous seismic data streams in Earth. We use as labels characteristics of the measured fault slip behavior in the laboratory such as the fault friction, shear displacement and fault thickness. In Earth, we use surface displacement as determined by Global Positioning Systems (GPS). We find that the laboratory acoustic data and the Earth seismic data are a type of Rosetta Stone revealing fault characteristics at all times and fault displacements. This is a surprising observation because previously we believed most or much of the signal was noise. Here we describe an overview of recent work in this area and also describe recent efforts on parallel problems such as volcanoes and geysers. |
Monday, March 4, 2019 8:36AM - 9:12AM |
A53.00002: Incipient Motion in Granular Beds Driven by Shear Flows Invited Speaker: Nicholas Ouellette Granular packings on Earth's surface are regularly subjected to shear flows, as they are typically exposed either to moving water or air. These flows agitate the packings, and may erode them by removing grains given sufficient flow strength. Incipient motion of the packings (that is, the point in parameter space when a nonzero net downstream flux of grains appears) is typically thought to occur at a critical value of the Shields number, which balances the shear stress delivered to the grains with the weight of a grain. However, such a simple framework cannot directly account for additional but common physics, including granular packing effects, turbulent flow fluctuations, or the stress delivered to the bed by other mobile grains. I will discuss both simple numerical models and laboratory experiments aimed at elucidating the consequences of these effects on describing incipient motion, with particular application to the armoring phenomenon observed in gravel-bedded rivers. |
Monday, March 4, 2019 9:12AM - 9:48AM |
A53.00003: Shear jamming in packings of frictional disks Invited Speaker: Mark Shattuck
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Monday, March 4, 2019 9:48AM - 10:24AM |
A53.00004: Yielding in granular materials: from riverbeds to renormalization group Invited Speaker: Abe Clark Granular media, like many other amorphous materials, possess a yield stress. The nature of yielding in granular materials is relevant to many industrial and geophysical problems, such as the onset of sediment motion in riverbeds or near-shore environments. When the applied shear stress Σ is below the yield stress Σc, grains move temporarily, but only until finding a mechanically stable (MS) configuration that is able to resist the applied shear stress. When Σ > Σc, the material is no longer able to find MS configurations. However, the geometrical reasons why MS states vanish at the yield stress is not well understood. In this talk, I will show evidence from molecular dynamics simulations, both in a riverbed-like geometry as well as simple shear, that yielding in granular materials is akin to a second-order critical point, where the behavior near the yield stress is dominated by a correlation length ξ that diverges at the yield stress as ξ ∼ |Σ - Σc|-ν. MS states exist above the yield stress for finite systems, but they vanish as the system size becomes large according to a critical scaling function. The packing fraction and coordination number for MS states are independent of the applied shear stress, implying that the critical behavior we observe is distinct from the jamming scenario. Additionally, the critical behavior persists for overcompressed systems, confirming that jamming and yielding are distinct. Instead, we observe that MS states at nonzero shear stress possess anisotropic force and contact networks, suggesting that the yield stress is set by the maximum anisotropy that can be realized in the large-system limit. |
Monday, March 4, 2019 10:24AM - 11:00AM |
A53.00005: Naturalistic Granular Flows: Using Experiments to Apply Granular Physics to Geophysical Shear Systems Invited Speaker: Emily Brodsky Granular flows are ubiquitous in nature and deviate in potentially important ways from idealized systems. Shearing in fault zones, landslides and debris flows involve water-saturated, rough particles of varying composition and particle size accelerating through a range of velocities while abutting stationary grains. In addition, the types of observations that can be made in nature also differ from laboratory and experimental situations. Here we present a range of experiments that attempt to isolate these naturalistic features and elucidate their implications. Specifically we find that: (1) Natural, rough particles emit acoustic energy during shear, which can affect the flow behavior at intermediate velocities, (2) The acoustic energy observed in the lab depends on the particle diameter cubed, (3) Mineralogy affects the intermediate to high velocity flow behavior primarily through the combination of elastic, fracture and plastic material properties captured by the critical slip distance required for fracture nucleation, (4) high and low shear rate regions of the flow are coupled through acoustic waves, creating a non-trivial boundary condition at the base of a landslide or other natural shear system, (5) Fluid-saturation can result in dilation at high velocities regardless of the initial compaction state and therefore dilatory drainage is expected even in repeated activation of the same shear flows, and (6) preserved, fine-grained apparent shear bands in the geological record can be indicative of fine grains migrating at high velocities. These experimental observations suggest important directions for theoretical development in order to bring granular physics to bear on natural systems. |
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