A Discrete Elements Study of the Frictional Behavior of Fault Gouges

A series of discrete elements simulations is presented for the study of fault gouges' frictional response. The gouge is considered to have previously undergone ultra-cataclastic flow and long-time consolidation loading. We explore the effect of different particle characteristics such as size, polydispersity, and also shearing velocities on gouge's response under the conditions met in the seismogenic zone. Monte-Carlo analyses suggest that the local stick-slip events disappear when averaging over a large number of numerical samples. Moreover, the apparent material frictional response remains almost unaffected by the spatial randomness of particles' position and by the particle's size distribution. On the contrary, the mean particle size controls the formation and thickness of the observed shear bands, which appear after the peak friction is met. Furthermore, the apparent friction evolution fits well to an exponential decay law with slip, which involves a particle size dependent critical slip distance. For the studied conditions and depth, the shearing velocity is found to play a secondary role on the apparent frictional response of the gouge, which highlights the importance of analyses involving multiphysics for studying the rheology of fault gouges. Besides improving the understanding of the underlying physics of the problem, the above findings are also useful for deriving pertinent constitutive models in the case of modeling with continuum theories.

Rain and small earthquakes maintain a slow-moving landslide in a persistent critical state

In tectonically active mountain belts, landslides contribute significantly to erosion. Statistical analysis of regional inventories of earthquake-triggered-landslides after large earthquakes (Mw > 5.5) reveal a complex interaction between seismic shaking, landslide material, and rainfall. However, the contributions of each component have never been quantified due to a lack of in-situ data for active landslides. We exploited a 3-year geodetic and seismic dataset for a slow-moving landslide in Peru affected by local earthquakes and seasonal rainfalls. Here we show that in combination, they cause greater landslide motion than either force alone. We also show the rigidity of the landslide’s bulk clearly decreasing during Ml ≥ 5 earthquakes. The recovery is affected by rainfall and small earthquakes (Ml < 3.6), which prevent the soil from healing, highlighting the importance of the timing between forcings. These new quantitative insights into the mechanics of landslides open new perspectives for the study of the mass balance of earthquakes.

In this study, the authors show the interaction between seismic activity and rainfalls on landslide movement and how their timing controls landslide stability and motion.

Volumetric and shear processes in crystalline rock approaching faulting

Understanding the approach to faulting in continental rocks is critical for identifying processes leading to fracturing in geomaterials and the preparation process of large earthquakes. In situ dynamic X-ray imaging and digital volume correlation analysis of a crystalline rock core, under a constant confining pressure of 25 MPa, are used to elucidate the initiation, growth, and coalescence of microfractures leading to macroscopic failure as the axial compressive stress is increased. Following an initial elastic deformation, microfractures develop in the solid, and with increasing differential stress, the damage pervades the rock volume. The creation of new microfractures is accompanied by propagation, opening, and closing of existing microfractures, leading to the emergence of damage indices that increase as powers of the differential stress when approaching failure. A strong spatial correlation is observed between microscale zones with large positive and negative volumetric strains, microscale zones with shears of opposite senses, and microscale zones with high volumetric and shear strains. These correlations are attributed to microfracture interactions mediated by the heterogeneous stress field. The rock fails macroscopically as the microfractures coalesce and form a geometrically complex 3D volume that spans the rock sample. At the onset of failure, more than 70% of the damage volume is connected in a large fracture cluster that evolves into a fault zone. In the context of crustal faulting dynamics, these results suggest that evolving rock damage around existing locked or future main faults influences the localization process that culminates in large brittle rupture events.

Collective Damage Growth Controls Fault Orientation in Quasibrittle Compressive Failure

The Mohr-Coulomb criterion is widely used in geosciences and solid mechanics to relate the state of stress at failure to the observed orientation of the resulting faults. This relation is based on the assumption that macroscopic failure takes place along the plane that maximizes the Coulomb stress. Here, this hypothesis is assessed by simulating compressive tests on an elastodamageable material that follows the Mohr-Coulomb criterion at the mesoscopic scale. We find that the macroscopic fault orientation is not given by the Mohr-Coulomb criterion. Instead, for a weakly disordered material, it corresponds to the most unstable mode of damage growth, which we determine through a linear stability analysis of its homogeneously damaged state. Our study reveals that compressive failure emerges from the coalescence of damaged clusters within the material and that this collective process is suitably described at the continuum scale by introducing an elastic kernel that describes the interactions between these clusters.

Abundant off-fault seismicity and orthogonal structures in the San Jacinto fault zone

The trifurcation area of the San Jacinto fault zone has produced more than 10% of all earthquakes in southern California since 2000, including the June 2016 M_w (moment magnitude) 5.2 Borrego Springs earthquake. In this area, the fault splits into three subparallel strands and is associated with broad V_P /V_S anomalies. We synthesize spatiotemporal properties of historical background seismicity and aftershocks of the June 2016 event. A template matching technique is used to detect and locate more than 23,000 aftershocks, which illuminate highly complex active fault structures in conjunction with a high-resolution regional catalog. The hypocenters form dipping seismicity lineations both along strike and nearly orthogonal to the main fault, and are composed of interlaced strike-slip and normal faults. The primary faults change dip with depth and become listric by transitioning to a dip of ~70° near a depth of 10 km. The M_w 5.2 Borrego Springs earthquake and past events with M > 4.0 occurred on the main faults, whereas most of the low-magnitude events are located in a damage zone (several kilometers wide) at seismogenic depths. The lack of significant low-magnitude seismicity on the main fault traces suggests that they do not creep. The very high rate of aftershocks likely reflects the large geometrical fault complexity and perhaps a relatively high stress due to a significant length of time elapsed since the last major event. The results provide important insights into the physics of faulting near the brittle-ductile transition.

Scaling of earthquake models with inhomogeneous stress dissipation

Natural earthquake fault systems are highly nonhomogeneous. The inhomogeneities occur because the earth is made of a variety of materials which hold and dissipate stress differently. In this work, we study scaling in earthquake fault models which are variations of the Olami-Feder-Christensen and Rundle-Jackson-Brown models. We use the scaling to explore the effect of spatial inhomogeneities due to damage and inhomogeneous stress dissipation in the earthquake-fault-like systems when the stress transfer range is long, but not necessarily longer than the length scale associated with the inhomogeneities of the system. We find that the scaling depends not only on the amount of damage, but also on the spatial distribution of that damage.

Localized and chaotic folding: the role of axial plane structures

Most natural fold systems are not sinusoidal in profile. A widely held view is that such irregularity derives solely from inherited initial geometrical perturbations. Although, undoubtedly, initial perturbations can contribute to irregularity, we explore a different (but complementary) view in which the irregular geometry results from some material or system softening process. This arises because the buckling response of a layer (or layers) embedded in a weaker matrix is controlled in a sensitive manner by the nature of the reaction forces exerted by the deforming matrix on the layer. In many theoretical treatments of the folding problem, this reaction force is assumed to be a linear function of some measure of the deformation or deformation rate. This paper is concerned with the influence of nonlinear reaction forces such as arise from nonlinear elasticity or viscosity. Localized folds arising from nonlinearity form in a fundamentally different way than the Biot wavelength selection process. As a particular example of nonlinear behaviour, we examine the influence of axial plane structures made up of layers of different mineralogy formed by chemical differentiation processes accompanying the deformation; they are referred to as metamorphic layering. The alternating mineralogical composition in the metamorphic layers means that the embedding matrix exerts a reaction force on the folded layers that varies not only with the deflection or the velocity of deflection of the layer, but also in a periodic manner along the length of the folded layers. The influence of this spatially periodic reaction force on the development of localized and chaotic folding is explored numerically.

The interaction of deformation and metamorphic reactions

Feedback relations between deformation and metamorphic mineral reactions, derived using the principles of non-equilibrium thermodynamics, indicate that mineral reactions progress to completion in high-strain areas, driven by energy dissipated from inelastic deformation. These processes, in common with other time-dependent geological processes, lead to both strain, and strain-rate, hardening/softening in rate-dependent materials. In particular, strain-rate softening leads to the formation of shear zones, folds and boudins by non-Biot mechanisms. Strain-softening alone does not produce folding or boudinage and results in low-strain shear zones; strain-rate softening is necessary to produce realistic strains and structures. Reaction–mechanical feedback relations operating at the scale of 10–100 m produce structures similar to those that arise from thermal–mechanical feedback relations at coarser (kilometre) scales and reaction–diffusion–mechanical feedback relations at finer (millimetre) scales. The dominance of specific processes at various length scales but the development of similar structures by all coupled processes leads to scale invariance. The concept of non-equilibrium mineral stability diagrams is introduced. In principle, deformation influences the position of mineral stability fields relative to equilibrium stability fields; the effect is negligible for the quartz→coesite reaction but may be important for others. Application of these results to the development of structures and mineral reactions in the Italian Alps is discussed.