Fault rock heterogeneity produces fault weakness and promotes unstable slip

Geological heterogeneity is abundant in crustal fault zones; however, its role in controlling the mechanical behaviour of faults is poorly constrained. Here, we present laboratory friction experiments on laterally heterogeneous faults, with patches of strong, rate-weakening quartz gouge and weak, rate-strengthening clay gouge. The experiments show that the heterogeneity leads to a significant strength reduction and decrease in frictional stability in comparison to compositionally identical faults with homogeneously mixed gouges typically used in the lab. We identify a combination of weakening effects, including smearing of the weak clay; differential compaction of the two gouges redistributing normal stress; and shear localization producing stress concentrations in the strong quartz patches. The results demonstrate that small-scale geological heterogeneity has pronounced effects on fault strength and stability, and by extension on the occurrence of slow-slip transients versus earthquake ruptures and the characteristics of the resulting events, and should be incorporated in lab experiments, fault friction laws, and earthquake source modelling.

Marine ice sheet experiments with the Community Ice Sheet Model

Ice sheet models differ in their numerical treatment of dynamical processes. Simulations of marine-based ice are sensitive to the choice of Stokes flow approximation and basal friction law and to the treatment of stresses and melt rates near the grounding line. We study the effects of these numerical choices on marine ice sheet dynamics in the Community Ice Sheet Model (CISM). In the framework of the Marine Ice Sheet Model Intercomparison Project 3d (MISMIP3d), we show that a depth-integrated, higher-order solver gives results similar to a 3D (Blatter–Pattyn) solver. We confirm that using a grounding line parameterization to approximate stresses in the grounding zone leads to accurate representation of ice sheet flow with a resolution of ∼2 km, as opposed to ∼0.5 km without the parameterization. In the MISMIP+ experimental framework, we compare different treatments of sub-shelf melting near the grounding line. In contrast to recent studies arguing that melting should not be applied in partly grounded cells, it is usually beneficial in CISM simulations to apply some melting in these cells. This suggests that the optimal treatment of melting near the grounding line can depend on ice sheet geometry, forcing, or model numerics. In both experimental frameworks, ice flow is sensitive to the choice of basal friction law. To study this sensitivity, we evaluate friction laws that vary the connectivity between the basal hydrological system and the ocean near the grounding line. CISM yields accurate results in steady-state and perturbation experiments at a resolution of ∼2 km (arguably 4 km) when the connectivity is low or moderate and ∼1 km (arguably 2 km) when the connectivity is strong.

The Mechanisms and Manifestations of Friction

This chapter reviews the types of friction that are of concern in tribological systems along with their associated causes and effects. It discusses some of the early discoveries that led to the development of friction laws and the understanding that friction is a system effect that can be analyzed based on energy dissipation. It describes the stick-slip behavior observed in wiper blades, the concept of asperities, and the significance of the shape, lay, roughness, and waviness of surfaces in sliding contact. It explains how friction forces are measured and how they are influenced by speed, load, and operating environment. It also covers rolling contact and fluid friction and the effect of lubrication.

Contact pressure, sliding velocity and viscosity dependent friction behavior of lubricants used in tube hydroforming processes

The final quality of sheet and tube metal formed components strongly depends of the tribology and friction conditions between the tools and the material to be formed. Furthermore, it has been recently demonstrated that friction is the numerical input parameter that has the biggest effect in the numerical models used for feasibility studies and process design. Industrial dedicated software packages have introduced friction laws which are dependent on sliding velocity, contact pressure and sometimes strain suffered by the sheet and currently, temperature dependency is being implemented as it has also major effect on friction. This last dependency on temperature is attributed to the viscosity change of the lubricant with temperature. In this work, three lubricant having different viscosity have been characterized using the tube sliding test. The final aim of the study is to obtain friction laws that are contact pressure and sliding velocity dependent for their use in tube hydroforming modelling. The tests, performed at various contact pressures and velocities, demonstrate that viscosity has a major effect on friction. As shown in the literature, the friction coefficient is also varying with the contact pressure and sliding velocity.Abs

Marine ice-sheet experiments with the Community Ice Sheet Model using the MISMIP+ experimental framework.

<p>Ice sheet models differ in their numerical treatment of dynamical processes. Simulations of marine-based ice are sensitive to the choice of Stokes flow approximation and basal friction law, and to the treatment of stresses and melt rates near the grounding line. We present the effects of these numerical choices on marine ice-sheet dynamics in the Community Ice Sheet Model (CISM). In the experimental framework of the Marine Ice Sheet Model Intercomparison Project (MISMIP+), we compare different treatments of sub-shelf melting near the grounding line. In contrast to recent studies arguing that melting should not be applied in partly grounded cells, it is usually beneficial in CISM simulations to apply some melting in these cells. This suggests that the optimal treatment of melting near the grounding line can depend on ice-sheet geometry, forcing, or model numerics. In the MISMIP+ framework, the ice flow is also sensitive to the choice of basal friction law. To study this sensitivity, we evaluate friction laws that vary the connectivity between the basal hydrological system and the ocean near the grounding line. CISM yields accurate results in steady-state and perturbation experiments at a resolution of ∼2 km (arguably 4 km) when the connectivity is low or moderate, and ∼1 km (arguably 2 km) when the connectivity is strong.</p>

Evaluating the influence of erosion and tectonic processes on California’s topography by measuring its fractal dimension and anisotropy across scales

<p>Roughness is paramount in Earth sciences, and landscapes, laboratory alluvial fans, river bed elevation, bedload transport and the friction laws of fluid mechanics all exhibit a fractal behavior described by a scale-persistent roughness. Yet, for a given landscape, the exact meaning of statistical roughness, or fractal dimension, remains unclear. The fractal dimension of topography is mainly understood as two end-members: at large spatial scales, it describes tectonic processes; at small spatial scales it describes erosion processes. In this study, we nuance this description by identifying the spatial scale at which erosion processes are inadequately described by fractal dimension and provide quantitative bounds on the meaning of the statistical roughness of topography at scales from 0.25 km to 100 km using three lines of evidence. First, we leverage spatial statistics to evaluate the auto-correlation structure of topographic statistical roughness across the physiographically diverse state of California, USA. Second, we identify the down-slope and across-slope directions using two-dimensional Fourier analysis, and measure the anisotropy of topography by evaluating statistical roughness in each direction. Third, we perform a spatial correlation analysis between statistical roughness and the Péclet number which describes the balance between diffusion and incision processes. Our preliminary results indicate that correlation between statistical roughness and Péclet number fades at scales greater than 4.6 km. In addition, auto-correlation saturation occurs for statistical roughness at scales greater than 16.5 km. Hence our analysis provides a more nuanced description of the statistical roughness of topography: it represents erosion processes at scales up to 4.6 km while being dominated by tectonics at scales greater than 16.5 km.</p>

Fundamentals of Sliding Interaction of Surfaces

This chapter reviews the laws governing the friction behavior of objects. The material starts with a historical view of the evolution of friction laws and how they shaped the science of tribology. The second part of the chapter provides a generalized overview of the mechanics of contact between complying solids. Major contact models are listed, and formulas for calculating area of contact, contact forces, contact stresses are also developed. Finally, the chapter applies the presented information to the contact of biological species. The information presents a summary of the major rules in biological attachments, their adhesion and friction, along with their contact mechanics.

Snow Avalanches

This work is a critical update of the most recent and innovative developments of the avalanche science. It aims at re-founding it on clear scientific bases, from field observations and experiments up to strong mathematical and physical analysis and modeling. It points out snow peculiarities, regarding both static mechanical properties and flow dynamics, that may strongly differ from those of compact solids for the former, and of Newtonian fluids for the latter. It analyzes the general processes involved in avalanche release, in terms of brittle fracture and ductile plasticity, specific friction laws, flow of healable granular materials, percolation concepts, cellular automata, scale invariance, criticality, theory of dynamical systems, bifurcations, etc. As a result, slab triggering (including remote triggering) can be summarized by the “slab avalanche release in 4 steps” concept, based on weak layer local collapse and subsequent propagation driven by slab weight. The frequent abortion of many incipient avalanches is easily explained in terms of snow grain dynamical healing. Sluffs and full-depth avalanches are also analyzed. Such advances pave the way for significant progress in risk evaluation procedures. In the present context of a speeding-up climate warming, possible evolutions of snow cover extent and stability are also tentatively discussed. We show how, in mountainous areas, the present analysis can be extended to other gravitational failures (rock-falls, landslides) that are likely to take over from avalanches in such circumstances. The text is supported by on-line links to field experiments and lectures on triggering mechanisms, risk management, and decision making.