Showing papers on "Slip (materials science) published in 1993"

A model for the motion of the Philippine Sea Plate consistent with NUVEL‐1 and geological data

Abstract: We investigate angular velocity vectors of the Philippine Sea (PH) plate relative to the adjacent major plates, Eurasia (EU) and Pacific (PA), and the smaller Caroline (CR) plate. Earthquake slip vector data along the Philippine Sea plate are inverted, subject to the constraint that EU-PA motion equals that predicted by the global relative plate model NUVEL-1. The resulting solution fails to satisfy geological constraints along the Caroline-Pacific boundary: convergence along the Mussau Trench and divergence along the Sorol Trough. We then seek solutions satisfying both the CR-PA boundary conditions and the Philippine Sea slip vector data, by adjusting the PA-PH and EU-PH best fitting poles within their error ellipses. We also consider northern Honshu to be part of the North American plate and impose the constraint that the Philippine Sea plate subducts beneath northern Honshu along the Sagmi Trough in a NNW-NW direction. Of the solutions satisfying these conditions, we select the best EU-PH as 48.2 deg N, 157.0 deg E, 1.09 deg/my, corresponding to a pole far from Japan and south of Kamchatka, and PA-PH, 1.2 deg N, 134.2 deg E, 1.00 deg/my. Predicted NA-PH and EU-PH convergence rates in central Honshu are consistent with estimated seismic slip rates. Previous estimates of the EU-PH pole close to central Honshu are inconsistent with extension within the Bonin backarc implied by earthquake slip vectors and NNW-NW convergence of the Bonin forearc at the Sagami Trough.

Internal structure and weakening mechanisms of the San Andreas Fault

Abstract: New observations of the internal structure of the San Gabriel fault (SGF) are combined with previous characterizations of the Punchbowl fault (PF) to evaluate possible explanations for the low frictional strength and seismic characteristics of the San Andreas fault (SAF). The SGF and PF are ancient, large-displacement faults of the SAF system exhumed to depths of 2 to 5 km. These fault zones are internally zoned; the majority of slip was confined to the cores of principal faults, which typically consist of a narrow layer (less than tens of centimeters) of ultracataclasite within a zone of foliated cataclasite several meters thick. Each fault core is bounded by a zone of damaged host rock of the order of 100 m thick. Orientations of subsidiary faults and other fabric elements imply that (1) the maximum principal stress was oriented at large angles to principal fault planes, (2) strain was partitioned between simple shear in the fault cores and nearly fault-normal contraction in the damaged zones and surrounding host rock, and (3) the principal faults were weak. Microstructures and particle size distributions in the damaged zone of the SGF imply deformation was almost entirely cataclastic and can be modeled as constrained comminution. In contrast, cataclastic and fluid-assisted processes were significant in the cores of the faults as shown by pervasive syntectonic alteration of the host rock minerals to zeolites and clays and by folded, sheared, and attenuated cross-cutting veins of laumontite, albite, quartz, and calcite. Total volume of veins and neocrystallized material reaches 50% in the fault core, and vein structure implies episodic fracture and sealing with time-varying and anisotropic permeability in the fault zone. The structure of the ultracataclasite layer reflects extreme slip localization and probably repeated reworking by particulate flow at low effective stresses. The extreme slip localization reflects a mature internal fault structure resulting from a positive feedback between comminution and transformation weakening. The structural, mechanical, and hydrologic characteristics of the Punchbowl and San Gabriel faults support the model for a weak San Andreas based on inhomogeneous stress and elevated pore fluid pressures contained within the core of a seismogenic fault. Elevated fluid pressures could be repeatedly generated in the core of the fault by a combination of processes including coseismic dilatancy and creation of fracture permeability, fault-valve behavior to recharge the fault with fluid, post-seismic self-sealing of fracture networks to reduce permeability and trap fluids, and time-dependent compaction of the core to generate high pore pressure. The localized slip and fluid-saturated conditions are wholly compatible with additional dynamic weakening by thermal pressurization of fluids during large seismic slip events, which can help explain both the low average strength of the San Andreas and seismogenic characteristics such as large stress relief. In addition, such a dynamic weakening mechanism is expected only in mature fault zones and thus could help explain the apparent difference in strength of large-displacement faults from smaller-displacement, subsidiary seismogenic faults.

Spatio-temporal complexity of slip on a fault

Abstract: Three-dimensional analyses are reported of slip on a long vertical strike-slip fault between steadily driven elastic crustal blocks. A rate- and state-dependent friction law governs motion on the fault; the law includes a characteristic slip distance L for evolution of surface state and slip weakening. Because temperature and normal stress vary with depth, frictional constitutive properties (velocity weakening/ strengthening) do also. Those properties are taken either as uniform along-strike at every depth or as perturbed modestly from uniformity. The governing equations of quasi-static elasticity and frictional slip are solved on a computational grid of cells as a discrete numerical system, and a viscous radiation damping term is included to approximately represent inertial control of slip rates during earthquakelike instabilities. The numerical results show richly complex slip, with a spectrum of event sizes, when solved for a grid with oversized cells, that is, with cell size h that is too large to validly represent the underlying continuous system of equations. However, in every case for which it has been feasible to do the computations (moderately large L only), that spatio-temporally complex slip disappears in favor of simple limit cycles of periodically repeated large earthquakes with reduction of cell size h. Further study will be necessary to determine whether a similar trhnsition occurs when the elastodynamics of rupture propagation is treated more exactly, rather than in the radiation damping approximation. The transition from complex to ordered fault response occurs as h is reduced below a theoretically derived nucleation size h* which scales with L but is 2 x 104 to 105 larger in cases considered. Cells larger than h* can fail independently of one another, whereas those much smaller than h* cannot slip unstably alone and can do so only as part of a cooperating group of cells. The results contradict an emergent view that spatio-temporal complexity is a generic feature of mechanical fault models. Such generic complexity does apparently result from models which are inherently discrete in the sense of having no well-defined continuum limit as h diminishes. Those models form a different class of dynamical systems from models like the present one that do have a continuum limit. Strongly oversized cells cause the model developed here to mimic an inherently discrete system. It is suggested that oversized cells, capable of failing independently of one another, may crudely represent geometrically disordered fault zones, with quasi-independent fault segments that join one another at kinks or jogs. Such geometric disorder, at scales larger than h*, may force a system with a well-defined continuum limit to mimic an inherently discrete system and show spatio-temporally complex slip at those larger scales.

Scaling of the critical slip distance for seismic faulting with shear strain in fault zones

Abstract: THEORETICAL and experimentally based laws for seismic faulting contain a critical slip distance1–5, Dc, which is the slip over which strength breaks down during earthquake nucleation. On an earthquake-generating fault, this distance plays a key role in determining the rupture nucleation dimension6, the amount of premonitory and post-seismic slip7–10, and the maximum seismic ground acceleration1,11. In laboratory friction experiments, D c has been related to the size of surface contact junctions2,5,12; thus, the discrepancy between laboratory measurements of Dc (∼10−5m) and values obtained from modelling earthquakes (∼10−2m) has been attributed to differences in roughness between laboratory surfaces and natural faults5. This interpretation predicts a dependence of Dc on the particle size of fault gouge2 (breccia and wear material) but not on shear strain. Here we present experimental results showing that Dc scales with shear strain in simulated fault gouge. Our data suggest a new physical interpretation for the critical slip distance, in which Dc is controlled by the thickness of the zone of localized shear strain. As gouge zones of mature faults are commonly 102–103 m thick12–17, whereas laboratory gouge layers are 1–10 mm thick, our data offer an alternative interpretation of the discrepancy between laboratory and field-based estimates of Dc.

Slip transition of a polymer melt under shear stress.

Abstract: We present the first direct measurements of the local velocity of a sheared polymer melt within the first 100 nm from the solid-liquid interface. For high enough shear rates we observe a sharp transition between weak and strong slip (i.e., a nonzero boundary fluid velocity) in the case of weak polymer-surface interactions [polydimethylsiloxane (PDMS) on silanated silica surfaces]. For strong polymersurface interactions the slip is strongly reduced. These results are compared to a theoretical model recently proposed by Brochard and de Gennes

Method and apparatus for providing a note for an application program

Abstract: A method and apparatus for providing a note on an application program includes noticing a note anchor object associated with a data file displayed by an application program on a computer screen and displaying a note slip image over the displayed data and images of the application program. Many anchor objects and note slips may be displayed on the screen at once, and a single anchor object is preferably associated with a single note slip. The note slip is preferably receptive to pen-based inputs, and may be resized or moved on the screen. The anchor object includes a visual picture data portion and a picture comment data portion that descibes the note slip associated with that anchor object. The anchor object preferably exists as a standard graphical picture in the application program and can be manipulated as such. The present invention thus allows note slips to be displayed in existing application programs, either pen-compatible or non-pen-compatible.

Elastic waves through a simulated fractured medium

Abstract: Ultrasonic velocities were measured on a block composed of lucite plates with roughened surfaces pressed together with a static normal stress to simulate a fractured medium. The measurements, normal, parallel, and oblique to the fractures, show that for wavelength much larger than the thickness of an individual plate, the block can be modeled as a particular type of transversely isotropic (TI) medium that depends on four parameters. This TI medium behavior is the same as that of an isotropic solid in which are embedded a set of parallel linear slip interfaces, specified by (1) the excess compliance tangential to the interfaces and (2) the excess compliance normal to the interfaces. At all static stress levels, the authors inverted the data for the background isotropic medium parameters and the excess compliances. The background parameters obtained were basically independent of stress level and agreed well with the bulk properties of the lucite. The excess compliances decreased with increasing static closing stress, implying that increasing static stress forces asperities on either side of a fracture into greater contact, gradually eliminating the excess compliance that gives rise to the anisotropy. A medium with such planes of excess compliance has been shown, theoretically, to describemore » the behavior of a medium with long parallel joints, as well as a medium with embedded parallel microcracks.« less

Earthquake Failure Sequences Along a Cellular Fault Zone in a Three-Dimensional Elastic Solid Containing Asperity and Nonasperity Regions

Abstract: Numerical simulations of earthquake failure sequences along a discrete cellular fault zone are performed for a three-dimensional (3-D) model representing approximately the central San Andreas fault. The model consists of an upper crust overlying a lower crust and mantle region, together defining an elastic half-space with a vertical half-plane fault. The fault contains a region where slip is calculated on a uniform grid of cells governed by a static/kinetic friction law and regions where slip is prescribed so as to represent tectonic loading, aseismic fault creep, and adjacent great earthquakes. The computational region models a 70-km-long and 17.5-km-deep section of the San Andreas fault to the NW of the great 1857 rupture zone. Different distributions of stress drops on failing computational cells are used to model asperity (“Parkfield asperity”) and nonasperity fault regions. The model is “inherently discrete” and corresponds to a situation in which a characteristic size of geometric disorder within the fault (i.e., cell size, here a few hundreds of meters) is much larger than the “nucleation size” (of the order of tens of centimeters to tens of meters) based on slip weakening or state evolution slip distances. The computational grid is loaded by a constant plate motion imposed at the lower crust, upper mantle, and creeping fault regions and by a “staircase” slip history imposed at the 1857 and 1906 rupture zones. Stress transfer along and outside the fault due to the imposed loadings and failure episodes along the computational grid is calculated using 3-D elastic dislocation theory. The resulting displacement field in the computational region is compatible with geodetic and seismological observations only when the asperity and nonasperity regions are characterized by significantly different average stress drops. The frequency-magnitude statistics of the simulated failure episodes are approximately self-similar for small events, with b ≈ 1.2 (the b value of statistics based on rupture area bA is about 1) but are strongly enhanced with respect to self-similarity for events larger than a critical size. This is interpreted as a direct manifestation of our 3-D elastic stress transfer calculations; beyond certain rupture area and potency (seismic moment divided by rigidity) release values, the event is usually unstoppable, and it continues to grow to a size limited by a characteristic model dimension. This effect is not accounted for by cellular automata and block-spring models in which the adopted simplified stress transfer laws fail to scale properly with increasing rupture size. The simulations suggest that local maxima in observed frequency-magnitude statistics correspond to dimensions of coherent brittle zones, such as the width of the seismogenic layer or the length of a fault segment bounded by barriers. The analysis indicates that a single cell size, representing approximately a single scale of geometric disorder, cannot induce self-similarity in a 3-D elastic model over a broad range of magnitudes. A representation of geometric disorder covering a range of scales may thus be required to generate a wide domain of self-similar Gutenberg-Richter statistics. Our simulations show a great diversity in the mode of failure of the Parkfield asperity; the earthquakes themselves define an irregular sequence of events. The modeling, like many other discrete fault models, suggests that expectations for periodic Parkfield earthquakes and/or simple precursory patterns repeating from one event to the other are unrealistic.

The rheology of concentrated dispersions of weakly attracting colloidal particles with and without wall slip

Abstract: It is demonstrated that weakly flocculated, concentrated colloidal dispersions show slip flow when sheared between smooth concentric cylinders. The precise pattern of behavior seen depends upon the stress and upon how long the flow is left to establish prior to measurement. With delay times of order hours, slip is not seen until a critical stress is exceeded (typically about 1 Pa) and, thus, the true low‐shear viscosity can be determined provided care is taken to ensure the stress does not exceed the critical level. With short delay times of order minutes, slip is seen irrespective of how small the stress is and the low‐shear viscosity can be underestimated by several orders of magnitude. Comparisons of flow curves obtained using smooth and roughened cylinders show that slip only occurs at the inner cylinder, and also that bulk flow is re‐established at higher stresses where the dispersions start to shear thin. The apparent low‐shear, relative viscosity measured in the presence of slip appears, to a first...

Paleoseismology along the 1980 surface rupture of the Irpinia Fault: Implications for earthquake recurrence in the southern Apennines, Italy

Abstract: The Irpinia fault was the source of the Ms 6.9 1980 Irpinia earthquake and produced the first unequivocal historical surface faulting in Italy. Trenching of the 1980 fault scarp at Piano di Pecore, a flat intermontane basin about 5 km south of the 1980 instrumental epicenter, provides the first data on earthquake recurrence intervals, slip per event, and slip rate on a major normal fault in the Southern Apennines fault zone. The trenches exposed evidence of four pre-1980 paleoearthquakes that occurred during the past 8600 years. A best estimate average recurrence interval is 2150 years, although the time interval between individual events varies by as much as a factor of 2. Each paleo earthquake is similar to the 1980 surface rupture in amount of slip and style of deformation, which suggests that the 1980 event is characteristic for the Irpinia fault. Slip per event values average 61 cm. The net vertical displacement of 2.12–2.36 m since 8600 cal year B.P. observed in the trenches gives a vertical slip rate of 0.25–0.35 mm/yr, a dip slip rate of 0.29–0.40 mm/yr, and an extension rate of 0.14–0.20 mm/yr. Although fault behavior data are only available for the Irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern Apennines. They suggest that (1) fault specific earthquake recurrence intervals based on the historical seismic record overestimates the occurrence of large magnitude (M7) earthquakes and (2) the Holocene rate of extension across the Apennines is ≤1 mm/yr. The 1980 earthquake and the paleoseismologic observations show that repeated and localized surface faulting occurs in southern Apennines and leaves subtle but distinct geomorphic evidence that can be detected with detailed and careful investigation.

Rheological behavior of a concentrated suspension: A solid rocket fuel simulant

Abstract: The rheological behavior of a very concentrated suspension (76.5 vol %), which serves as a widely used solid rocket fuel simulant, was characterized employing both torsional and capillary flows. No comprehensive studies of the rheology of concentrated suspensions have been carried out previously at such a high solids content. The suspension exhibited shear thinning over the apparent shear rate range of 30–3000 s−1. Significant slip at the wall was observed in both torsional and capillary flows with the slip velocity increasing from about 0.001 mm/s at a shear stress of 4 Pa to as high as 60 mm/s at 100 kPa. A flow visualization technique was applied for the first time to determine the wall slip velocities in torsional flow directly, to also provide the true deformation rate and feedback on yielding. The contribution of the slip of the suspension at the wall to the volumetric flow rate in capillary flow was found to increase with decreasing shear stress, giving rise to plug flow at sufficiently low shear s...

Stick slip and control in low-speed motion

Abstract: Dimensional and perturbation analysis are applied to the problem of stick slip encountered during the motion of machines. The friction model studied is motivated by current tribological results and is appropriate for lubricated metal contacts. The friction model incorporates Coulomb, viscous, and Stribeck friction with frictional memory and rising static friction. Through dimensional analysis an exact model of the nonlinear system can be formed using five parameters rather than ten, greatly facilitating study and explicitly revealing the interaction of parameters. By converting the system of differential equations into a set of integrations, the perturbation technique makes approximate analysis possible where only numerical techniques had been available before. The analysis predicts the onset of stick slip as a function of plant and controller parameters; these results are compared with experimental data. >

Plugging of the Flow of Granular Materials during the Discharge from a Silo

Abstract: Laboratory experiments and numerical simulations were performed to find the mechanism of the plugging during the gravitationally emptying a silo. Some interesting results were given by this investigation: 1) Two major slip lines can be seen symmetrically about a vertical midline in the continuous flow, but particles flow only on a one-side slip line was found at each moment; 2) At transition of the flow from one-side to the other, two-side flows collided each other on a vertical midline. And this collision possibly resulted in the formation of the arches of granular materials; 3) In the special case for 2) when arches were stable enough under certain geometrical condition the plugging of the flow occurred. For computer simulation on this issue, the Discrete Element Method specifically taking into account of rolling friction effect was used. Results of numerical simulations were in good agreement with the experimental measures not only in the flow pattern but also in the occurrence of the plugging.

Depth distribution of coseismic slip along the Nankai Trough, Japan, from joint inversion of geodetic and tsunami data

Abstract: Coseismic slip distribution on the fault plane, particularly in the downdip direction, associated with large subduction earthquakes can be estimated by joint inversion of geodetic and tsunami data. Two large earthquakes, the 1944 Tonankai earthquake (Mw=8.1) and the 1946 Nankaido earthquake (Mw=8.3), occurred on the Nankai trough, southwestern Japan, where the Philippine Sea plate is subducting beneath the Eurasian plate. The source areas of these events extended over both land and ocean. Coseismic crustal movements on land were measured by leveling, while those in ocean were recorded as tsunami waveforms on tide gauges. The coseismic slip distribution inverted from these data shows that the slip on the shallower part of the fault plane is comparable to that on the deeper parts. This indicates that large coseismic slip can occur beneath an accretionary wedge where current seismicity is low. The result has implications for other subduction zones having a similar tectonic environment such as for the Pacific Northwest region of the United States. Although the possibility of a large earthquake there is still debated, should a large subduction earthquake occur in this region, coseismic slip on the shallow part could be large, and the potential for large tsunamis is high.

Velocity dependent friction of granite over a wide range of conditions

Abstract: Direct shear sliding experiments on bare ground surfaces of Westerly Granite have been conducted over an exceptionally wide range of sliding rates (10-4 µm/s to 103 µm/s) at unconfined normal stresses (sn) of 5, 15, 30, 70, and 150 MPa. A new sample configuration was developed that permitted measurements at normal stresses of 70 and 150 MPa without immediate sample failure. Measurements of steady-state velocity dependence of friction at velocities between 10-4 and 1 µm/s show similar velocity weakening behavior at all normal stresses, with more negative dependence at lower slip rates. However, at rates above 10 µm/s, velocity weakening is observed only at σn = 30, 70 and 150 MPa, while velocity neutral behavior is observed at σ n = 15 MPa and velocity strengthening is observed at (5, = 5 MPa. The greater velocity weakening observed at velocities below 10-2 µm/s may suggest a transition in competing deformation mechanisms, or the influence of additional mechanisms. The transition to velocity strengthening at high velocity and low normal stress implies that rapid slip on shallow faults could be arrested before resulting in true stick-slip behavior. Stable fault creep and creep events observed at shallow levels on some natural faults may result from this transition in velocity dependence.

Viscosity determinations of some frictionally generated silicate melts : implications for fault zone rheology at high strain rates

Abstract: Analytical scanning electron microscopy has been used to determine the major element compositions of some natural and artificial silicate glasses and their microcrystalline equivalents derived by the frictional melting of intermediate to acid protoliths. The data show that the matrices of the friction melts (which cool to form pseudotachylytes) are relatively basic and hydrous, even when their protoliths are intermediate to acid. This is because frictional fusion involves the selective comminution and nonequilibrium melting of minerals based on their individual mechanical properties and melting points, not the formation of minimum melts through equilibrium mineral interaction. This means that hydrous ferromagnesian minerals (e.g., micas and amphiboles) melt preferentially to form the liquid matrix, while feldspars and especially quartz more readily survive as clasts. Pseudotachylytes generated by frictional melting are therefore not bulk melts, and as clast-melt suspensions, they cannot be considered as simple Newtonian fluids. The calculated viscosities of the friction melts are low. For example, at 1200°C, most friction melts possess zero-shear suspension viscosities of 102–104 dPa s (1 dPa s = 1 P). This is equivalent to the viscosities of tholeiitic and alkaline basaltic magmas at the same temperature. These viscosities are maximum determinations because, as clast-melt suspensions, friction melts may undergo shear thinning and exhibit pseudoplasticity at high shear rates (i.e., during slip on a fault surface). Contrary to earlier suggestions, where the bulk melting of intermediate to acid protoliths was believed to result in the generation of viscous friction melts that could act to inhibit continued sliding, this work shows that most pseudotachylytes are partial melts possessing low viscosities. The formation of highly fluid suspensions during slip may have profound effects on the dissipation of stored strain energy in the rocks surrounding a fault. Interface lubrication could facilitate an increase in the slip rate and the rate of energy dissipation. This would be manifest as an increase in high-frequency seismic wave radiation and vibrational.

Fault strain and seismic coupling on mid‐ocean ridges

Abstract: The contribution of extensional faulting to seafloor spreading along the East Pacific Rise (EPR) axis near 3°S and between 13°N and 15°N is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data. It is found that faulting may account for of the order of 5–10% of the total spreading rate, which is comparable to a previous estimate from the EPR near 19°S. Given the paucity of normal faulting earthquakes on the EPR axis, a maximum estimate of the seismic moment release shows that seismicity can account for only 1% of the strain due to faulting. This result leads us to conclude that most of the slip on active faults must be occurring by stable sliding. Laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding, while increasing temperature promotes stable sliding. By applying a simple frictional model to mid-ocean ridge faults it is shown that at fast spreading ridges (≥90 mm/yr) the seismic portion of a fault (Ws) is a small proportion of the total downdip width of the fault (Wƒ). The ratio Ws/ Wƒ interpreted as the seismic coupling coefficient X, and in this case X≈ 0. In contrast, at slow spreading rates (≤40 mm/yr), Ws≈Wƒ, and therefore X≈ 1, which is consistent with the occurrence of large-magnitude earthquakes (mb= 5.0 to 6.0) occurring, for example, along the Mid-Atlantic Ridge axis.

Deformation processes related to interfacial boundaries in two-phase γ-titanium aluminides

Abstract: The deformation behaviour of two-phase (α2 + γ) titanium aluminide alloys with a lamellar microstructure was investigated by conventional and high resolution electron microscopy. With regard to the mechanical properties, the structures of the interfacial boundaries occurring in these materials are characterized and the interaction mechanisms of the deformation processes with these interfaces is investigated. Accordingly, the misfit dislocations present at semicoherent α2/γ and γ/γ interfaces assist the generation of dislocations and deformation twins. On the other hand, the semicoherent interfaces act as very effective barriers impeding the propagation of slip across the lamellae.

Solid-solution hardening

Abstract: Recent advances in the understanding of solid-solution hardening (SSH) of crystalline materials, as well as some basic early papers are briefly reviewed. This survey shows that models of SSH based on the concept of a frictional drag on dislocations migrating through fields of point-like obstacles, whether randomly dispersed or clustered, do not encompass the principal features of SSH, e.g. the temperature dependence of the yield stress, the stress and temperature dependences of the activation volume, and the phenomenon of stress equivalence. However, a model based on the nucleation of slip, involving the breakaway of dislocation segments from several pinning points, formulated in closed form, is shown to account satisfactorily for the principal observations.

Structural and thermal constraints on the initiation angle of detachment faulting in the southern Basin and Range: The Chemehuevi Mountains case study

Abstract: The Cenozoic normal fault system exposed in the Chemehuevi Mountains of the southern Cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated extreme crustal stretching. There, three stacked, brittle, low-angle normal faults that formed at depths as great as 10-12 km cut gently down section northeastward through deformed Proterozoic and Mesozoic crystalline basement. Hanging-wall blocks are displaced relatively northeastward. The upper crust above the Chemehuevi detachment fault was pulled apart along high-angle normal faults that rotated to more gentle dips through time. In contrast, rocks of originally mid-crustal affinity in the footwall were only gently rotated and accommodated minor extension ( Application of 40 Ar/ 39 Ar and fission-track thermochronology to rocks in the footwall of the Chemehuevi detachment fault system provides further constraints on the timing and initiation angle of regional detachment faulting. At the onset of extension between 22 and 24 Ma, granitic rocks exposed in the southwestern and northeastern portions of the footwall were at ∼200 °C and 350-400 °C, respectively, separated by a distance of some 23 km down the known slip direction. This gradual increase in temperature with original depth is attributed to the gentle southwest tilting of broadly planar pre-extension isothermal surfaces and constrains the exposed part of the Chemehuevi detachment fault to have had a regional dip initially of about 15° to 30°. The fault system apparently cut gently down through the upper crust, to a minimum depth of ∼10-12 km, the deepest exposed parts of the system today, and was domed from midcrustal depths and locally denuded during continued slip. Together the structural and thermochronologic data confirm the suggestion that faults accommodating large-magnitude slip can be initiated and move within the seismogenic regime at moderate to low angles (that is, ≤30°).

Estimates from atomic models of tension-shear coupling in dislocation nucleation from a crack tip

Abstract: The normal stress distribution across a slip plane has the effect of reducing the critical loading required for dislocation emission from a crack tip. The reduction by normal stresses was found to be very significant for Si, based on properties estimated for it using density functional theory, to be large for Fe as modeled by the embedded atom method (EAM), and to be smaller in AI, Ni and ordered Ni3A1, estimated using the EAM. The general dependence over a wide range on parameters characterizing the tension-shear coupling was also determined. In the context of a Peierls model for dislocation nucleation at a crack tip (J. R. Rice, J. Mech. Phys. Solids, 40 (1992) 239), our approach was to search for onset of the dislocation nucleation instability based on the numerical solution of the system of non-linear integral equations describing an incipient dislocation. The incipient dislocation consists of a distribution of sliding and opening displacements along a slip plane emanating from the crack tip; these displacements are related to the shear and tensile stresses across the slip plane by constitutive relations based on the atomic models mentioned. Results from the atomic models are used to parametrize constitutive relations involving a Frenkel sinusoidal dependence of shear stress on sliding displacement at any fixed opening displacement, and a Rose-Ferrante-Smith universal binding form of dependence of tensile stress on opening displacement at any fixed shear displacement. These relations then enter the system of integral equations, solved numerically, which describe the elasticity solution for a non-uniform distribution of sliding and opening along the slip plane. The results show that tension-shear coupling will often significantly reduce the loading for dislocation emission from the value estimated on the basis of an unstable stacking energy Yu~ determined with neglect of such coupling, in a shear-only type analysis. For the EAM models of the metals considered, a simple and approximate method to account for the tension effects is to use a modified quantity yo iu*~, which is an unstable stacking energy for lattice planes which are constrained to a fixed opening A0*, corresponding to that for vanishing normal stress at the unstable shear equilibrium position. Moreover, it is found that the normal stress effect can be described well in these cases by replacing the unstable stacking energy 7u~ in the shear-only model by a tension softened ~'u~(q~), which depends on the phase angle ~p of the combined tension-shear loading along the slip plane according to the stress intensity factors of the elastic singular solution. The same simple procedures for accounting for tension effects on nucleation are less suitable for lattices with strong coupling such as Si.

Mechanism diversity of the loma prieta aftershocks and the mechanics of mainshock-aftershock interaction.

Abstract: The diverse aftershock sequence of the 1989 Loma Prieta earthquake is inconsistent with conventional models of mainshock-aftershock interaction because the aftershocks do not accommodate mainshock-induced stress changes. Instead, the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane. This orientation implies that (i) stress drop in the mainshock was nearly complete, (ii) mainshock-induced decreases of fault strength helped were important in controlling the occurrence of after-shocks, and (iii) mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip.

Identification of a Second Dynamic State During Stick-Slip Motion

Abstract: Stick-slip, or interrupted, motion rather than smooth uninterrupted motion occurs in many different phenomena such as friction, fluid flow, material fracture and wear, sound generation, and sensory "texture." During stick-slip, a system is believed to undergo transitions between a static (solid-like) state and a kinetic (liquid-like) state. The stick-slip motion between various types of pretreated surfaces was measured, and a second, much more kinetic state that exhibits ultra-low friction was found. Transitions to and from this super-kinetic state also give rise to stick-slip motion but are fundamentally different from conventional static-kinetic transitions. The results here suggest practical conditions for the control of unwanted stick-slip and the attainment of ultra-low friction.

An augmented Lagrangian method for discrete large‐slip contact problems

Abstract: Reference LMAF-ARTICLE-1993-001View record in Web of Science Record created on 2005-09-14, modified on 2017-05-10