Showing papers on "Fault (geology) published in 1998"

Laboratory-derived friction laws and their application to seismic faulting

Abstract: This paper reviews rock friction and the frictional properties of earthquake faults. The basis for rate- and state-dependent friction laws is reviewed. The friction state variable is discussed, including its interpretation as a measure of average asperity contact time and porosity within granular fault gouge. Data are summarized showing that friction evolves even during truly stationary contact, and the connection between modern friction laws and the concept of “static” friction is discussed. Measurements of frictional healing, as evidenced by increasing static friction during quasistationary contact, are reviewed, as are their implications for fault healing. Shear localization in fault gouge is discussed, and the relationship between microstructures and friction is reviewed. These data indicate differences in the behavior of bare rock surfaces as compared to shear within granular fault gouge that can be attributed to dilation within fault gouge. Physical models for the characteristic friction distance are discussed and related to the problem of scaling this parameter to seismic faults. Earthquake afterslip, its relation to laboratory friction data, and the inverse correlation between afterslip and shallow coseismic slip are discussed in the context of a model for afterslip. Recent observations of the absence of afterslip are predicted by the model.

Crustal thickening in Gansu-Qinghai, lithospheric mantle subduction, and oblique, strike-slip controlled growth of the Tibet plateau

Abstract: Fieldwork complemented by SPOT image analysis throws light on current crustal shortening processes in the ranges of northeastern Tibet (Gansu and Qinghai provinces, China). The ongoing deformation of Late-Pleistocene bajada aprons in the forelands of the ranges involves folding, at various scales, and chiefly north-vergent, seismogenic thrusts. The most active thrusts usually break the ground many kilometres north of the range-fronts, along the northeast limbs of growing, asymmetric ramp-anticlines. Normal faulting at the apex of other growing anticlines, between the range fronts and the thrust breaks, implies slip on blind ramps connecting distinct active decollement levels that deepen southwards. The various patterns of uplift of the bajada surfaces can be used to constrain plausible links between contemporary thrusts downsection. Typically, the foreland thrusts and decollements appear to splay from master thrusts that plunge at least 15–20 km down beneath the high ranges. Plio-Quaternary anticlinal ridges rising to more than 3000 m a.s.l. expose Palaeozoic metamorphic basement in their core. In general, the geology and topography of the ranges and forelands imply that structural reliefs of the order of 5–10 km have accrued at rates of 1–2 mm yr−1 in approximately the last 5 Ma. From hill to range size, the elongated reliefs that result from such Late-Cenozoic, NE–SW shortening appear to follow a simple scaling law, with roughly constant length/width ratio, suggesting that they have grown self-similarly. The greatest mountain ranges, which are over 5.5 km high, tens of kilometres wide and hundreds of kilometres long may thus be interpreted to have formed as NW-trending ramp anticlines, at the scale of the middle–upper crust. The fairly regular, large-scale arrangement of those ranges, with parallel crests separated by piggy-back basins, the coevality of many parallel, south-dipping thrusts, and a change in the scaling ratio (from ≈5 to 8) for range widths greater than ≈30 km further suggests that they developed as a result of the northeastward migration of large thrust ramps above a broad decollement dipping SW at a shallow angle in the middle–lower crust. This, in turn, suggests that the 400–500 km-wide crustal wedge that forms the northeastern edge of the Tibet–Qinghai plateau shortens and thickens as a thick-skinned accretionary prism decoupled from the stronger upper mantle underneath. Such a thickening process must have been coupled with propagation of the Altyn Tagh fault towards the ENE because most thrust traces merge northwestwards with active branches of this fault, after veering clockwise. This process appears to typify the manner in which the Tibet–Qinghai highlands have expanded their surface area in the Neogene. The present topography and structure imply that, during much of that period, the Tibet plateau grew predominantly towards the northeast or east-northeast, but only marginally towards the north-northwest. This was accomplished by the rise, in fairly fast succession, of the Arka Tagh, Qiman Tagh, Mahan shan, Tanghenan Shan, and other NW-trending mountain ranges splaying southeastwards from the Altyn Tagh, isolating the Aqqik-Ayakkum Kol, Qaidam, Suhai and other catchments and basins that became incorporated into the highland mass as intermontane troughs. The tectonic cut-off of catchments and the ultimate infilling of basins by debris from the adjacent ranges, a result of tectonically forced internal drainage, have thus been essential relief-smoothing factors, yielding the outstandingly flat topography that makes Tibet a plateau. Using Late-Mesozoic and Neogene horizons as markers, the retrodeformation of sections across the West Qilian Ranges and Qaidam basin implies at least ≈150 km of N30°E Neogene shortening. On a broader scale, taking erosion into account, and assuming isostatic compensation and an initial crustal thickness comparable to that of the Gobi platform (47.5±5 km), minimum amounts of Late-Cenozoic crustal shortening on NE sections between the Kunlun fault and the Hexi corridor are estimated to range between 100 and 200 km. In keeping with the inference of a deep crustal decollement and with the existence of Mid-Miocene to Pliocene plutonism and volcanism south of the Kunlun range, such values suggest that the lithospheric mantle of the Qaidam plunged obliquely into the asthenosphere south of that range to minimum depths of the order of 200–300 km. A minimum of ≈150 km of shortening in the last ≈10 Ma, consistent with the average age of the earliest volcanic–plutonic rocks just south of the Kunlun (≈10.8 Ma) would imply average Late-Cenozoic rates of shortening and regional uplift in NE Tibet of at least ≈15 mm yr−1 and ≈0.2 mm yr−1, respectively. Such numbers are consistent with a cumulative sinistral offset and slip rate of at least ≈200 km and ≈2 cm yr−1, respectively, on the Altyn Tagh fault east of 88°E. The fault may have propagated more than 1000 km, to 102°E, in the last 10 Ma. Our study of ongoing tectonics in northeast Tibet is consistent with a scenario in which, while the Himalayas-Gangdese essentially ‘stagnated’ above India’s subducting mantle, much of Tibet grew by thickening of the Asian crust, as propagation of large, lithospheric, strike-slip shear zones caused the opposite edge of the plateau to migrate far into Asia. The Asian lithospheric mantle, decoupled from the crust, appears to have subducted southwards along the two Mesozoic sutures that cut Tibet north of the Gangdese, rather than to have thickened. The Bangong-Nujiang suture was probably reactivated earlier than the Jinsha-Kunlun suture, located farther north. Overall, the large-scale deformation bears a resemblance to plate tectonics at obliquely convergent margins, including slip-partioning along large strike-slip faults such as the Altyn Tagh and Kunlun faults. Simple mechanisms at the level of the lithospheric mantle are merely hidden by the broader distribution and greater complexity of strain in the crust.

The 1997 Umbria‐Marche, Italy, Earthquake Sequence: A first look at the main shocks and aftershocks

Abstract: A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure consists of a NW-SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE-SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (∼40°) to the southwest, and confined in the upper ∼8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km-long fault segments, adjacent and slightly offset from one another.

Origin of extensional core complexes: Evidence from the Mid‐Atlantic Ridge at Atlantis Fracture Zone

Abstract: The contrast in geologic structure observed on opposing flanks of the Mid-Atlantic Ridge, where it is offset by the Atlantis transform fault, illustrates how significant differences in crustal structure can result from tectonic processes that operate near the ends of slow spreading segments. New geophysical and geological data provide information on the nature of large massifs that punctuate the strips of crust fonned at the inside corner of ridge-transform intersections (RTI), as well as of the low-relief volcanic morphology that typifies the outside corners. The geological relations mapped at the inside corner of the eastern Atlantis RTI are strikingly similar to those seen in the Basin and Range where metamorphic core complexes are unroofed through asymmetric detachment faulting. The core of the eastern RTI massif exposes deep-seated rocks beneath a shallow-dipping, corrugated surface which is interpreted as a fault surface. On the median valley side of the massif, this seafloor detachment is overlain by upper crustal blocks bounded by steeper fault scarps. The western side of the 15-km-wide massif is characterized by en echelon faults which face away from the ridge axis. Similar features are mapped at two fossil massifs that are interpreted to have formed at the inside corners of each RTI and to have rafted off-axis as plate spreading proceeded. Analysis of new and preexisting shipboard gravity data indicates that high-density material is not continuously emplaced at the inside corner. Rather, peaks in the gravity anomaly map are patchily distributed along the transform valley walls. The gravity highs associated with the three massifs (oceanic core complexes) in this area are not centered with respect to their morphology but are located toward their spreading axis and transform sides. Gravity modeling suggests that the western boundary of a high-density wedge at the eastern RTI massif is steeply dipping, whereas the eastern boundary may dip about 15° toward the median valley. In contrast to the inside comers of the RTls in our study area, the outside corner seafloor is characterized by volcanic constructions similar to those found on either side of the spreading axis at the center of the segments and inferred to be typical basaltic upper crust. Kinematic analysis at the Mid-Atlantic Ridge-Atlantis Transform RTI suggests that the formation of seafloor detachments may occur when the rate of extension not accommodated by magmatic input exceeds about 4 mm/yr. Isolated volcanic ridges that extend into the fracture zone domain, curving as they approach the fault trace, mark times of abundant magma supply at the segment ends. The apparent interplay between magmatic and tectonic strain accommodation at a mid-ocean ridge, as well as the overall structure of oceanic core complexes, may provide important kinematic constraints on core complex formation and the development of shallow-dipping detachment faults.

A mechanism to explain rift-basin subsidence and stratigraphic patterns through fault-array evolution

Abstract: Rift-basin stratigraphy commonly records an early stage of slow subsidence followed by an abrupt increase in subsidence rate. The physical basis for this transition is not well understood, although an increase in extension rate is commonly implied. Here, a numerical fault-growth model is used to investigate the influence of segment linkage on fault-displacement-rate patterns along an evolving normal fault array. The linkage process we describe is controlled by a stress feedback mechanism, which leads to enhanced growth of optimally positioned faults. Model results indicate that, even with constant extension rates, slow displacement rates prevail during an initial phase of distributed extension, followed by an increase in displacement rates as strain becomes localized on linked fault arrays. This is due to the dynamics of fault interactions rather than mechanical weakening. Comparison of model simulations with rift-basin subsidence and stratigraphic patterns in the Gulf of Suez and North Sea suggests that the occurrence and timing of rapid basin deepening can be explained by the mechanics of fault-zone evolution, without invoking a change in regional extension rates.

FINSIM--a FORTRAN Program for Simulating Stochastic Acceleration Time Histories from Finite Faults

Abstract: INTRODUCTION Ground motions from earthquakes are created by ruptures on tectonic faults. The causative faults can be considered point sources at distances large compared to the fault dimensions. At closer distances, the finite-fault effects become important. These effects are primarily related to the finite speed of rupture propagation, which causes certain parts of the fault to radiate energy much earlier than do other parts; the delayed waves then interfere, creating significant directivity effects. The duration and amplitude of ground motion become dependent on the angle of observation. Finite-source modeling has been an important part of ground-motion prediction near the epicenters of large earthquakes (Hartzell, 1978; Irikura, 1983; Joyner and Boore, 1986; Heaton and Hartzell, 1989; Somerville et al., 1991; Hutchings, 1994; Tumarkin and Archuleta, 1994; Zeng et al. , 1994). In the approach adopted in most studies, the fault plane is discretized into elements, each element is treated as a small...

Suppression of large earthquakes by stress shadows: A comparison of Coulomb and rate-and-state failure

Abstract: Stress shadows generated by California's two most recent great earthquakes (1857 Fort Tejon and 1906 San Francisco) substantially modified 19th and 20th century earthquake history in the Los Angeles basin and in the San Francisco Bay area. Simple Coulomb failure calculations, which assume that earthquakes can be modeled as static dislocations in an elastic half-space, have done quite well at approximating how long the stress shadows, or relaxing effects, should last and at predicting where subsequent large earthquakes will not occur. There has, however, been at least one apparent exception to the predictions of such simple models. The 1911 M>6.0 earthquake near Morgan Hill, California, occurred at a relaxed site on the Calaveras fault. We examine how the more complex rate-and-state friction formalism based on laboratory experiments might have allowed the 1911 earthquake. Rate-and-state time-to-failure calculations are consistent with the occurrence of the 1911 event just 5 years after 1906 if the Calaveras fault was already close to failure before the effects of 1906. We also examine the likelihood that the entire 78 years of relative quiet (only four M≥6 earthquakes) in the bay area after 1906 is consistent with rate-and-state assumptions, given that the previous 7 decades produced 18 M≥6 earthquakes. Combinations of rate-and-state variables can be found that are consistent with this pattern of large bay area earthquakes, assuming that the rate of earthquakes in the 7 decades before 1906 would have continued had 1906 not occurred. These results demonstrate that rate-and-state offers a consistent explanation for the 78-year quiescence and the 1911 anomaly, although they do not rule out several alternate explanations.

Dynamics of subduction initiation at preexisting fault zones

Abstract: We show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces, consistent with what is observed in the western Pacific. The dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method. We investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion. The viscosity of the medium is non-Newtonian and temperature-dependent. We also examine the influence of a perfectly plastic yield stress. A new method to model the preexisting weak zone as a graded fault with remeshing is introduced. The first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics. Initially, topography across the plate boundary is characterized by over 1000 m of uplift close to the trench. Subsequently, a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m/Myr for a slab subducting at 2 cm/yr. Tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration. For a plate force of 4×10^(12) N/m, subduction initiates even with a fault shear stress of 3 MPa, the resistance most consistent with seismological observations. However, subduction will not initiate when this fault shear stress is >5 MPa. Our results are consistent with plate reconstructions, which predict the initiation of subduction across preexisting weak zones within oceanic basins.

Ground-motion amplification in the Santa Monica area: Effects of shallow basin-edge structure

Abstract: The 1994 Northridge earthquake produced ground motions in the northwest portion of the Los Angeles basin that were significantly larger than rock-site motions observed at locations just north of the basin. The Santa Monica area was hit particularly hard, with numerous structures being damaged or destroyed by the strong ground shaking. In this region, the basin-edge geology is controlled by the active strand of the east-west-striking Santa Monica fault, and virtually all of the structural damage occurred at or south of the fault location. We have used 2D finite-difference ground-motion simulations to investigate the effect of the basin-edge structure in amplifying ground response. Constraints on the basin-edge structure come from geologic cross sections, geophysical data, and seismological observations. Our simulations indicate that the shallow basin-edge structure (1 km deep) formed by the active strand of the Santa Monica fault creates a large amplification in motions immediately south of the fault scarp, in very good agreement with mainshock damage patterns, recorded ground motions, and locations of elevated site response. This large amplification results from constructive interference of direct waves with the basin-edge-generated surface waves and is quite similar to the basin-edge effect associated with the 1995 Kobe earthquake. In addition, we find that focusing effects created by the deeper basin structure (3 to 4 km deep) cannot explain the large motions observed immediately south of the fault scarp. This strongly suggests that the deep-basin focusing models proposed by Gao et al. (1996) and Alex and Olsen (1998) are not likely explanations of the observed pattern of ground-motion amplification in the Santa Monica area.

Fault sealing processes in siliciclastic sediments

Abstract: Abstract The microstructure and petrophysical properties of fault rocks from siliciclastic hydrocarbon reservoirs of the North Sea are closely related to the effective stress, temperature and sediment composition at the time of deformation, as well as their post-deformation stress and temperature history. Low permeability fault rocks may develop due to a combination of processes including: the deformation induced mixing of heterogeneously distributed fine-grained material (principally clays) with framework grains, pressure solution, cataclasis, clay smear, and cementation. Fault rocks can be classified into various types (disaggregation zones, phyllosilicate-framework fault rocks, cataclasites, clay smears, and cemented faults/fractures) based upon their clay and cement content as well as the amount of cataclasis experienced. In the absence of extensive cementation, the distribution of fault rock types along a fault plane can often be predicted from a detailed knowledge of the reservoir sedimentology. The permeability of fault rocks can vary by over six orders of magnitude, depending on the extent to which the porosity reduction processes have operated. Utilizing the strong link between the petrophysical properties of fault rocks and their geohistory allows the risks associated with fault seal evaluation to be reduced.

Geomorphic assessment of active tectonics in the Acambay graben, Mexican Volcanic Belt

Abstract: Spatial variations of Quaternary deformation and tectonic activity of faults along the Acambay graben are assessed using geomorphic and morphometric approaches. The Acambay graben is an east–west trending structure of apparent Quaternary age, located in the central part of the Mexican Volcanic Belt, which gives rise to pronounced scarps over a distance of about 80 km. Continuing tectonic activity in the Acambay graben is confirmed by recent well documented seismic episodes. The intensity of active tectonics has been interpreted through a detailed geomorphic study of the fault-generated mountain fronts and fluvial systems. The combined geomorphic and morphometric data provide evidence for relative variations in tectonic activity among the Acambay graben faults. Geomorphic indices suggest a relatively high degree of tectonic activity along the Venta de Bravo and the Acambay–Tixmadeje faults, followed, in order of decreasing activity, by the Pastores, Temascalcingo and Tepuxtepec faults. Spatial variations within faults have also been identified, suggesting a higher level of tectonic activity at the tips of the faults. This pattern of variation in the relative degree of tectonic activity is consistent with field evidence and seismic data for the Acambay graben. Geomorphic evaluation of the Acambay graben faults suggests that the Acambay–Tixmadeje and Venta de Bravo faults, and specifically the tips of these faults and a central segment near the town of Venta de Bravo, should be considered as areas of potentially high earthquake risk. © 1998 John Wiley & Sons, Ltd.

Earthquakes on Dipping Faults: The Effects of Broken Symmetry

Abstract: Dynamic simulations of earthquakes on dipping faults show asymmetric near-source ground motion caused by the asymmetric geometry of such faults. The ground motion from a thrust or reverse fault is larger than that of a normal fault by a factor of 2 or more, given identical initial stress magnitudes. The motion of the hanging wall is larger than that of the footwall in both thrust (reverse) and normal earthquakes. The asymmetry between normal and thrust (reverse) faults results from time-dependent normal stress caused by the interaction of the earthquake-generated stress field with Earth's free surface. The asymmetry between hanging wall and footwall results from the asymmetric mass and geometry on the two sides of the fault.

Layer-bound compaction faults in fine-grained sediments

Abstract: This paper describes examples of a recently recognized type of soft-sediment deformation associated with early compaction of fine-grained sediments. This type of deformation was originally described from the North Sea Basin, where Paleogene slope and basin-floor claystones are deformed over an area of >150 000 km 2 by a layer-bound system of minor extensional faults arranged in polygonal patterns in map view. The development of this regionally extensive polygonal fault system has been attributed to volumetric contraction during early compactional dewatering on the basis of detailed strain measurements carried out using high-resolution three-dimensional seismic data. A comprehensive review of published two-dimensional and three-dimensional seismic data from 27 other layer-bound fault systems from many different sedimentary basins is presented in this paper. The only factors common to all 28 examples of layer-bound faults are that the deformed units are only found in marine depositional settings, are dominantly composed of ultrafine-grained smectitic claystones or carbonate chalks, and are characterized by high porosity and extremely low permeability. Other factors such as sedimentation rate, organic carbon content, age, depth of burial, methane content, and pore-fluid chemistry are not systematically correlated with this deformational response. The correlation between distribution of deformed units and ultrafine grain size suggests that the deformation mechanism is related to colloidal properties as part of this type of compactional response. The restricted distribution of layer-bound fault systems to predominantly pelagic depositional units with often low sedimentation rates is compatible with a recently presented model of volumetric contraction during early burial. We build on this model of fully three-dimensional compaction to propose that layer-bound faulting is an expression of the process of syneresis, whereby pore fluid is expelled from sedimentary gels under the spontaneous action of osmotic or electrochemical forces.

Faulting, fault sealing and fluid flow in hydrocarbon reservoirs: an introduction

Abstract: Abstract A predictive knowledge of fault zone structure and transmissibility can have an enormous impact on the economic viability of exploration targets and generate considerable benefits during reservoir management. Understanding the effects of faults and fractures on fluid flow behaviour and distribution within hydrocarbon provinces has therefore become a priority. To model fluid flow in hydrocarbon reservoirs, it is essential to gain a detailed insight into the evolution, structure and properties of faults and fractures. Generation of realistic flow models also requires calibration with data on the fluid distributions and flow rates from hydrocarbon fields. Most hydrocarbon geologists at one time or another have asked the question ‘What is the behaviour of this fault?’. This question, as emphasized by the contributions to this volume, should more fundamentally be phrased; ‘What is the geometry of this fault zone, what are the nature and petrophysical properties of any fault rocks developed and how are they distributed in the subsurface?’. An additional important question is ‘What impact could the fault zone have on fluid flow through time?’. The properties and evolution of fault zones can be evaluated using the combined results of structural core and down-hole logging, microstructural and physical property characterization, together with analysis of faults from seismic and outcrop studies and well test data. Successful fault analysis depends upon the amalgamation of these data and incorporation into robust numerical flow models.

Absence of earthquake correlation with Earth tides: An indication of high preseismic fault stress rate

Abstract: Because the rate of stress change from the Earth tides exceeds that from tectonic stress accumulation, tidal triggering of earthquakes would be expected if the final hours of loading of the fault were at the tectonic rate and if rupture began soon after the achievement of a critical stress level. We analyze the tidal stresses and stress rates on the fault planes and at the times of 13,042 earthquakes which are so close to the San Andreas and Calaveras faults in California that we may take the fault plane to be known. We find that the stresses and stress rates from Earth tides at the times of earthquakes are distributed in the same way as tidal stresses and stress rates at random times. While the rate of earthquakes when the tidal stress promotes failure is 2% higher than when the stress does not, this difference in rate is not statistically significant. This lack of tidal triggering implies that preseismic stress rates in the nucleation zones of earthquakes are at least 0.15 bar/h just preceding seismic failure, much above the long-term tectonic stress rate of 10−4 bar/h.

Evidence of Shallow Fault Zone Strengthening After the 1992 M7.5 Landers, California, Earthquake

Abstract: Repeated seismic surveys of the Landers, California, fault zone that ruptured in the magnitude (M) 75 earthquake of 1992 reveal an increase in seismic velocity with time P, S, and fault zone trapped waves were excited by near-surface explosions in two locations in 1994 and 1996, and were recorded on two linear, three-component seismic arrays deployed across the Johnson Valley fault trace The travel times of P and S waves for identical shot-receiver pairs decreased by 05 to 15 percent from 1994 to 1996, with the larger changes at stations located within the fault zone These observations indicate that the shallow Johnson Valley fault is strengthening after the main shock, most likely because of closure of cracks that were opened by the 1992 earthquake The increase in velocity is consistent with the prevalence of dry over wet cracks and with a reduction in the apparent crack density near the fault zone by approximately 10 percent from 1994 to 1996

Common faults and their impacts for rooftop air conditioners

Abstract: This paper identifies important faults and their performance impacts for rooftop air conditioners. The frequencies of occurrence and the relative costs of service for different faults were estimated through analysis of service records. Several of the important and difficult to diagnose refrigeration cycle faults were simulated in the laboratory. Also, the impacts on several performance indices were quantified through transient testing for a range of conditions and fault levels. The transient test results indicated that fault detection and diagnostics could be performed using methods that incorporate steady-state assumptions and models. Furthermore, the fault testing led to a set of generic rules for the impacts of faults on measurements that could be used for fault diagnoses. The average impacts of the faults on cooling capacity and coefficient of performance (COP) were also evaluated. Based upon the results, all of the faults are significant at the levels introduced, and should be detected and diagnosed ...

Estimation of current plate motions in Papua New Guinea from Global Positioning System observations

Abstract: Plate tectonic motions have been estimated in Papua New Guinea from a 20 station network of Global Positioning System sites that has been observed over five campaigns from 1990 to 1996. The present velocities of the sites are consistent with geological models in which the South Bismarck, Woodlark, and Solomon Sea Plates form the principal tectonic elements between the Pacific and Australian Plates in this region. Active spreading is observed on the Woodlark Basin Spreading Centre but at a rate that is about half the rate determined from magnetic reversals. The other major motions observed are subduction on the New Britain Trench, seafloor spreading across the Bismarck Sea Seismic Lineation, convergence across the Ramu-Markham Fault and left-lateral strike slip across the Papuan Peninsula. These motions are consistent with a 8.2° Myr -1 clockwise rotation of the South Bismarck Plate about a pole in the Huon Gulf and a rotation of the Woodlark Plate away from the Australian Plate. Second order deformation may also be occurring; in particular, Manus Island and northern New Ireland may be moving northward relative to the Pacific Plate at ∼5-8 mm yr -1 (significant at the 95% but not at the 99% confidence level) which may suggest the existence of a North Bismarck Plate.

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.

Relative motion between the Caribbean and North American plates and related boundary zone deformation from a decade of GPS observations

Abstract: Global Positioning System (GPS) measurements in 1986, 1994, and 1995 at sites in Dominican Republic, Puerto Rico, Cuba, and Grand Turk define the velocity of the Caribbean plate relative to North America. The data show eastward motion of the Caribbean plate at a rate of 21 ± 1 mm/yr (1 standard error ) in the vicinity of southern Dominican Republic, a factor of 2 higher than the NUVEL-1A plate motion model prediction of 11 ± 3 mm/yr. Independent measurements on San Andres Island, and an Euler vector derived from these data, also suggest a rate that is much higher than the NUVEL-1A model. Available data, combined with simple elastic strain models, give the following slip rate estimates for major left-lateral faults in Hispaniola: (1) the North Hispaniola fault offshore the north coast of Hispaniola, 4 ± 3 mm/yr; (2) the Septentrional fault in northern Dominican Republic, 8 ± 3 mm/yr; and (3) the Enriquillo fault in southern Dominican Republic and Haiti, 8 ± 4 mm yr. The relatively high plate motion rate and fault slip rates suggested by our study, combined with evidence for strain accumulation and historical seismicity, imply that seismic risk in the region may be higher than previous estimates based on low plate rate/low fault slip rate models and the relatively low rate of seismicity over the last century.

Miocene-Pliocene tectonic evolution of the Slovenian Periadriatic fault: Implications for Alpine-Carpathian extrusion models

Abstract: The Periadriatic Line (PAL) is a remarkable, several hundred kilometer long fault system of the Alpine orogen. Its dextral character was documented by several authors using diverse criteria, but detailed kinematics and timing of movements had not been investigated along its whole length. Structural and paleomagnetic measurements, mapping, and stratigraphic and sedimentological studies have helped to unravel the Miocene-Pliocene evolution of the Slovenian segment of the PAL. Brittle deformation was characterized by NW-SE to N-S compression and perpendicular tension. Deformation has resulted in dextral strike-slip faulting, folding, and tilting of beds. The first transpressional event corresponds to the first phase of lateral extrusion of the East Alpine-Western Carpathian-Northern Pannonian block in the early Miocene (24–17.5 Ma). After a short period of transtension during the Karpatian (17.5–16.5 Ma), dextral transpression reoccurred during the middle Miocene to Pliocene and lasted up to the Quaternary. Middle Miocene dextral slip can be connected to the second phase of extrusion. The highly deformed rocks within the dextral shear zones show variable clockwise, sometimes counterclockwise, rotations. The mechanism of rotation seems to be complex, ranging from regional rotation to local folding due to pure or simple shear (domino-type rotation).