Showing papers on "Slip (materials science) published in 1990"

Inversion of field data in fault tectonics to obtain the regional stress—III. A new rapid direct inversion method by analytical means

Abstract: SUMMARY A new method for determining the reduced stress tensor with four degrees of freedom (the orientations of the three principal stress axes as well as the ratio of principal stress differences) using fault slip data (or focal mechanisms of earthquakes) is presented. From a computational point of view, the inversion of fault slip data is made in a direct way by purely analytical means; as a result, the determination process is extremely fast and adaptable on small microcomputers. From a physical point of view, the method aims at simultaneously (i) minimizing the angles between theoretical shear stress and actual slip vector and (ii) having relative magnitudes of shear stress large enough to induce slip despite rock cohesion and friction. Examples of application to actual fault slip data sets with good or poor variety of fault slip orientations are shown. The double significance of the basic criterion adopted results in a more realistic solution of the inverse problem than the single minimization of the shear-stria angle.

Shear flow near solids: Epitaxial order and flow boundary conditions.

Abstract: We report on molecular-dynamics simulations of Lennard-Jones liquids sheared between two solid walls. The velocity fields, flow boundary conditions, and fluid structure were studied for a variety of wall and fluid properties. A broad spectrum of boundary conditions was observed including slip, no-slip, and locking. We show that the degree of slip is directly related to the amount of structure induced in the fluid by the periodic potential from the solid walls. For weak wall-fluid interactions there is little ordering and slip was observed. At large interactions, substantial epitaxial ordering was induced and the first one or two fluid layers became locked to the wall. This epitaxial ordering was enhanced when the wall and fluid densities were equal. For unequal densities, high-order commensurate structures formed in the first fluid layer creating slip within the fluid.

Frictional behavior and constitutive modeling of simulated fault gouge

Abstract: This paper presents an investigation of the factional properties and stability of frictional sliding for simulated fault gouge. In these experiments we sheared gouge layers (quartz sand) under saturated drained conditions and at constant normal stress (50–190 MPa) between either rough steel surfaces or Westerly granite surfaces in a triaxial apparatus. Surface roughness (60 to 320 grit) and gouge layer thickness (0–4.0 mm) were varied in the experiments with granite samples. Porosity ϕ was monitored continuously during shear. Our measurements indicate that granular gouge exhibits strain hardening and net compaction for shear strains γ less than 0.5–1.0. For γ > 0.5–1.0, sliding occurs at approximately constant shear stress and net compaction from one load/unload cycle to the next ceases. Dilatancy occurs at 1/3 to 1/2 the shear stress required for sliding and d2ϕ/dγ2 becomes negative at about the peak stress in a given loading cycle, indicating the onset of shear localization. Oblique shear bands appear in the layers at γ = 1.3–1.5. Experiments with an initial gouge layer exhibit velocity strengthening (the coefficient of friction increases with slip velocity), and initially bare granite surfaces exhibit velocity weakening. The magnitude of velocity strengthening varies inversely with normal stress and directly with gouge thickness and surface roughness. In the gouge experiments the dilatancy rate dϕ/dγ also varies with slip rate. Using a simple energy balance to relate volume change and frictional resistance, we find quantitative agreement between the measured change in dilatancy rate and friction following changes in slip rate. This indicates that velocity strengthening within granular gouge is the result of dilatancy. The slip rate dependence of dϕ/dγ increases with gouge thickness and surface roughness, in agreement with the friction data. Our data therefore suggest that slip within unconsolidated granular material, such as some natural fault gouges, is inherently stable. The results thus provide an explanation for (1) the tendency of gouge accumulation to stabilize slip in laboratory samples, and (2) the tendency for aseismic slip within shallow (< 3–5 km) unconsolidated fault gouge and within unconsolidated sediments such as shallow alluvium and accretionary prisms.

Origin of stick-slip motion in boundary lubrication.

Abstract: Molecular dynamics simulations of atomically thin, fluid films confined between two solid plates are described. For a broad range of parameters, a generic stick-slip motion is observed, consistent with the results of recent boundary lubrication experiments. Static plates induce crystalline order in the film. Stick-slip motion involves periodic shear-melting transitions and recrystllization of the film. Uniform motion occurs at high velocities where the film no longer has time to order. These results indicate that the origin of stick-slip motion is thermodynamic instability of the sliding state, rather than a dynamic instability as usually assumed.

Determination of total strain from faulting using slip measurements

Abstract: IN the past twenty years it has become commonplace in seismology to sum seismic moments for large earthquakes to determine the contemporary fault slip rate1 or regional strain rate produced by earthquakes2. The method has several drawbacks: it often greatly underestimates deformation rates predicted from plate tectonics, either because the seismic history is of insufficient length or a substantial amount of fault slip is aseismic. Also, it can be used to calculate only current deformation rates, and cannot be applied to earlier geological eras or to estimate total strain. These problems can be overcome by applying the same methods to geological measures of fault displacement. Complete fault data sets, however, are generally not available. Here we show that faults obey general scaling laws in their size frequency distribution and in the relation between displacement and fault length. Combining these scaling relations, we demonstrate that the calculation of strain can be successfully applied to sparse geological data sets, because most strain is produced by the largest faults so that the data set need not be complete for small fault sizes.

Temperatures, heat flux, and frictional stress near major thrust faults

Abstract: We examine the two-dimensional advective and conductive transport of heat in a region of thrust faulting. Both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor, S=1+b(zƒVsin δ)/κ below what they would be with the same heat sources but in the absence of advection. In this expression zƒ is the depth to the fault, V is the slip rate, δ is the component of the dip of the fault in the direction of underthrusting, κ is thermal diffusivity, and b is a dimensionless factor that is essentially equal to one for most forms of heating. Initial changes in temperatures near the fault are given by T(zƒ,t)=T(zƒ,0)-12Vsinδ∂T(z,0)/∂z. These simple formulae are successful because of the neglible influence of lateral conduction of heat, at least for slip rates of a few mm/yr or more. Two time constants govern the transition from the initial change in temperature to steady state: t1 = uƒ/V, where uƒ is the distance along the fault in the direction of underthrusting from the surface to the depth in question, and t2=zƒ2/κπ2, where zƒ is the depth to the fault. When elapsed times exceed the sum of these two time constants, temperatures differ from their steady state values by only about 10 percent. The numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted. Putain. An application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 MPa and are greater than 30 MPa. Calculations of temperatures appropriate for the Himalaya suggest that shear stresses of 100 MPa on the Main Central Thrust probably are required to account for the Tertiary granites of the region, if melting took place after slip began on the thrust. Similarly, the cut-off in seismicity at a depth of about 15 km in the Himalaya, if due to temperatures exceeding 350° to 450°C, implies a deviatoric stress close to 100 MPa.

Fault-propagation folds: Geometry, kinematic evolution, and hydrocarbon traps

Abstract: Fault-propagation folding, a common folding mechanism in fold and thrust belts, occurs when a propagating thrust fault loses slip and terminates upsection by transferring its shortening to a fold developing at its tip. Area-balanced theoretical models that relate the footwall cutoff angle (theta) to the fold interlimb half-angles (gamma* and gamma) show that open folds (high gamma* and gamma) are characterized by relative thickening of stratigraphically higher units, whereas tight folds (low gamma* and gamma) are characterized by relative thinning of these units. The propagation of thrusts is commonly characterized by the progressive tightening of the fold hinge and steepening of the front limb. Thickening of stratigraphically higher units in the early stages of folding i followed by localized thinning of the front limb in the late stages. The geometry of a fault-propagation fold can be modified by subsequent translation on propagating thrusts. The thrust fault may propagate through the undeformed units, along the synclinal axial plane, or through the forelimb of the anticline, depending on the tightness of the fold. Deeper thrusts commonly are abandoned upsection, and the slip transferred to steeper imbricates, resulting in the listric geometries of many thrust faults. The fold also can be transported over a ramp and onto an upper detachment, resulting in a transition to fault-bend folding. Fault-propagation folds with or without additional fold translation can be distinguished from translated detachment folds by the detailed geometries of the hanging wall and footwall structures, and by the characteristic differences n their relations between fault slip and depth to detachment. Some important characteristics of fault-propagation folds are that they require no slip transfer in or out of the structure, involve a minimum amount of shortening, and have a relatively large depth to detachment, compared to other types of fault-related folds. Fault-propagation folds form important hydrocarbon traps in fold and thrust belts. Some common trap types include fold traps in the crestal area, and fault traps in the footwall and along imbricates on the forelimb and the backlimb of major basement-detached and basement-involved anticlines. Secondary traps also occur in intraplate and leading-edge structures within major thrust sheets.

Rupture model of the 1989 Loma Prieta earthquake from the inversion of strong motion and broadband teleseismic data

Abstract: We have used 24 broadband teleseismic and 48 components of local strong motion velocity records of the 1989 Loma Prieta earthquake in a formal inversion to determine the temporal and spatial distribution of slip. Separate inversions of the teleseismic data (periods 3-30 sec) or strong motion data (periods 1-5 sec) result in similar models. The data require bilateral rupture with relatively little slip in the region directly updip from the hypocenter. Slip is concentrated in two patches; one centered 6 km northwest of the hypocenter at a depth of 12 km and with a maximum slip of 350 cm, and the other centered about 5 km southeast of the hypocenter at a depth of 16 km and with a maximum slip of 460 cm. The bilateral nature of the rupture results in large amplitude ground motions at sites located along the fault strike, both to the northwest and the southeast. However, the northwestern patch has a larger moment and overall stress drop and is, consequently, the source of the largest ground motion velocities, consistent with the observed recordings. This bilateral rupture also produces relatively modest ground motion amplitudes directly updip from the hypocenter, which is in agreement with the velocity ground motions observed at Corralitos. There is clear evidence of a foreshock (magnitude between 3.5 and 5.0) or a slow rupture nucleation about 2 seconds before the main part of the rupture; the origin time implied by strong motion trigger times is systematically 2 seconds later than the time predicted from the high-gain regional network data. The seismic moment obtained from either of the separate data sets or both sets combined is about 3.0 x 10^(26) dyne-cm and the potency is 0.95 km^3.

The 1960 Chile earthquake: inversion for slip distribution from surface deformation

Abstract: SUMMARY A total of 166 observations of sea-level change, 130 measurements of elevation difference, and 16 determinations of horizontal strain provide an excellent view of the (quasi-)static source process of the great 1960 Chilean earthquake. These surface deformation data were employed in classical uniform slip fault models as well as more recently developed models that allow spatial variability of slip. The best uniform slip planar (USP) model is 850km long, 130km wide, and dips 20°. Seventeen metres of fault displacement contributed to a USP moment of 9.4 times 1022 N m. The variable slip planar (VSP) model concentrates slip on a 900 km long, 150 km wide band parallel to the coast. Several peaks of slip with dimensions of 50–100 km appear in this band and are thought to represent major subduction zone asperities. Important fractures of the oceanic lithosphere bound the 1960 rupture and are offered as a potential source of fault segmentation within the Chilean subduction zone. The VSP moment for 1960 earthquake totals 9.5 times 1022 N m, about one fifth of the value estimated for the foreshock-mainshock sequence from seismic methods. Except for areas out to sea, geodetic resolution on the fault is fairly uniform. Thus, it is unlikely that slip missed by the network could increase the VSP moment much beyond 1.8 times 1023 N m. Several patches of moment, isolated from the main body at 80–110 km depth, are found down dip in the VSP model and are presumably indicative of aseismic slip. One patch at the northern end of the rupture is probably associated with the initiation phase of the mainshock, although the time sequence of the relationship is unknown. Tide gauge records suggest that another patch between 40° and 43° S, responsible for the observed strain and uplifts inland at those latitudes, is not of coseismic origin, but derives from in-place, post-seismic creep over several years. Apparently, great 1960-type events are not typical members of the ∼ 128 yr earthquake cycle in south-central Chile. The Nazca-South America boundary here is characterized by a variable rupture mode in which major asperities are completely broken by great earthquakes only once in four or five earthquake cycles. The more frequent large earthquakes, that geographically overlap the great events, fill in between the locked zones.

Mechanisms of slip mode modification in F.C.C. solid solutions

Abstract: While it is widely recognized that alloy factors other than stacking fault energy play a role in promoting planarity of slip, no detailed model has been advanced to explain the mechanism of planar vs wavy slip mode Therefore, friction stress effects of alloying on disolcation behavior are reviewed, as well as the role of stacking fault structure in inhibiting the clustering of dislocations in planar slip metal A model of cross-slip inhibition (and thus planar slip behavior) is developed from the idea that the joining of partials is resisted by frictional effects Planarity of slip is promoted not only by low stacking fault energy but by increase in shear modulus, atomic size misfit and solute content A critical solute concentration is predicted by the model for the transition from wavy slip to planar slip and this is shown to be in good agreement with observations for copper base solid solutions and other alloy systems

Right-lateral shear and rotation as the explanation for strike-slip faulting in eastern Tibet

Abstract: Bounds are placed here on the rate of rotation proposed by Cobbold and Davy (1988) for the major strike-slip faults in the eastern Tibetan Plateau. It is also concluded here that the image of lateral transport on such faults, known also as continental escape, extrusion, or expulsion, is an illusion, and that instead the left-lateral slip on east-striking plates in eastern Tibet is a manifestation of north-striking right-lateral simple shear. If this conclusion is correct, the east-striking left-lateral faults and the crustal blocks between them are rotating clockwise at 1-2 deg/Myr, the east-west dimension of eastern Tibet is shortening at 10-20 mm/yr, and little material is moving eastward out of India's path into Eursasia by left-lateral simple shear.

Tactile detection of slip: surface microgeometry and peripheral neural codes.

Abstract: 1. The role of the microgeometry of planar surfaces in the detection of sliding of the surfaces on human and monkey fingerpads was investigated. By the use of a servo-controlled tactile stimulator to press and stroke glass plates on passive fingerpads of human subjects, the ability of humans to discriminate the direction of skin stretch caused by friction and to detect the sliding motion (slip) of the plates with or without micrometer-sized surface features was determined. To identify the associated peripheral neural codes, evoked responses to the same stimuli were recorded from single, low-threshold mechanoreceptive afferent fibers innervating the fingerpads of anesthetized macaque monkeys. 2. Humans could not detect the slip of a smooth glass plate on the fingerpad. However, the direction of skin stretch was perceived based on the information conveyed by the slowly adapting afferents that respond differentially to the stretch directions. Whereas the direction of skin stretch signaled the direction of impending slip, the perception of relative motion between the plate and the finger required the existence of detectable surface features. 3. Barely detectable micrometer-sized protrusions on smooth surfaces led to the detection of slip of these surfaces, because of the exclusive activation of rapidly adapting fibers of either the Meissner (RA) or the Pacinian (PC) type to specific geometries of the microfeatures. The motion of a smooth plate with a very small single raised dot (4 microns high, 550 microns diam) caused the sequential activation of neighboring RAs along the dot path, thus providing a reliable spatiotemporal code. The stroking of the plate with a fine homogeneous texture composed of a matrix of dots (1 microns high, 50 microns diam, and spaced at 100 microns center-to-center) induced vibrations in the fingerpad that activated only the PCs and resulted in an intensive code. 4. The results show that surprisingly small features on smooth surfaces are detected by humans and lead to the detection of slip of these surfaces, with the geometry of the microfeatures governing the associated neural codes. When the surface features are of sizes greater than the response thresholds of all the receptors, redundant spatiotemporal and intensive information is available for the detection of slip.

Rigid body collisions with friction

Abstract: An energetically consistent theory is presented for dynamics of partly elastic collisions between somewhat rough rigid bodies with friction that opposes slip. This theory is based on separately accounting for frictional and non-frictional sources of dissipation. Alternative theories derived from Newton’s impact law or Poisson’s impact hypothesis are shown to be valid only for central (collinear) or non-frictional collisions; generally the latter theories yield erroneous energy dissipation if small initial slip stops during collision between eccentric bodies. Collision processes are complex when small slip is stopped by friction; then either the direction of slip reverses or contact points roll without slip. An inconsistent theory based on Newton’s impact law can yield erroneous energy increases when slip stops during collision; the consistent theory always dissipates energy. The impact law that specifies a simple proportionality between normal components of contact velocity for incidence and rebound is not applicable in any range of incident velocities with small slip if the collision is non-collinear with friction. In Percussion the force or Impetus whereby one body is moved may cause another body against which it strikes to be put in motion, and withal lose some of its strength or swiftness. (J. Wallis, 1668)

Deformation of polysynthetically twinned crystals of TiAl with a nearly stoichiometric composition

Abstract: Polysynthetically twinned crystals of TiAl with a nearly stoichiometric composition have been grown and deformed in compression at room temperature. The yield stress and deformation behaviour depend strongly on the angle between the twin boundaries and the compression axis rather than on the crystallographic orientation of the compression axis. The yield stress is high when compression is perpendicular or parallel to the twin boundaries, and is generally very low for specimens where the twin boundaries are at an intermediate angle to the compression axis. The ratio of the highest to the lowest values of yield stress is almost 8:1. This large difference in yield stress has been found to be related to the difference in the deformation mode; for compression perpendicular or parallel to the twin boundaries, shear deformation always occurs across them while, for compression at an intermediate angle, shear deformation is parallel to the boundaries. The former mode of deformation is much harder than the...

An In Situ Transmission Electron Microscope Deformation Study of the Slip Transfer Mechanisms in Metals

Abstract: The slip transfer mechanisms across grain boundaries in 310 stainless steel, high-purity aluminum, and a Ni-S alloy have been studied by using thein situ transmission electron microscope (TEM) deformation technique. Several interactions between mobile lattice dislocations and grain boundaries have been observed, including the transfer and generation of dislocations at grain boundaries and the nucleation and propagation of a grain boundary crack. Quantitative conditions have been established to correctly predict the slip transfer mechanism.

Late Cenozoic strike-slip faulting in the Mojave Desert, California

Abstract: Recent tectonic models for southern California treat the entire Mojave Desert Block as the site of distributed simple shear during late Cenozoic time. These models consider that much of the region is composed of a series of narrow blocks, bounded by active NW striking, right-slip faults that have facilitated the distortion and rotation of the region about vertical axes during translations. As much as 100 km of cumulative right slip is predicted for these faults by some of these models. These kinematic models require that the faults of the Mojave Desert Block merge with the Garlock fault, which is viewed as the intact northern boundary that served to accommodate the distortion of the Mojave Desert Block by simple shear. Map-scale structural relations are used to test explicit and implicit features of kinematic models proposed for the region. These relationships indicate that late Cenozoic NW striking, right-slip faults of the Mojave Desert Block possess the following characteristics: (1) the faults are discontinuous, with only the Calico-Blackwater fault spanning the entire Mojave Desert; (2) the faults terminate before reaching the Garlock fault; (3) faults south of an irregular line extending from near Barstow eastward to Ludlow and to Soda Lake are continuous and well developed and have a cumulative net slip of >40 km, whereas faults to the north are discontinuous and display <12 km of right slip; and (4) there is a northwestward decrease in net slip along most of the faults. A new kinematic model is proposed to reconcile these new observations with existing data. We assert that integrated strain within the province since middle Miocene time is not regionally homogeneous as predicted by simple shear models but is instead partitioned into six major domains. The domains probably have deformed and rotated about vertical axes independently of each other and are separated by zones of shortening or extension or by strike-slip faults. Strike-slip faults and folding have likely accommodated internal deformation and rotation of some of the domains. The model predicts that the Mojave Desert has been the site of ∼65 km of right shear since middle Miocene time. The broad network of faults of the Mojave Desert Block along with similar strike-slip faults of the Death Valley region constitute a regional zone of right shear, named here, the Eastern California shear zone. Because of its probable physical connection to the San Andreas fault system, the Eastern California shear zone may have accommodated a significant portion of Pacific-North American transform motion. The Eastern California shear zone accounts for 9–14% of the total shear, predicted from plate tectonic reconstructions, along the Pacific-North American transform boundary since ∼10.6 Ma. The kinematic connection of the normal faults of the Death Valley region, with the San Andreas fault system via the faults of the Mojave Desert accords with the deduction of Atwater (1970) that late Cenozoic extension in portions of the Basin and Range province is related to Pacific-North American transform shear. Finally, the present arcuate trace of the Garlock fault is ascribed to oroclinal folding within the broad zone of distributed shear of the Eastern California shear zone.

Basal slip and mechanical anisotropy of biotite

Abstract: The basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on (001). Samples loaded at 45° to (001) exhibit gentle external rotations associated with dislocation glide. High-angle kink bands in these samples, unlike those developed in micas loaded parallel to (001), are limited to sample corners. Samples shortened perpendicular to (001) show no evidence of nonbasal slip and fail by fracture over all conditions tested. The mechanical response of biotite shortened at 45° to (001) is nearly perfectly elastic-plastic; stress-strain curves are characterized by a steep elastic slope, a sharply defined yield point, and continued deformation at low (mostly 1%. Stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 MPa and exhibit weak dependencies upon strain rate and temperature. Assuming an exponential relationship between differential stress σd and strain rate e˙ of the form e˙=Cexp(ασd)exp(−Q/RT), the data collected over strain rates and temperatures of 10−7 to 10−4 s−1 and 20° to 400°C, respectively, are best fit by an exponential constant α of 0.41±0.08 MPa-1 and an activation energy Q of 82±13 kJ/mol. A power law fits the data equally well with n = 18±4 and Q = 51±9 kJ/mol. Samples oriented favorably for slip in directions [100] and [110] are measurably weaker than those shortened at 45° to [010] and [310], consistent with the reported Burgers vectors 〈100〉, 1/2 〈110〉, and 1/2 〈110〉. The anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to (001). Previous studies have shown that biotite loaded in the (001) plane is strong prior to the nucleation of kink bands. The strength of biotite shortened perpendicular to (001) exceeds that measured parallel to (001) and is pressure dependent. Application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions. Its presence within foliated rocks and shear zones may limit locally the stresses that can be supported.

TEM in situ deformation study of the interaction of lattice dislocations with grain boundaries in metals

Abstract: The passage of dislocations across grain boundaries in metals has been studied by using the in situ TEM deformation technique. A detailed analysis of the interaction of glissile matrix dislocations with grain-boundary dislocations has been performed. The results show that the dislocations piled-up at the grain boundary can: (1) be transferred directly through the grain boundary into the adjoining grain; (2) be absorbed and transformed into extrinsic grain-boundary dislocations; (3) be accommodated in the grain boundary, followed by the emission from the grain boundary of a matrix dislocation; and (4) be ejected back into their original grain. To predict which slip system is favourable for slip transfer, three criteria have been considered, namely: (1) the angle between the lines of intersection of the incoming and outgoing slip planes with the grain boundary, this should be as small as possible; (2) the resolved shear stress acting on the possible slip systems in the adjoining grain, this should ...

Equivalent strike‐slip earthquake cycles in half‐space and lithosphere‐asthenosphere earth models

Abstract: By virtue of the images used in the dislocation solution, the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike-slip fault in an Earth model consisting of an elastic plate (lithosphere) overlying a viscoelastic half-space (asthenosphere) can be duplicated by prescribed slip on a vertical fault embedded in an elastic half-space. For the case in which each earthquake ruptures the entire lithosphere (thickness H), the half-space equivalent slip rate is as follows: Depth interval 0-H, slip identical to that in lithosphere-asthenosphere model (i.e., abrupt coseismic slip and no subsequent slip); depth interval (2n−1)H to (2n+1)H (n = 1,2,…), slip rate uniform in space and dependent upon time as Fn(t) exp (−t/τa) where Fn is a (n - 1) degree polynomial in t, τa is twice the asthenosphere relaxation time (η/μ), and t is measured from the instant after the preceding earthquake. The slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion. For reasonable values of τa, slip rates below 5H do not vary much from that mean value and can be treated as constant. Thus the surface deformation due to the earthquake cycle in the lithosphere-asthenosphere model can be calculated very simply from the half-space model with time-dependent slip in the two depth intervals H−3H and 3H−5H, and uniform slip at a rate equal to the secular relative plate velocity below depth 5H. Inversion of 1973–1988 geodetic measurements of deformation across the segment of the San Andreas fault in the Transverse Ranges north of Los Angeles for the half-space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm/yr below 30 km. One equivalent lithosphere-asthenosphere model would have a 30-km thick lithosphere and an asthenosphere relaxation time greater than 33 years, but other models are possible.

Stress and the direction of slip on fault planes

Abstract: The shear stress direction on a fault plane depends only on four of the six components of the stress tensor. Assuming only that the slip direction marks the shear stress direction on any fault plane (and that stress is homogeneous), it is possible to estimate these four stress parameters from populations of fault planes with known slip directions, as several workers have observed. Different formulations of the problem may yield varying best-fitting stresses and estimates of uncertainty. In the simplest case, no assumptions are made regarding the orientations of fault planes relative to the stress tensor; thus the technique allows for the possibility that the fault planes may be very weak. Here we present simple analytical and graphical descriptions of the field of admissible fault geometries relative to any four-parameter stress model, which can be used to illustrate the significance of various inverse strategies. In particular, this paper explores the effects of using two alternative measures of misfit between an observed fault datum and stress model: (1) the pole rotation (the angle between the observed and predicted slip direction on the observed fault plane), and (2) the minimum rotation (the smallest angle between the observed fault geometry and any fault geometry which is consistent with the model). By allowing for variation of the fault plane as well as the slip vector, the minimum rotation procedure generally achieves a more stable and (presumably) realistic estimate of the actual discrepancy between a fault observation and stress model than the pole rotation procedure. In a test case using 17 earthquake focal mechanisms from the YuIi region of eastern Taiwan, separate inversions based on the two misfit criteria yield different optimum stress models and uncertainty estimates. Additional constraints on the stress tensor, such as the effect of friction, can be superimposed on the ones used here.

Order and diversity in the modes of Circum-Pacific Earthquake recurrence

Abstract: Recurrence characteristics of great circum-Pacific earthquakes and determinations of spatial distribution of seismic moment release are surveyed in order to delineate their general features and relate earthquake slip distribution to models of recurrent rupture. As noted by others, the pattern of moment release is typically very irregular, with strong concentrations in a few isolated regions of a much larger aftershock zone. Despite this complexity, rupture nucleation is notably systematic, with mainshock epicenter showing a strong tendency to locate in or immediately adjacent to identified regions of high moment release. This generalization suggests that earthquake recurrence is more likely to be controlled by maximum rather than average fault slip. Well-documented case histories from twelve plate boundary regions demonstrate that seismic strain release tends to be temporally well-ordered, while source dimensions, slip, and cumulative moment release vary considerably from cycle to cycle. On none of the segments studied do earthquakes consistently recur in nearly identical events. Instead, individual great earthquakes differ significantly from cycle to cycle or rerupture takes place in a sequence of two or more smaller events. Despite these differences the duration of the seismic cycle is approximately uniform. Furthermore, when strain release occurs in a sequence of large earthquakes, these events take place towards the end of the cycle and occur with increasing rupture length and magnitude. These ordered and irregular features of earthquake recurrence argue for the existence of corresponding elements on plate boundary faults. The ordered characteristics are identified with zones of concentrated moment release that slip comparable amounts in each cycle and have high shear strength. The irregular features are associated with intervening weaker regions that move in response to the stress concentration of dynamic rupture and slip by differing amounts in each event. Although these mechanistic associations are indirect and tentative, the features of recurrent behavior documented here have implications for long-term earthquake hazard assessment that are not dependent on the models proposed to explain them. Chief among these are the cycle-to-cycle differences among gap-filling events and the absence of shocks that fill major slip-deficient regions early in the seismic cycle.

Stress tensor and fault plane solutions for a population of earthquakes

Abstract: An algorithm for the simultaneous estimation of the orientation and shape of the stress tensor and the individual fault plane solutions for a population of earthquakes is studied. It corresponds to a synthesis of the methods used by Brillinger et al. (1980) to obtain focal mechanisms and by Armijo and Cisternas (1978) for stress tensor analysis in microtectonics. The input data are the polarities of the P arrival and take-off angles for the set of source-station pairs. The method distinguishes, in general, which one of the nodal planes corresponds to the fault and gives the direction of the slip. The application to the aftershock sequence of the 1980 Arudy earthquake (Western Pyrenees) shows that the observations may be explained by a single stress tensor producing a N32°E extension, with a likelihood of 95 per cent.

Polycrystalline plastic deformation and texture evolution for crystals lacking five independent slip systems

Abstract: W e clearly elucidate the kinematic constraints, and the corresponding kinematic indeterminacy of part of the deviatoric stress tensor, in a rigid-viscoplastic single crystal lacking five independent slip systems. The indeterminate stress component is a Lagrange multiplier enforcing the kinematic constraint, and it must be determined from equilibrium considerations. A simple polycrystalline model is constructed which precisely satisfies local kinematic constraints as well as global compatibility. Volume-average global stresses are obtained by approximating the local constraint stress as the corresponding projection of the (to-be-determined) global stress. Applications of the model to hexagonal crystals without pyramidal slip, and to large deformation and texturing of orthorhombic polycrystalline materials (olivine; HDPE) are made.

Amorphous material in high strain experimental fault gouges

Abstract: The microstructures of gouges produced in room temperature, rotary shear sliding experiments were examined by transmission electron microscopy. Gouges were produced by sliding on ground surfaces of granite, quartzite, or marble except for one experiment in which a 1-mm-thick simulated gouge layer was used. Water was added to the sliding surfaces of all but one sample. Crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles. All of the gouges consist of mostly submicron crystalline fragments; in addition, the granite gouges contain 5–60% amorphous material, and the quartzite gouge contains −50% amorphous material. In the granite samples the composition of the amorphous material commonly lies between K-rich and Na,Ca-rich feldspars, although portions may be silica-rich. The microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting. The amount of amorphous material increases, and the size of the largest crystalline fragments decreases, with an increase in average shear strain, although the microstructure is nearly uniform throughout each granite gouge layer. These observations suggest that after slip becomes localized on “Y” shear surfaces and/or R1 Riedel shears the entire gouge layer must continue to undergo deformation. It is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces.

Bookshelf faulting and horizontal block rotations between overlapping rifts in southern Afar

Abstract: Lateral slip on initially rift-parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts. It occurs in southern Afar, where clockwise block rotations result from distributed dextral shear between the overlapping Ghoubbet Asal-Manda Inakir and Manda Hararo-Abhe Bad rifts. Faulting observed during the 1969, Serdo earthquakes and on SPOT images is consistent with the shear being taken up by left-lateral slip on steep NW-SE striking faults, which formed as normal faults before extensional strain became localized in the two rifts. This bookshelf faulting accounts quantitatively for the 14.5°± 7.5° rotation documented by paleomagnetism in the 1.8 ± 0.4 Ma old Afar stratoid basalts, given the 17.5 ± 5 mm/yr rate of separation between Arabia and Somalia.

Boundary condition for fluid flow: Curved or rough surfaces.

Abstract: The curvature of the boundary is shown to alter the fluid's slip length, which may even become negative as a result. Due to the mesoscopic curvature of surface roughness, the microscopically calculated and the macroscopically measured slip lengths can be quite different.