Showing papers on "Slip (materials science) published in 1998"

Earthquakes and friction laws

Abstract: The traditional view of tectonics is that the lithosphere comprises a strong brittle layer overlying a weak ductile layer, which gives rise to two forms of deformation: brittle fracture, accompanied by earth- quakes, in the upper layer, and aseismic ductile flow in the layer beneath Although this view is not incorrect, it is imprecise, and in ways that can lead to serious misunderstandings The term ductility, for example, can apply equally to two common rock deformation mechanisms: crystal plasticity, which occurs in rock above a critical temperature, and cataclastic flow, a type of granular deformation which can occur in poorly consolidated sediments Although both exhibit ductility, these two deformation mechanisms have very different rheologies Earthquakes, in turn, are associated with strength and brittleness—associations that are likewise sufficiently imprecise that, if taken much beyond the generality implied in the opening sentence, they can lead to serious misinterpretations about earthquake mechanics Lately, a newer, much more precise and predictive model for the earthquake mechanism has emerged, which has its roots in the observation that tectonic earthquakes seldom if ever occur by the sudden appearance and propagation of a new shear crack (or 'fault') Instead, they occur by sudden slippage along a pre-existing fault or plate interface They are therefore a frictional, rather than fracture, phenomenon, with brittle fracture playing a secondary role in the lengthening of faults 1 and frictional wear 2 This distinction was noted by several early workers 3 , but it was not until 1966 that Brace and Byerlee 4 pointed out that earthquakes must be the result of a stick-slip frictional instability Thus, the earthquake is the 'slip', and the 'stick' is the interseismic period of elastic strain accumula- tion Subsequently, a complete constitutive law for rock friction has been developed based on laboratory studies A surprising result is that a great many other aspects of earthquake phenomena also now seem to result from the nature of the friction on faults The properties traditionally thought to control these processes— strength, brittleness and ductility—are subsumed within the over- arching concept of frictional stability regimes Constitutive law of rock friction In the standard model of stick-slip friction it is assumed that sliding begins when the ratio of shear to normal stress on the surface reaches a value ms, the static friction coefficient Once sliding initiates, frictional resistance falls to a lower dynamic friction coefficient, md, and this weakening of sliding resistance may,

Influence of processing on microstructure and mechanical properties of (α+β) titanium alloys

Abstract: The present paper tries to summarize the relationship between processing, microstructure, and mechanical properties of two-phase (α+β) titanium alloys. Although for most structural applications of titanium alloys a variety of important mechanical properties (yield stress, ductility, HCF, LCF, da/dN of micro- and macrocracks, KIC, and creep) have to be optimized or balanced, and although both processing as well as microstructure contain many variables, it can be shown that from the numerous correlation possibilities only a few underlying basic principles are really important. One of them is the relationship between cooling rate, colony size, and slip length leading directly to the advantages of a bi-modal (duplex) type of microstructure usable for most applications and involving a reproducible and insensitive processing route.

A general theory of curved deformable interfaces in solids at equilibrium

Abstract: We discuss the deformation of a curved interface between solid phases, assuming small strains in the bulk phases and neglecting accretion at the interfaces. Such assumptions are relevant to the deformation of solid microstructures when atomic diffusion and the formation of defects such as dislocations are negligible. We base our theory on a constitutive equation giving the (excess) free energy ψ of the interface when the interfacial limits of the displacement fields in the abutting phases as well as the limits of the displacement gradients are known. Using general considerations of frame invariance, we show that ψ can depend on these quantities at most through: firstly the normal and tangential components of the jump in displacement at the interface (stretch and slip), secondly the average of the projected strain in the tangent plane (average tangential strain), thirdly the tangential component of the jump in the projected displacement gradient at the interface (relative tangential strain and rel...

Plasticity Induced by Shock Waves in Nonequilibrium Molecular-Dynamics Simulations

Abstract: Nonequilibrium molecular-dynamics simulations of shock waves in three-dimensional 10-million atom face-centered cubic crystals with cross-sectional dimensions of 100 by 100 unit cells show that the system slips along all of the available {111} slip planes, in different places along the nonplanar shock front. Comparison of these simulations with earlier ones on a smaller scale not only eliminates the possibility that the observed slippage is an artifact of transverse periodic boundary conditions, but also reveals the richness of the nanostructure left behind. By introducing a piston face that is no longer perfectly flat, mimicking a line or surface inhomogeneity in the unshocked material, it is shown that for weaker shock waves (below the perfect-crystal yield strength), stacking faults can be nucleated by preexisting extended defects.

The effect of loading rate on static friction and the rate of fault healing during the earthquake cycle

Abstract: The seismic cycle requires that faults strengthen (heal) between earthquakes, and the rate of this healing process plays a key role in determining earthquake stress drop1,2,3,4, rupture characteristics5,6 and seismic scaling relations2,3,4,7. Frictional healing (as evidenced by increasing static friction during quasi-stationary contact between two surfaces1,8,9,10,11,12) is considered the mechanism most likely to be responsible for fault strengthening2,3,13,14. Previous studies, however, have shown a large discrepancy between laboratory and seismic (field) estimates of the healing rate2,3,4,14,15; in the laboratory, rock friction changes by only a few per cent per order-of-magnitude change in slip rate, whereas seismic stress drop increases by a factor of 2 to 5 per order-of-magnitude increase in earthquake recurrence interval. But in such comparisons, it is assumed that healing and static friction are independent of loading rate. Here, I summarize laboratory measurements showing that static friction and healing vary with loading rate and time, as expected from friction theory16,17,18. Applying these results to seismic faulting and accounting for differences in laboratory, seismic and tectonic slip rates, I demonstrate that post-seismic healing is expected to be retardedfor a period of several hundred days following an earthquake, in agreement with recent findings from repeating earthquakes13,14,19,20.

Quantitative measure of the variation in fault rheology due to fluid-rock interactions

Abstract: We analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature (T) and pore water pressure (Pp) on the sliding behavior of faults. Rate-stepping sliding tests were performed on laboratory faults in granite containing “gouge” (granite powder), both dry at 23° to 845°C [Lockner et al., 1986], and wet (Pp = 100 MPa) at 23° to 600°C [Blanpied et al., 1991, 1995]. Imposed slip velocities (V) ranged from 0.01 to 5.5 μm/s, and effective normal stresses were near 400 MPa. For dried granite at all temperatures, and wet granite below ∼300°C, the coefficient of friction (μ) shows low sensitivity to V, T, and Pp. For wet granite above ∼350°, μ drops rapidly with increasing T and shows a strong, positive rate dependence and protracted strength transients following steps in V, presumably reflecting the activity of a water-aided deformation process. By inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate- and state-dependent constitutive law. One or two state variables were used to represent slip history effects. Each velocity step yielded an independent set of values for the nominal friction level, five constitutive parameters (transient parameters a, b1, and b2 and characteristic displacements Dc1 and Dc2), and the velocity dependence of steady state friction ∂μss/∂ ln V = a-b1−b2. Below 250°, data from dry and most wet tests are adequately modeled by using the “slip law” [Ruina, 1983] and one state variable (a = 0.003 to 0.018, b = 0.001 to +0.018, Dc ≈ 1 to 20 μm). Dried tests above 250° can also be fitted with one state variable. In contrast, wet tests above 350° require higher direct rate dependence (a = 0.03 to 0.12), plus a second state variable with large, negative amplitude (b2 = −0.03 to −0.14) and large characteristic displacement (Dc2 = 300 to >4000 μm). Thus the parameters a, b1, and b2 for wet granite show a pronounced change in their temperature dependence in the range 270° to 350°C, which may reflect a change in underlying deformation mechanism. We quantify the trends in parameter values from 25° to 600°C by piecewise linear regressions, which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip. The modeling results suggest that the succeptibility for unstable (stick-slip) sliding is maximized between 90° and 360°C, in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust.

Analysis of fault slip inversions: Do they constrain stress or strain rate?

Abstract: Fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates, which are not necessarily parallel or equal. At the local scale of an individual fault, the shear plane and slip direction define the orientations of the local principal strain rate axes but not, in general, the local principal stress axes. At a large scale, the orientations of P and T axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress. The quantitative inversion of such fault slip data, however, provides direct constraints on the orientations and relative magnitudes of the global principal strain rates. To interpret the inversion solution as constraining the global principal stresses requires that (1) the fault slip pattern must have a characteristic symmetry no lower than orthorhombic; (2) the material must be mechanically isotropic; and (3) there must be a linear constitutive relationship between the global stress and the global strain rate. Isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity. Fault slip inversions, however, constrain the characteristics of a large-scale cataclastic flow, which is described by constitutive equations that are probably, but to an unknown degree, anisotropic and nonlinear. Such material behavior would not strictly satisfy the requirements for the stress interpretation. Thus, at the present state of knowledge, fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates.

Holocene left-slip rate determined by cosmogenic surface dating on the Xidatan segment of the Kunlun fault (Qinghai, China)

Abstract: Cosmogenic dating, using in situ {sup 26}Al and {sup 10}Be in quartz pebbles from alluvial terrace surfaces, constrains the late Holocene slip rate on the Xidatan segment of the Kunlun fault in northeastern Tibet Two terrace risers offset by 24 {+-} 3 and 33 {+-} 4 m, having respective ages of 1799 {+-} 388 and 2914 {+-} 471 yr, imply a slip rate of 121 {+-} 26 mm/yr The full range of ages obtained ({le}228 ky, most of them between 67 and 14 ky) confirm that terrace deposition and incision, hence landform evolution, are modulated by post-glacial climate change Coupled with minimum offsets of 9--12 m, this slip rate implies that great earthquakes (M {approximately}8) with a recurrence time of 800--1000 yr, rupture the Kunlun fault near 94 E

Conditions under which Velocity-Weakening Friction Allows a Self-healing versus a Cracklike Mode of Rupture

Abstract: Slip rupture processes on velocity-weakening faults have been found in simulations to occur by two basic modes, the expanding crack and self-healing modes. In the expanding crack mode, as the rupture zone on a fault keeps expanding, slip continues growing everywhere within the rupture. In the self-healing mode, rupture occurs as a slip pulse propagating along the fault, with cessation of slip behind the pulse, so that the slipping region occupies only a small width at the front of the expanding rupture zone. We discuss the determination of rupture mode for dynamic slip between elastic half-spaces that are uniformly prestressed at background loading level τ b 0 outside a perturbed zone where rupture is nucleated. The interface follows a rate and state law such that strength τ strength approaches a velocity-dependent steady-state value τ ss ( V ) for sustained slip at velocity V , where d τ ss ( V )/ dV ≦ 0 (velocity weakening). By proving a theorem on when a certain type of cracklike solution cannot exist, and by analyzing the results of 2D antiplane simulations of rupture propagation for different classes of constitutive laws, and for a wide range of parameters within each, we develop explanations of when one or the other mode of rupture will result. The explanation is given in terms of a critical stress level τ pulse and a dimensionless velocity-weakening parameter T that is defined when τ b 0 ≥ τ pulse . Here τ pulse is the largest value of τ b 0 satisfying τ b 0 − (μ/2 c ) V ≤ τ ss ( V ) for all V > 0, where μ is the shear modulus and c is the shear wave speed. Also, T = [− d τ ss ( V )/ dV ]/(μ/2 c ) evaluated at V = V dyna , which is the largest root of τ b 0 − (μ/2 c ) V = τ ss ( V ); T = 1 at τ b 0 = τ pulse , and T diminishes toward 0 as τ b 0 is increased above τ pulse . We thus show that the rupture mode is of the self-healing pulse type in the low-stress range, when τ b 0 pulse or when τ b 0 is only slightly greater than τ pulse , such that T is near unity (e.g., T > 0.6). The amplitude of slip in the pulse diminishes with propagation distance at the lowest stress levels, whereas the amplitude increases for τ b 0 above a certain threshold τ arrest , with τ arrest pulse in the cases examined. When τ b 0 is sufficiently higher than τ pulse that T is near zero (e.g., T Thus rupture under low stress is in the self-healing mode and under high stress in the cracklike mode, where our present work shows how to quantify low and high. The results therefore suggest the possibility that the self-healing mode is common for large natural ruptures because the stresses on faults are simply too low to allow the cracklike mode.

Role of oblique convergence in the active deformation of the Himalayas and southern Tibet plateau

Abstract: Noting similarities with subduction along curved oceanic trenches and using a simple block model, we show that radial vergence evident in earthquake slip vectors along the Himalayan deformation front, east-west extension on north-trending normal faults in the Himalayas and southern Tibet, and right-lateral strike slip on the Karakorum-Jiali fault zone can all result from basal shear caused by the Indian plate sliding obliquely beneath Tibet along a gently dipping, arcuate plate boundary. Within the framework of this mechanism, the normal faults in the Himalayas and southern Tibet are not proxies for the uplift history of Tibet. The distribution and style of the faults in the Himalayas and southern Tibet suggest that the basal drag from the underthrusting Indian lithosphere extends northward beneath most of southern Tibet.

Transpression (or transtension) zones of triclinic symmetry: natural example and theoretical modelling

Abstract: We describe a natural shear zone with triclinic symmetry, present a general model for triclinic shear zones based on natural examples, and investigate the kinematics and strain geometry within such zones. In the Roper Lake shear zone in the Canadian Appalachians, the orientation of a stretching lineation is oriented approximately down-dip near the shear zone boundary and becomes gradually shallower towards the centre. The structures in the central portion of the shear zone exhibit approximately monoclinic symmetry where the poles to both the S- and C-surfaces, the stretching lineation on the S-surfaces and the striations on the C-surfaces all plot in a great circle girdle. However, the lineations from the marginal portion do not plot in the same girdle, and the bulk symmetry of the shear zone is triclinic. Theoretical modelling shows that the observed strain geometry can be interpreted by an oblique transpression with a larger ratio of simple shear to pure shear in the centre of the shear zone than in the margin. The latter suggests a higher degree of localization of the zone boundary-parallel movement component relative to the boundary-normal compression component. We emphasize that since the imposed boundary displacements for most natural shear zones lie between dip slip and strike slip, their movement pictures are generally triclinic; monoclinic shear zones are special end members. Structural data that exhibit monoclinic symmetry do not necessarily mean that they resulted from a monoclinic movement picture; the present modelling demonstrates that a triclinic movement picture with a high ratio of boundary-parallel movement to boundary-normal movement can result in apparent monoclinic structural geometry. The results of the modelling also show that the simple statement made for simple shear zones that stretching lineations will align with, and therefore indicate, the shear direction cannot be extrapolated to three dimensional transpressional(-transtensional) shear zones.

Mechanical deformation of atomic-scale metallic contacts: Structure and mechanisms

Abstract: We have simulated the mechanical deformation of atomic-scale metallic contacts under tensile strain using molecular dynamics and effective medium theory potentials. The evolution of the structure of the contacts and the underlying deformation mechanisms are described along with the calculated electronic conductance. Various defects such as intersecting stacking faults, local disorder, and vacancies are created during the deformation. Disordered regions act as weak spots that reduce the strength of the contacts. The disorder tends to anneal out again during the subsequent atomic rearrangements, but vacancies can be permanently present. The transition states and energies for slip mechanisms have been determined using the nudged elastic band method, and we find a size-dependent crossover from a dislocation-mediated slip to a homogeneous slip when the contact diameter becomes less than a few nm. We show that the results measured in a nanocontact experiment depend significantly on the elastic stiffness of the experimental apparatus. For a soft setup, some of the atomic rearrangements might not be detected, whereas others are amplified.

Models for long-/short-range interactions and cross slip in 3D dislocation simulation of BCC single crystals

Abstract: Models and rules for short-range interactions, cross slip and long-range interactions of dislocation segments for implementation in a 3D dislocation dynamics (3DD) model are developed. Dislocation curves of arbitrary shapes are discretized into sets of straight segments of mixed dislocations. Long-range interactions are evaluated explicitly based on results from the theory of dislocations. Models for short-range interactions, including, annihilation, formation of jogs, junctions, and dipoles, are developed on the basis of a `critical-force' criterion that captures the effect of the local fields from surrounding dislocations. In addition, a model for the cross-slip mechanism is developed and coupled with a Monte Carlo type analysis to simulate the development of double cross slip and composite slip. The model is then used to simulate stage I (easy glide) stress-strain behaviour in BCC single crystals, illustrating the feasibility of the 3DD model in predicting macroscopic properties such as flow stress and hardening, and their dependence on microscopic parameters such as dislocation mobility, dislocation structure, and pinning points.

The motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike-slip faults

Abstract: Geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere. If the dominant forces acting on crustal blocks are tractions at their bases, then the long-term motion of each block will be given by the average velocity of the underlying lithosphere. Slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the South Island of New Zealand and Southern California are in good agreement with slip rates estimated geologically.

Modeling dynamic rupture in a 3D earthquake fault model

Abstract: We propose a fourth-order staggered-grid finite-difference method to study dynamic faulting in three dimensions. The method uses an implementation of the boundary conditions on the fault that allows the use of general friction models including slip weakening and rate dependence. Because the staggered-grid method defines stresses and particle velocities at different grid points, we preserve symmetry by implementing a two-grid-row "thick" fault zone. Slip is computed between points located at the borders of the fault zone, while the two components of shear traction on the fault are forced to be symmetric inside the fault zone. We study the properties of the numerical method comparing our simulations with well-known properties of seismic ruptures in 3D. Among the properties that are well modeled by our method are full elastic-wave interactions, frictional instability, rupture initiation from a finite initial patch, spontaneous rupture growth at subsonic and supersonic speeds, as well as healing by either stopping phases or rate-dependent friction. We use this method for simulating spontaneous rupture propagation along an arbitrarily loaded planar fault starting from a localized asperity on circular and rectangular faults. The shape of the rupture front is close to elliptical and is systematically elongated in the in- plane direction of traction drop. This elongation is due to the presence of a strong shear stress peak that moves ahead of the rupture in the in-plane direction. At high initial stresses the rupture front becomes unstable and jumps to super-shear speeds in the direction of in-plane shear. Another interesting effect is the development of relatively narrow rupture fronts due to the presence of rate-weakening friction. The solutions for the "thick fault" boundary conditions scale with the slip-weakening distance (Do) and are stable and reproducible for Do greater than about 4 in terms of 2T,//.t × Ax. Finally, a comparison of scalar and vector boundary conditions for the friction shows that slip is dominant along the direction of the prestress, with the largest deviations in slip-rate direction occurring near the rupture front and the edges of the fault.

Active antenna for contact sensing

Abstract: This paper proposes a new active sensor system (active antenna) that can detect not only the contact location between an insensitive flexible beam and an environment but also the information of the environment's surface where the beam makes contact. The active antenna is simply composed of a flexible beam, actuators to move the beam, position sensors to measure the rotational angle of the beam, and a moment sensor. We first show that the contact distance under no lateral slip is proportional to the rotational compliance that the beam can sense at the rotational center. The lateral slip, which possibly occurs according to the pushing direction and the environment's geometry, overestimates the rotational compliance, and as a result, brings a large sensing error for the localizing contact point. The goal of this paper is to find the contact location under such conditions. We explore how to detect a lateral slip and how to determine the new pushing direction to avoid it. We show an algorithm that can search for the pushing direction which can avoid any lateral slip. The convergence of this algorithm is shown and a practical utilization of this algorithm is also discussed with a trade-off between the number of trials and the sensing accuracy.

Properties and implications of dynamic rupture along a material interface

Abstract: We perform two-dimensional plane-strain finite-difference calculations of dynamic rupture along an interface separating different elastic media. The calculations extend earlier results of Andrews and Ben-Zion (1997) who found a self-sustaining narrow slip pulse associated with dynamic reduction of normal stress along a material interface governed by constant friction, in agreement with Weertman (1980). The pulse propagates in a wrinklelike mode having remarkable dynamic properties that may be relevant to many geophysical phenomena. Here we examine the range of values of elastic parameters, friction coefficient, and strength heterogeneities allowing for the existence of the wrinklelike pulse. Rupture is initiated in the simulations by imposed slip in a limited space-time domain. Outside the region of the imposed slip, the pulse becomes narrower and higher with propagation distance along the interface. The strength of the wrinklelike pulse increases with S -wave velocity contrast up to a maximum at about 35% contrast. Beyond such a velocity contrast, there is no solution for a generalized Rayleigh wave along a material interface, and the strength of the pulse decreases. However, the wrinklelike pulse can still propagate in a self-sustaining manner for larger velocity contrasts. For a fixed S -wave velocity contrast, the strength has little dependence on density contrast or Poisson9s ratio, but the pulse strength increases rapidly with increasing coefficient of friction. Stress and strength heterogeneities with small correlation length have little effect on the pulse, while long wavelength heterogeneities reduce the strength of the pulse. The high mechanical efficiency of the wrinklelike pulse suggests that earthquake ruptures may favor such mode of failure when possible.

Source parameters of large African earthquakes: implications for crustal rheology and regional kinematics

Abstract: Summary Source parameters for 38 African earthquakes have been determined using P and SH body-waveform inversion of analogue and digital waveforms. The results of this modelling are combined with 15 other earthquakes whose source parameters are also well constrained by body-waveform inversion. This data set shows that parts of East Africa have a seismogenic thickness of up to ∼ 35 km, and that seismicity occurs throughout the upper and lower crust. 1-D heat-flow calculations suggest temperatures of ∼ 325–475 ° C at 35 km depth in the Archaean and Proterozoic crust, requiring that the lower crust has a dry, mafic bulk composition in order to deform seismogenically. The slip vectors associated with these 53 earthquakes are combined with an additional 28 solutions from the Harvard CMT catalogue in order to investigate plate kinematics. In the Red Sea/Gulf of Aqaba the slip vectors agree closely with the Africa–Arabia extension direction predicted by the current best-fit plate model of Jestin, Huchon & Gaulier (1994). In the southern East African Rift System motion is split between a southeast direction in Zambia and a northeast direction in Malaŵ i and Rukwa, which combine to achieve the predicted Africa–Somalia east–west extension. Thus the current best-fit plate model of Jestin et al. (1994) adequately describes the active kinematics of Africa.

Hairpin river loops and slip-sense inversion on southeast asian strike-slip faults

Abstract: In the Golden Triangle region of southeast Asia (northern Thailand, Laos and Burma, southern Yunnan), the Mekong, Salween, and neighboring rivers show hairpin geometries where they cross active strike-slip faults. Restoration of young, left-lateral offsets of these rivers leaves residual right-lateral bends of many kilometers. We interpret these hairpins as evidence of late Cenozoic slip-sense inversion on these faults, about 5 to 20 Ma. Near the Red River fault, stress field and slip-sense inversion occurred ca. 5 Ma. This implies that the present course of these large rivers has existed for at least several million years. Pliocene–Quaternary slip rates, possibly on the order of 1 mm/yr, are inferred on each of the strike-slip faults of the Golden Triangle.

Time-resolved studies of stick-slip friction in sheared granular layers

Abstract: Sensitive and fast force measurements are performed on sheared granular layers undergoing stick-slip motion, along with simultaneous optical imaging. A full study has been done for spherical glass particles with a 20% size distribution. Stick-slip motion due to repetitive fluidization of the granular layer occurs for low driving velocities. Between major slip events, slight creep occurs that is highly variable from one event to the next. The effects of varying the stiffness k of the driving system and the driving velocity V are studied in detail. The stick-slip motion is almost periodic for spherical particles over a wide range of parameters, whereas it becomes irregular when k is large and V is relatively small. At larger V, the motion becomes smoother and is affected by the inertia of the upper plate bounding the layer. Measurements of the period and amplitude of the relative motion are presented as a function of V. At a critical value ${V}_{c}$ a transition to continuous sliding motion occurs. The transition is discontinuous for k not too large, and large fluctuations occur in the neighborhood of the transition. The time dependence of the instantaneous velocity of the upper plate and the frictional force produced by the granular layer are determined within individual slipping events. The frictional force is found to be a multivalued function of the instantaneous velocity during slip, with pronounced hysteresis and a sudden drop just prior to resticking. Measurements of vertical displacement reveal a very small dilation of the material (about one-tenth of the mean particle size in a layer 20 particles deep) associated with each slip event; the dilation reaches its maximum amplitude close to the time of maximum acceleration. Finally, optical imaging reveals that localized microscopic rearrangements precede (and follow) each macroscopic slip event; their number is highly variable and the accumulation of these local displacements is associated with macroscopic creep. The behavior of smooth particles is contrasted qualitatively with that of rough particles.

Dike‐induced earthquakes: Theoretical considerations

Abstract: Earthquakes of magnitude 1 and greater seem to be ubiquitous features of dike propagation, but their origin is not well understood. We examine the elastic stress field surrounding propagating fluid-filled cracks, with an emphasis on assessing the ambient stress required to produce earthquakes with linear dimensions of ∼100 m near dikes with linear dimensions of a few kilometers. An important feature of the solutions is the dike “tip cavity,” a low-pressure region where magma cannot penetrate and where the stress field differs most from the classical near-tip stress field. Two regions are considered: near the dike tip but away from the tip cavity and near the tip cavity. The stress state most conducive to failure occurs near the tip cavity when the cavity pressure is maintained by influx of host rock pore fluids rather than by exsolution of magmatic volatiles. Even in this case, however, shear fracture of previously intact rock seems unlikely. Thus most dike-induced seismicity with a frequency content typical of “tectonic” earthquakes should be interpreted as resulting from slip along suitably aligned existing fractures. Production of magnitude 1 earthquakes appears to require either large ambient differential stresses or low ambient confining pressures; in the latter case, the effective normal stress on prospective faults may be low enough for slip to be aseismic. We conclude that the distribution of (recorded) dike-induced seismicity reflects the distribution of ambient stresses that are near to failure and does not necessarily reflect the extent of the dike. This result is consistent with recent images of the seismicity associated with the 1983 dike intrusion at Kilauea.

Dynamic recrystallization and texture development in ice as revealed by the study of deep ice cores in Antarctica and Greenland

Abstract: The preferred c axis orientation of ice from polar ice sheets develops essentially as a result of intracrystalline slip; but dynamic recrystallization appears to alter the kinetics of the development of deformation textures and is, at high temperature, at the origin of recrystallization textures. The purpose of this work is to obtain a better understanding of recrystallization processes that occur in polar ice sheets and to clarify the relationship between dynamic recrystallization and textures. The study was based on two deep ice cores from Greenland and Antarctica, the GReenland Ice core Project (GRIP) and Vostok ice cores. The structure along the GRIP core displays normal grain growth in the first 100 m of the ice sheet and rotation recrystallization and migration recrystallization near the bottom. Only grain growth and rotation recrystallization appear to occur in the Vostok ice core. The transition between these recrystallization regimes was studied, estimating, for interglacial ice, the evolution with depth of the dislocation density. This calculation has shown the efficiency of grain boundary migration for the absorption of dislocations. At Vostok, the highest value of the dislocation density is found at a depth of about 1000 m and the continuous decrease in the dislocation density below this depth is related to the increase of the grain boundary migration rate. It is shown that the driving force required to initiate migration recrystallization is not reached in interglacial ice at Vostok. The observed textures were compared with those predicted by the self-consistent approach. Recrystallization textures are interpreted by assuming that the less stressed grains, i.e., the best oriented for basal slip, are favored by the size advantage of subgrains. The recrystallization textures are compared with those of other materials.