Geometric Asperities Research Articles

Laboratory Earthquakes Simulations—Emergence, Structure, and Evolution of Fault Heterogeneity

AbstractSeismic faults are known to exhibit a high level of spatial and temporal complexity, and the causes and consequences of this complexity have been the topic of numerous research works in the past decade. In this paper, we investigate the origins and the structure of this complexity by considering a numerical model of laboratory earthquake experiment, where we introduce a fault with homogeneous mechanical properties but allow it to evolve spontaneously to its natural level of complexity. This is achieved by coupling the elastic deformability of the off‐fault medium (and therefore allowing for heterogeneous stress fields to develop) and the discrete degradation and gouge formation at the fault plane (and therefore allowing for structural heterogeneity to develop). Numerical results show the development of persistent stress, damage, and gouge thickness heterogeneities, with a much larger variability in space than in time. Strong positive correlations are found between these quantities, which suggest a positive feedback between local normal stress and damage rate, only mildly mitigated by the mobility of the granular gouge in the interface. For a wide range of confining stresses, after a sufficient number of seismic cycles, the fault reaches a state of established disorder with a constant roughness, a certain amount of periodicity at the millimetric scale, and a power law decay of the Power Spectral Density at smaller spatial scales. The typical height‐to‐wavelength ratio of geometrical asperities and the correlations between stress and damage profiles are in good agreement with previous field or lab estimates.

The evolution of stresses and shear resistance on rough faults at large slip

The deviation of natural faults from planarity results in geometric asperities and a heterogeneous stress field, affecting the shear resistance and slip behavior. This study numerically examines the effects of macro-scale roughness, local friction, and wear on the evolution of the fault stresses and shear resistance with fault slip. In contrast to previous analytical and numerical studies, our quasistatic simulations allow for slip at the scale of roughness wavelengths, large roughness levels, and fault opening. The simulations indicate that analytical solutions, which predict a linear increase in shear resistance with slip, significantly overestimate the additional shear resistance from roughness, i.e., the roughness drag, at slips larger than several percent of the minimum roughness wavelength. Starting with mated fault surfaces, the average shear resistance on the fault initially rises rapidly, but the rate of increase diminishes significantly as slip increases and the faults open. While initially showing a larger increase in shear resistance than self-similar faults, at large slip, self-affine faults exhibit significantly smaller shear resistance, and the background stresses are bounded without the need for off-fault inelastic deformation. Although smaller than predicted by analytical solutions, the simulation results show that the additional resistance from macro-scale geometrical irregularities is important, enabling rough faults to operate under shear resistance significantly larger than that from local friction only. The impact of macro-scale roughness increases with the roughness amplitude and the friction coefficient, while wear reduces fault stresses and average shear resistance. The combined effect of local friction and roughness is nonlinear, involving a direct contribution to shear resistance from roughness, as observed in prior studies, and an additional contribution from the increasing average normal stress with slip, which amplifies the resistance from friction and becomes more pronounced at large slip.

Unraveling the roles of fault asperities over earthquake cycles

Fault asperities can produce concentrated slips during large earthquakes and intensify damage. However, how asperities control fault behavior during other phases of the earthquake cycle remains poorly known. Here, we conduct friction experiment on a laboratory fault featuring two prominent geometric asperities to directly image their influences on long-term and short-term seismicity, and the nucleation and propagation of the mainshock. The laboratory observations and supporting numerical simulations reveal that one asperity located in the fault interior behaves firstly as a mechanical attractor to long-term seismicity, then a barrier to preseismic slow slip, and ultimately a source of large coseismic slip. In contrast, another asperity located near the fault margin primarily undergoes persistent aseismic slip throughout most phases of the earthquake cycle. These results provide new insights into how asperities partition strain across a broad spatiotemporal domain, establishing a physical link between long-term and short-term fault behaviors and the occurrence of large earthquakes.

Microscale Spatial Variations in Coseismic Temperature Rise on Hematite Fault Mirrors in the Wasatch Fault Damage Zone

AbstractCoseismic temperature rise activates fault dynamic weakening that promotes earthquake rupture propagation. The spatial scales over which peak temperatures vary on slip surfaces are challenging to identify in the rock record. We present microstructural observations and electron backscatter diffraction data from three small‐displacement hematite‐coated fault mirrors (FMs) in the Wasatch fault damage zone, Utah, to evaluate relations between fault properties, strain localization, temperature rise, and weakening mechanisms during FM development. Millimeter‐ to cm‐thick, matrix‐supported, hematite‐cemented breccia is cut by ∼25–200 μm‐thick, texturally heterogeneous veins that form the hematite FM volume (FMV). Grain morphologies and textures vary with FMV thickness over μm to mm lengthscales. Cataclasite grades to ultracataclasite where FMV thickness is greatest. Thinner FMVs and geometric asperities are characterized by particles with subgrains, serrated grain boundaries, and(or) low‐strain polygonal grains that increase in size with proximity to the FM surface. Comparison to prior hematite deformation experiments suggests FM temperatures broadly range from ≥400°C to ≥800–1100°C, compatible with observed coeval brittle and plastic deformation mechanisms, over sub‐mm scales on individual slip surfaces during seismic slip. We present a model of FM development by episodic hematite precipitation, fault reactivation, and strain localization, where the thickness of hematite veins controls the width of the deforming zones during subsequent fault slip, facilitating temperature rise and thermally activated weakening. Our data document intrasample coseismic temperatures, resultant deformation and dynamic weakening mechanisms, and the length scales over which these vary on slip surfaces.

Mid-crustal ramping of the Main Himalayan Thrust in Nepal to Bhutan Himalaya: New insights from analogue and numerical experiments

The mid-crustal ramp on the Main Himalayan Thrust (MHT) in the Lesser Himalaya Sequence (LHS) is the most critical geometrical asperity to trigger major seismic events in the Himalaya, including the 2015 Mw 7.8 Gorkha, Nepal earthquake. However, it is still not well understood what caused the MHT to ramp up at laterally varying locations in central to eastern Himalaya during the southward propagation of the Himalayan tectonic wedge. Based on laboratory and numerical model experiments, approximated to the central (Nepal) and eastern (Bhutan) Himalayan geological settings, we show that the coal-bearing Gondwana horizon in the stratigraphic sequence has played a crucial role in producing the mid-crustal ramp on the MHT. During the southward propagation of the Himalayan wedge, the mechanically weak Gondwana sequence resulted in strain localization at its northern edge, and it eventually caused the MHT to ramp from a depth of >20 km to ~6–7 km. Combining our experimental and field observations with the geophysical sections, we interpret that a strike-wise non-uniform occurrence of the Gondwana horizon is responsible for the MHT to ramp at different locations, separated by N-S trending natural barriers, like tear faults. The weak-zone model also explains the rock uplift versus erosion pattern recorded in central Nepal. The ramp-induced surface uplift rates, calculated from our model, are in good agreement with those reported from central Nepal (U* ~ 0.28, normalized to the convergent rate).

Melt Welding and Its Role in Fault Reactivation and Localization of Fracture Damage in Seismically Active Faults

AbstractLow displacement fracture damage plays an important role in influencing the behavior and mechanical evolution of faults. Fracture damage zones influence slip behavior through changing near‐field stress orientations, altering fluid pathways and modifying fault structure. Here we use small displacement triaxial experiments to explore the development of fault zone damage, frictional lock‐up, and the generation of new faults using samples with preground faults, oriented in 5° increments between 25° and 65° relative to the shortening direction. With increasing reactivation angle, faults support higher peak normal stresses (104–845 MPa) and behavior transitions from stable sliding to stick slip. Frictional melting occurs on surfaces where stick slip is initiated, forming micron‐thick layers that locally weld asperity contacts. The extent of melt welding is correlated with normal stress and melt‐welded zones increase fault cohesion. Distribution of fracture damage adjacent to the fault is spatially correlated with melt‐welded zones and the corresponding concentrations of stress and elastic strain. In a process referred to as “adhesive wear,” fractures bypassing welded zones transfer melt‐adhered material from one side of the fault to the other, forming new geometric asperities. On faults with high reactivation angles (55°–60°) the increase in cohesive strength resulting from melt‐welded contacts drives fault lock‐up after an initial slip event; subsequent slip localizes on new, favorably oriented faults. Given their size, melt‐welded zones are likely to be short‐lived in nature but may play a significant and previously unrecognized role in the development of fault‐related damage.

Thermochronometric and textural evidence for seismicity via asperity flash heating on exhumed hematite fault mirrors, Wasatch fault zone, UT, USA

Exhumed faults record the temperatures produced by earthquakes. We show that transient elevated fault surface temperatures preserved in the rock record are quantifiable through microtextural analysis, fault-rock thermochronometry, and thermomechanical modeling. We apply this approach to a network of mirrored, minor, hematite-coated fault surfaces in the exhumed, seismogenic Wasatch fault zone, UT, USA. Polygonal and lobate hematite crystal morphologies, coupled with hematite (U–Th)/He data patterns from these surfaces and host rock apatite (U–Th)/He data, are best explained by friction-generated heat at slip interface geometric asperities. These observations inform thermomechanical simulations of flash heating at frictional contacts and resulting fractional He loss over generated fault surface time–temperature histories. Temperatures of >∼700–1200 °C, depending on asperity size, are sufficient to induce 85–100% He loss from hematite within 200 μm of the fault surface. Spatially-isolated, high-temperature microtextures imply spatially-variable heat generation and decay. Our results reveal that flash heating of asperities and associated frictional weakening likely promote small earthquakes (Mw≈−3 to 3) on Wasatch hematite fault mirrors. We suggest that similar thermal processes and resultant dynamic weakening may facilitate larger earthquakes.

Slickenline orientations as a record of fault rock rheology

Contact of geometric asperities across rough faults causes perturbations to the shear traction resolved on a fault surface that could potentially deflect the local slip direction. Slickenlines, which record the relative displacement of opposing sides of a fault, may therefore be sensitive to fault surface geometry. To investigate the relationship between fault geometry, shear traction and slip, we use ground-based LiDAR to measure the orientations of slickenlines defined by centimeter-amplitude corrugations on three fault surfaces. Slickenline rakes measured in the mean plane of each fault rarely vary by more than a few degrees. Deviations from the mean rake do not correlate with the fault surface topography at scales up to a few meters, but a weak correlation may be present at larger scales. Slip directions are therefore insensitive to any shear traction perturbations from the contact of geometric asperities at small scales. This observation is consistent with scale-dependent deformation. We show that the roughness of fault surfaces implies that short wavelength asperities fail inelastically because of flattening during fault slip. A crossover to elastic deformation occurs at lengths of 10−2 to 100 m, which defines the minimum dimension of a strength asperity that influences the displacement field. The roughness at the crossover length scale corresponds to the typical thickness of fault rocks in the fault core suggesting that fault rock thickness is related to the crossover length scale. The data requires that multiple processes combine to produce the fault surface geometry. We reconcile the variety of processes with the consistent surface roughness scaling by noting that all of the processes are governed in different ways by the elastic limit of rock.

Hot faults: Iridescent slip surfaces with metallic luster document high-temperature ancient seismicity in the Wasatch fault zone, Utah, USA

and localized iridescence in the footwall dam- age zone of the Wasatch fault (Utah, USA) and document elevated temperatures on these faults. We propose that the formation of these iridescent slip surfaces requires frictional heat generated at geometric asperities at seismic slip rates. WASATCH FAULT ZONE ABSTRACT We document new geological indicators of ancient seismicity in the form of highly reflec- tive, iridescent, hematite-coated fault surfaces. Small faults that cut the Paleoproterozoic Farmington Canyon Complex in the footwall damage zone of the Brigham City segment of the Wasatch fault (Utah, USA) are smooth to striated surfaces, tens of square centimeters to 30 m 2 in area. The dull-rusty to high-metallic luster and moderate- to high-gloss surfaces exhibit multicolored elliptical iridescent patches ~0.5-3 cm across. Preexisting hematite crystals were deformed during slip on 1-200-mm-thick slip surfaces. Textural observations, X-ray diffraction, X-ray photoelectron spectroscopy, electron backscattered diffraction analysis, surface metrology, and similarity to experimentally formed iridescent spots in rocks and metals indicate that iridescence is associated with high-temperature (>300 °C) reduction of iron (Fe 3+ to Fe 2+ ) and associated conversion of hematite to magnetite. We pro- pose that the iridescent slip surfaces in the Wasatch fault damage zone are the result of seismic slip and flash heating at asperities along the small faults. The thousands of these sur - faces represent coseismic or aftershock deformation down to magnitude -3 in the exhumed footwall damage zone of the Wasatch fault.

Fault architecture and deformation mechanisms in exhumed analogues of seismogenic carbonate-bearing thrusts

Faults in carbonates are well known sources of upper crustal seismicity throughout the world. In the outer sector of the Northern Apennines, ancient carbonate-bearing thrusts are exposed at the surface and represent analogues of structures generating seismicity at depth. We describe the geometry, internal structure and deformation mechanisms of three large-displacement thrusts from the km scale to the microscale. Fault architecture and deformation mechanisms are all influenced by the lithology of faulted rocks. Where thrusts cut across bedded or marly limestones, fault zones are thick (tens of metres) and display foliated rocks (S-CC′ tectonites and/or YPR cataclasites) characterized by intense pressure-solution deformation. In massive limestones, faulting occurs in localized, narrow zones that exhibit abundant brittle deformation. A general model for a heterogeneous, carbonate-bearing thrust is proposed and discussed. Fault structure, affected by stratigraphic heterogeneity and inherited structures, influences the location of geometrical asperities and fault strain rates. The presence of clay minerals and the strain rate experienced by fault rocks modulate the shifting from cataclasis-dominated towards pressure-solution-dominated deformation. Resulting structural heterogeneity of these faults may mirror their mechanical and seismic behaviour: we suggest that seismic asperities are located at the boundaries of massive limestones in narrow zones of localized slip whereas weak shear zones constitute slowly slipping portions of the fault, reflecting other types of “aseismic” behaviour.

Isostaticity at Frictional Jamming

Amorphous packings of frictionless, spherical particles are isostatic at jamming onset, with the number of constraints (contacts) equal to the number of degrees of freedom. Their structural and mechanical properties are controlled by the interparticle contact network. In contrast, amorphous packings of frictional particles are typically hyperstatic at jamming onset. We perform extensive numerical simulations in two dimensions of the geometrical asperity (GA) model for static friction to further investigate the role of isostaticity. In the GA model, interparticle forces are obtained by summing up purely repulsive central forces between periodically spaced circular asperities on contacting grains. We compare the packing fraction, contact number, mobilization distribution, and vibrational density of states (in the harmonic approximation) using the GA model to those generated using the Cundall-Strack approach. We find that static packings of frictional disks obtained from the GA model are mechanically stable and isostatic when we consider interactions between asperities on contacting particles. The crossover in the structural and mechanical properties of static packings from frictionless to frictional behavior as a function of the static friction coefficient coincides with a change in the type of interparticle contacts and the disappearance of a peak in the density of vibrational modes for the GA model. These results emphasize that mesoscale features of the model for static friction play an important role in determining the properties of granular packings.

Identifying fault heterogeneity through mapping spatial anomalies in acoustic emission statistics

Seismicity clusters within fault zones can be connected to the structure, geometric complexity and size of asperities which perturb and intensify the stress field in their periphery. To gain further insight into fault mechanical processes, we study stick‐slip sequences in an analog, laboratory setting. Analysis of small scale fracture processes expressed by acoustic emissions (AEs) provide the possibility to investigate how microseismicity is linked to fault heterogeneities and the occurrence of dynamic slip events. The present work connects X‐ray computer tomography (CT) scans of faulted rock samples with spatial maps of b values (slope of the frequency‐magnitude distribution), seismic moments and event densities. Our current experimental setup facilitates the creation of a series of stick‐slips on one fault plane thus allowing us to document how individual stick‐slips can change the characteristics of AE event populations in connection to the evolution of the fault structure. We found that geometric asperities identified in CT scan images were connected to regions of low b values, increased event densities and moment release over multiple stick‐slip cycles. Our experiments underline several parallels between laboratory findings and studies of crustal seismicity, for example, that asperity regions in lab and field are connected to spatial b value anomalies. These regions appear to play an important role in controlling the nucleation spots of dynamic slip events and crustal earthquakes.

A New Model for Constriction Resistance of Rough Contacts between Nominally Flat Surfaces

Thermal contact resistance plays an important role in many domains, such as microelectronics and nuclear reactors. This paper proposes a more comprehensive model for the prediction of constriction resistance of rough contact between nominally flat surfaces in vacuum. Firstly, a 3D geometrical asperity contact model is proposed based on the analysis of the profile of actual engineering surface. In this model, the contact is not simplified as a rough surface contacting with a perfectly smooth surface, but described as two rough surfaces. Oblique contact is considered and the effects of several parameters such as the shape of the asperity, the depth of interference, and the radial distance between the centerlines of the contacting asperities are investigated. Some mathematical derivations for constriction resistance are performed, and a series of numerical simulations are also carried out, covering a wide range of values of these parameters in practice applications. A comprehensive correlation for constriction resistance as a function of these parameters is finally obtained by nonlinear curve fitting, and it is validated through some comparisons and it can be used to predict more accurately the thermal contact resistance between rough surfaces.

Himalayan tectonic model and the great earthquakes: an appraisal

The best known conceptual tectonic model of the Himalayan Seismic Belt (HSB) suggests that the Basement Thrust Front (BTF) lies beneath the Main Central Thrust (MCT) with a prominent ‘ramp’. The ‘ramp’ is viewed as a geometrical asperity that accumulates the stress due to the Himalayan collision tectonics, and it has been suggested that the past great earthquakes occurred on the plane of detachment. The plane of detachment is the interface between the Indian shield and the Himalayan sedimentary wedge, also known as the Main Himalayan Thrust (MHT). The recent earthquake data from the local permanent and temporary networks and a re-examination of source processes of the great earthquakes in the Himalaya, however, do not support this model for the entire HSB. The four known instrumentally recorded great (M ∼8.0–8.7) earthquakes in the foothills of the Himalaya in India, from west to east – the 1905 Kangra, 1934 Bihar, 1897 Shillong and the 1950 Assam earthquakes – occurred by different tectonic processes, and possibly none can be explained as a plane of detachment earthquake; each occurred in its own unique complex tectonic environment. The 1905 as well as the 1934 great event may have a deeper source to the south of the MBT. The 1897 great event is argued to be a shield earthquake rather than a Himalayan earthquake and it occurred by pop-up tectonics of the Shillong plateau. The 1950 great event is argued to be caused by transform tectonics in the eastern syntaxis zone rather than by thrusting on the plane of detachment.

Slip heterogeneity on a corrugated fault

Abstract Slip heterogeneity reflects the fundamental physics of earthquake rupture and has been attributed to strong fault patches termed asperities or barriers. We propose that variations in fault-surface orientation due to slip-parallel corrugations may act as geometric asperities and barriers, generating variations in incremental (i.e. due to a single earthquake) slip across a fault surface. We evaluate this hypothesis using observations from the Arkitsa normal fault exposure in central Greece. A scan of the Arkitsa fault surface with 1-m spatial resolution and mm-scale precision reveals corrugations made up of 1–5 m wide synforms, antiforms, and nearly planar fault sections with long axes that extend in the slip direction across the entire exposed surface. The surface is thus more than an order of magnitude smoother in the direction of slip than in the perpendicular direction. Slip-perpendicular profiles exhibit nearly self-similar scaling across the range of observed wavelengths (~ 2–50 m), whereas slip-parallel profiles are significantly smoother at shorter wavelengths (

Geometric and rheological asperities in an exposed fault zone

Earthquake dynamics are strongly affected by fault zone structure and fault surface geometry. Here we investigate the interplay of bulk deformation and surface topography using detailed structural analysis of a fault zone near Klamath Falls, Oregon, combined with LiDAR measurements of the fault surface. We find that the fault zone has a layered damage architecture. Slip primarily occurs inside a 1–20 mm wide band that contains principal slip surfaces with individual widths of ∼100 μm. The slip band sits atop a cohesive layer which deforms by granular flow. Several fault strands with total slips of 0.5–150 m also have cohesive layers with thicknesses increasing monotonically with slip. The thickness added to the cohesive layer per unit slip decreases with increasing displacement indicating that slip progressively localizes. The main fault is a continuous surface with 10–40 m long quasi‐elliptical geometrical asperities, i.e., bumps. The bumps reflect variations of the thickness of the granular cohesive layer and can be generated by a pinch‐and‐swell instability. As the granular layer is rheological distinct from its surroundings, the asperities are both geometrical and rheological inhomogenities. Modeling slip along wavy faults shows that slip on a surface with a realistic geometry requires internal yielding of the host rock. Our observations suggest that the internal deformation processes in the fault zone include ongoing fracture, slip along secondary faults, and particle rotation. Granular flow is an important part of faulting in this locale. Slip surfaces localize on the border of the granular cohesive layer. The ongoing slip smoothes the surfaces and thus the structural and geometrical evolution of the granular layer creates a preference for continued of slip on the same surface. There is a feedback cycle between slip on the surface and the generation of the granular layer that then deforms and controls the locus of future slip.

Numerical simulation of fault development in fold-and-thrust belt of Nepal Himalaya

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 111

Smart electrodes for ultralarge-area thin film capacitors

A process suitable for preparing metal-insulator-metal thin film capacitors with submicron insulating layers and top electrodes with cm-scale dimensions is presented. Most importantly, this process does not require sophisticated deposition equipment or a clean room environment. The key to large area yield is co-firing the insulator film with a non-dewetting electrode during the dielectric crystallization/densification anneal. We propose a mechanism of electrode dewetting during the high temperature anneal where the metal laterally retreats from geometric asperities that compromise the integrity of the insulating layer. This behavior is driven by surface energy minimization, which promotes metal migration away from the regions of high curvature. This methodology is not material specific, and only requires a top electrode with a large contact angle to the dielectric in question. Using this technique, functional thin film capacitors with 2.5 cm lateral dimensions and 1 μm dielectric thicknesses can be routinely prepared.

Fatigue threshold studies in Fe, Fe-Si, and HSLA steel: Part I. Effect of strength and surface asperities on closure

The fatigue threshold was found to increase as the temperature decreased from 300 K to 123 K for a sequence of Fe and Fe-S binary alloys and an HSLA steel. A prominent closure was observed with the magnitude of closure stress intensity higher at lower temperature. The correlation between closure and threshold stress intensity was examined first with residual plasticity models. With geometrical asperity closure arguments, the closure stress intensity was also correlated with the yield stress and the grain size of the material. By considering the reverse slip, geometry, and oxide-induced closure simultaneously, a model for the closure at fatigue threshold was proposed and compared to the data from this study and the literature. All three mechanisms of residual plasticity, geometrical asperity, and oxide formation are shown to be relevant to the observed closure phenomena in this study.

On the Generating Mechanism of Surface Roughness : Forming Process of Single Asperity by Two Grooves' Interference

It is clear that pile-up phenomena of groove sides produced by means of abrasive grains have some effects on the roughness of ground surface. In this paper, for the purpose of making clear the effect of pile-up in the case of two grooves' interference on the surface roughness, the ratio of an actual generated asperity height to a geometrical asperity one, ni is measured experimentally and then the effect of the change of cutting edges pitch is estimated statistically. The ratio of the standard deviation in the distribution of the actual generated asperity heights to geometrical asperity values has a range between 2.82 and 3.64 for the same depth of cut. The value of ni increases with an increase in the apical angle of edge, in the ratio of cutting depth of the first passing edge to he second passing edge, and with a decrease in the hardness of workpieces.