Slip Style Research Articles

A range of fault slip styles on progressively misoriented planes during flexural-slip folding, Cape Fold Belt, South Africa

Flexural slip folds are distinctive of mixed continuous-discontinuous deformation in the upper crust, as folding is accommodated by continuous bending of layers and localized, discontinuous slip along layer interfaces. The mechanism of localized, layer-parallel slip and the stress and fluid pressure conditions at which flexural slip occurs are therefore distinctive of shear localization during distributed deformation. In the Prince Albert Formation mudstone sequence of the Karoo Basin, the foreland basin to the Cape Fold Belt, folds are well developed and associated with incrementally developed bedding-parallel quartz veins with slickenfibers oriented perpendicular to fold hinge lines, locally cross-cutting axial planar cleavage, and showing hanging wall motion toward the fold hinge. Bedding-parallel slickenfiber-coated veins dip at angles from 18° to 83°, implying that late increments of bedding-parallel shear occurred along unfavorably oriented planes. The local presence of tensile veins, in mutually cross-cutting relationship with bedding-parallel, slickenfiber-coated veins, indicate local fluid pressures in excess of the least compressive stress.Slickenfiber vein microstructures include a range of quartz morphologies, dominantly blocky to elongate-blocky, but in places euhedral to subhedral; the veins are commonly laminated, with layers of quartz separated by bedding-parallel slip surfaces characterized by a quartz-phyllosilicate cataclasite. Crack-seal bands imply incremental slickenfiber growth, in increments from tens of micrometers to a few millimeters, in some places, whereas other vein layers lack evidence for incremental growth and likely formed in single slip events. Single slip events, however, also involved quartz growth into open space, and are inferred to have formed by stick-slip faulting. Overall, therefore, flexural slip in this location involved bedding-parallel faulting, along progressively misoriented weak planes, with a range of slip increments.

Brittle‐viscous deformation, slow slip, and tremor

AbstractGeophysical observations have illuminated a spectrum of fault slip styles from continuous aseismic sliding to fast earthquake slip. We study exhumed intercalated lenses of oceanic crust and sedimentary rocks, deformed to high shear strains. Deformation was partitioned between fractured, rigid blocks, with lengths of tens to hundreds of meters, and surrounding metapelites characterized by interconnected phyllosilicate networks. Under inferred conditions of low effective stress at temperatures > 500°C, locally and transiently elevated shear strain rate in phyllosilicates deforming by dislocation creep can reach those needed for transient slow slip. Concurrently, increased matrix strain rate likely stimulates brittle failure in rigid lenses. The ubiquitous presence of quartz veins and microfractures within rigid material provides evidence for brittle deformation occurring coincident with viscous shearing flow. We suggest that geophysically observed tremor and slow slip may be a manifestation of strain partitioning, where deformation is accommodated viscously in a matrix enveloping rigid lenses.

Enabling 3D geomechanical restoration of strike- and oblique-slip faults using geological constraints, with applications to the deep-water Niger Delta

We present a new approach of using local constraints on fault slip to perform three-dimensional geomechanical restorations. Geomechanical restoration has been performed previously on extensional and contractional systems, yet attempts to restore strike- and oblique-slip fault systems have generally failed to recover viable fault-slip patterns. The use of local measures of slip as constraints in the restoration overcomes this difficulty and enables restorations of complex strike- and oblique slip-systems. To explore this approach, we develop a synthetic restraining bend system to evaluate different ways that local slip constraints can be applied. Our restorations show that classical boundary conditions fail to reproduce the fault offset and strain pattern. In contrast, adding piercing points and/or properly constraining lateral walls enables restoration of the structure and resolves the correct pattern of slip along the faults. We then restore a complex system of tear-faults in the deepwater Niger Delta basin. We use channel offsets imaged by the seismic data to define local fault-slip constraints. Balancing these constraints equally on both sides of the major faults yields the most consistent restoration outcomes. This approach resolves reasonable slip styles on the complex set of thrust, normal, and strike-slip faults in the structure. This suggests that limited geologic fault slip constraints can be effectively incorporated in geomechanical restorations, yielding accurate restoration kinematics and thereby forecasting faults slip patterns within the structures.

隣接する活断層の連結性と変位様式

島根県南西部の弥畝山西断層と都茂断層の性状と分布を詳細に再検討するために，それらの断層に沿う地域と延長部で地形・地質調査を実施した．得られた結果と既存の重力異常図および震源分布データを組み合わせ，弥畝山西断層と都茂断層の連結性と変位様式を議論する． リニアメント判読と地表調査に基づいて，これまで指摘されてきたよりも，弥畝山西断層は南西方向に10km，都茂断層は東方向に 1 km長く連続することを指摘した．弥畝山西断層北東部と都茂断層が接合する益田市美都町板井川地域において横ずれデュープレックスが確認される．その横ずれデュープレックスに囲まれた領域は周囲に比べて標高が低く，低地になっている．断層露頭の変形構造は，都茂断層の変位センスが左横ずれから右横ずれにインバージョンしたことを示す．この変位センスは，横ずれデュープレックスに囲まれた低地がプルアパート (板井川プルアパート) としてNE-SW方向に拡大を続けるとともに，それによって低地が沈降することを示唆している．

Non‐volcanic tremor and discontinuous slab dehydration

Non‐volcanic tremor is a recently discovered fault slip style occurring with remarkable regularity in space near the down‐dip end of the locked zone on several subduction thrust interfaces. The physical mechanisms and the controls on the location of tremor have not yet been determined. We calculate the stable mineral assemblages and their water content in the subducting slab, and find that slab dehydration is not continuous, but rather restricted to a few reactions localised in pressure‐temperature space. Along geothermal gradients applicable to Shikoku and Cascadia ‐ where tremor has been relatively easy to detect ‐ tremor locations correlate with discontinuous and localised voluminous water release from the breakdown of lawsonite and chlorite + glaucophane respectively. The shape of the pressure‐temperature path for subducting slabs prevents fluid release at depths above and below where these dehydration reactions occur. We conclude that abundant tremor activity requires metamorphic conditions where localised dehydration occurs during subduction, and this may explain why tremor appears more abundant in some subduction zones than others.

Modeling monthlong slow slip earthquakes

The slow crawl of the Earth's tectonic plates is periodically punctuated by the release of decades or centuries of accumulated stress. Standard earthquakes, lasting from fractions of a second to minutes in duration, are known to range from imperceptible to those that can flatten a city, but recent geodetic measurements have led to the discovery of a previously unrecognized style of fault slip—episodic slow earthquakes. These slow earthquakes recur roughly annually, involve a weeklong or monthlong speeding up of a plate's motion by a factor of 10—100, and are associated with low‐level seismic tremors. First detected about 10 years ago in Japan and western North America, they occur along the same faults that produce devastating magnitude 9 earthquakes but at greater depths. How and why slow earthquakes differ from regular earthquakes, and the extent to which they might foreshadow the shallower truly great earthquakes, are currently topics of wide interest. (Geochemistry, Geophysics, Geosystems, doi:10.1029/2010GC003386, 2011)

Unique deformation processes involving the recrystallization of chrysotile within serpentinite: implications for aseismic slip events within subduction zones

AbstractSerpentinite bodies within the Franciscan Complex, a Mesozoic accretionary prism located in California, USA, display a unique form of deformation that involves the recrystallization of chrysotile and the formation of a block‐in‐matrix structure. The phacoidal‐shaped blocks have a preferred orientation, and result from the local replacement of serpentine minerals by chrysotile grains that are aligned parallel to ductile shear planes such as S–C foliation; ultimately, some of the rocks evolved into chrysotile schist. The relic blocks are also fragmented into multiple parts, with the spaces between fragments being infilled by recrystallized chrysotile. The low coefficient of friction of chrysotile means that this deformation process acts to suppress the frictional properties of the entire serpentinite body within the forearc mantle. This phenomenon can be attributed to the slip style that occurs in aseismic regions of subduction zones in areas shallower than the stability field of antigorite.

Effects of fault geometry and slip style on near-fault static displacements caused by the 1999 Chi-Chi, Taiwan earthquake

We investigated the fault geometry effects and the corresponding coseismic slip distribution using various proposed earthquake fault models for the Chi-Chi earthquake of 21 September 1999. The types of fault geometries are threefold: a simple planar fault plane, a two segmented planar fault plane and a three dimensional (3D) curved fault surface rupture propagation model. We derived the estimated spatial slip distribution from an inversion analysis of GPS coseismic displacement data and show that the 3D fault model is the preferred solution. The simple and segmented fault models lead to significant artificial slip distributions associated with the pre-defined fault geometry and the spatial distribution of GPS stations. The spatial distribution of coseismic slip deduced from the 3D fault model has three observable features: (1) the overall slip is concentrated at depth of less than 12 km, which may well correspond to a shallow-dipping detachment; (2) the maximum slip of about 10 m is located 45 km to the north of the epicenter; and (3) the slip vector is dominated by the dip-slip component. In addition, the results from the inversion of GPS data are consistent with those from the inversion analysis of teleseismic broadband data. A resolution analysis, further, demonstrates that the results are highly correlated with field GPS data studies when we used synthetic test data. The inversion of spatially distributed GPS data is highly sensitive to fault geometry. We conclude that the use of the 3D fault model is not only necessary but also certainly competent enough to well explain the inferred slip style and the observed static coseismic displacements. D 2005 Elsevier B.V. All rights reserved.

How Sensitive Are Fault-Slip Rates in the Los Angeles Basin to Tectonic Boundary Conditions?

Geodetic studies have produced different interpretations of the relative vertical thickening versus escape tectonics occurring within the Los Angeles basin based on calculated horizontal strain rates ranging from 56 10 9 /yr contraction at N36 Et o 85 10 9 /yr north-south contraction and 60 10 9 /yr east-west extension. Three-dimensional forward mechanical models of faults in the Los An- geles basin are used to compare modeled slip rates and slip styles, represented as the rakes of average net slip vectors, under four representative horizontal strain condi- tions with geologically determined late-Quaternary slip rates and observed slip styles. The comparisons strive to identify the most mechanically viable tectonic boundary conditions for the Los Angeles basin. The three-dimensional fault surfaces used within the mechanical models have been defined by Southern California Earthquake Center (SCEC) workers based on surface geology and subsurface data. The model results suggest that northeast-southwest principal contraction is inappropriate for the Los Angeles basin because this tectonic boundary condition produces left-lateral slip along the Whittier and San Gabriel faults in contrast with abundant geologic evidence of right-lateral slip. Of the four tectonic boundary conditions evaluated, north-south contraction accompanied by either zero or slow east-west contraction gives the low- est mismatch of modeled and observed fault slip style. These results suggest that vertical thickening may be more predominant than escape tectonics in this region.

Three-dimensional strain tensor estimation by GPS observations: methodological aspects and geophysical applications

The aim of this paper is to describe the theoretical fundamentals, the main features and some geophysical applications of a software (STRAINGPS) suitably implemented to estimate the strain tensor from repeated GPS surveys of deformation control networks. Current softwares developed for geophysical applications generally estimate or compute bi-dimensional strain, since this is the most requested use. On the contrary, this software allows for a three-dimensional (3D) estimate of the strain tensor and does not need a subdivision of the network in triangles. It accounts for all the significant coordinate differences (or velocities) coming from repeated surveys and estimates the strain tensor components by the least squares method, starting from the hypothesis of one homogeneous strain field. Moreover, some tests to control both model adequacy and detecting outliers are performed, allowing a subdivision of the field into sub-domains with homogeneous strain field. We applied this software to three real geophysical situations. The first concerns the geodynamic (long term) induced deformation at regional scale, by estimating extensional strain rate ((0.22±0.06)×10−7) per year in central-southern Italy across the Apennines chain; the second is co-seismic strain by the assessment of the strike slip style of the Molise (central-southern Italy) earthquakes (31 October and 1 November 2002); the last regards the subsidence induced strain in the Travale-Radicondoli (central Italy) exploitation area.

Effect of complex fault geometry and slip style on near-fault strong motions and static displacement

Although there are many studies that deal with complex slip distribution or rupture propagation on an earthquake fault, they usually regard a fault system as a fault of simple geometry. Actual fault systems have highly heterogeneous slip distribution and very complicated shapes, as is often observed through field surveys of surface breaks. In this study, we synthesize seismograms including static displacement near a fault using the discrete wavenumber method in order to estimate the effects of the above types of fault complexity in a quantitative manner. We introduce a complex slip distribution based on the Nojima Fault associated with the 1995 Hyogo-ken Nanbu earthquake. As a result, we show that strong motions at a frequency of lower than 1.0 Hz are strongly affected by the complexity of the fault geometry, at a scale of not more than several km, rather than the rupture propagation style. Distributions of static displacement fluctuate, depending on the fault geometry characterized by the length of each fault segment. Such small-scale variations in fault geometry (≤1 km) have been mostly ignored prior to this work. Our results also suggest that details of fault segmentation and bending can be determined by dense observations (e.g., GPS or geological surveys) of static displacement near a fault system, indicating the importance of simultaneous studies on static and dynamic near-fault motions.

Interaction between shallow and subcrustal dislocations on a normal fault

We propose a model which may explain seismic sequences which are often observed in seismogenic regions, as for example in the Apenninic chain (Italy). In particular, we consider a normal fault and earthquakes taking place at different depths: a first shock in a shallower layer and a second in a deeper one. The normal fault is embedded in a viscoelastic half-space. As a consequence of the rheology, there are two different brittle layers, a shallower and a deeper one, where earthquakes can nucleate. Between these two layers, the rheological behavior is ductile. The thicknesses of the layers depend on the geothermal profile that is calculated taking into account the profile of the thermal and rheological parameters with depth. The fault plane, crossing layers with different rheological behavior, is heterogeneous in respect to the slip style: seismic in the brittle layers, aseismic in the ductile layer. Dislocations in the shallower layer are assumed to produce aseismic slip in the area of the fault belonging to the ductile layer. The stress concentrated, by the seismic and aseismic dislocations, on the fault plane section in the deeper brittle layer is evaluated. It is compared with the tectonic stress rate in order to calculate how much earlier the second earthquake would occur compared to if just the bare tectonic sstress was acting. It results that such an advance is comparable with typical recurrence times of earthquakes and so a mechanism of interaction between different seismic sources, mediated by aseismic slip, can be supposed. The strains and displacements at the Earth’s surface due to seismic and aseismic slip are calculated. They are large enough to be detected by present geodetic techniques.

Slip styles in a Spring‐Slider Model with a laboratory‐derived constitutive law for serpentinite

A spring‐slider model with a laboratory‐derived constitutive law for serpentinite generates seismic slip, aseismic slip, or a combination of both. Such behavior is observed on oceanic ridge‐transform systems and portions of the San Andreas Fault, and may be due to the presence of serpentinite. The constitutive law combines two sets of equations: a rate‐dependent flow equation which dominates at low velocities, and a rate‐ and state‐dependent set of equations which dominate at high velocities. The combined model produces transient behavior with maximum slip velocities well in excess of the loading velocity. Metastable slip prior to the onset of high‐speed slip is produced in many simulations and may be analogous to the slow precursors observed for some oceanic ridge‐transform earthquakes. This precursory slip results from the double‐valued nature of the steady‐state curve of the two‐mechanism model in which the state‐variable equations are rate weakening.

Paleoseismology of the 1912 Acambay earthquake and the Acambay‐Tixmadejé fault, Trans‐Mexican Volcanic Belt

The Acambay Graben is a major intra‐arc basin of the Trans‐Mexican Volcanic Belt. Trenching studies at four sites along the Acambay‐Tixmadejé fault reveal evidence for the 1912 Acambay earthquake and at least four late Pleistocene and Holocene ground‐rupturing earthquakes with similar displacements to the 1912 event. Evidence for multiple events was recognized by a combination of upward terminating faults, fissure fills, erosionally truncated fissures, colluvial wedge fills, and downward increasing separation of stratigraphic units. Sites along the central part of the ESE‐WNW trending rangefront fault indicates large dip slip separations across a multi event scarp in Holocene colluvium with a penultimate event that occurred since ∼5230 calibrated years before present (cal. yr B.P.). On the Huapango Plain, near the eastern end of the 1912 rupture where slip was oblique, the penultimate earthquake occurred after ∼4700 cal. yr B.P., and there is evidence for at least four more earthquakes in the last ∼34,000 cal. yr B.P. Three‐dimensional excavation of lacustrine and channel deposits at Las Lomas yields a left‐normal slip vector of S40±5°E with 105±10 cm of slip for the last three events there. This slip vector is consistent with NNE‐oriented extension across the graben. The amount and style of slip documented for prehistoric ruptures are also consistent with the 1912 rupture, with a mean of ∼60 cm/event of dip slip on the rangefront and a mean of ∼35 cm/event of oblique slip on the N48°W trending Huapango Plain. Assuming these values represent a characteristic behavior yields an average recurrence interval of ∼3600 years for the last four large earthquakes (since ∼11,570 cal. yr B.P.), and a slip rate of ∼0.17 mm/yr across the rangefront. Empirical earthquake parameters calculated from our paleoseismic data support instrumental measures indicating that the 1912 Acambay earthquake was ofM6.8–7.0. The 1912 event is representative of ground‐rupturing earthquakes on the Acambay‐Tixmadejé fault and demonstrates that large seismic events (M∼7) can be generated by faults in the Trans‐Mexican Volcanic Belt and in other intra‐arc settings.

Late Quaternary slip on the Santa Cruz Island fault, California

The style, timing, and pattern of slip on the Santa Cruz Island fault were investigated by trenching the fault and by analysis of offset late Quaternary landforms. A trench excavated across the fault at Christi Beach, on the western coast of the island, exposed deformation of latest Pleistocene to Holocene sediments and pre-Quaternary rocks, recording repeated large-magnitude rupture events. The most recent earthquake at this site occurred ca. 5 ka. Coastal terraces preserved on western Santa Cruz Island have been dated using the uranium-series technique and by extrapolation using terrace elevations and the eustatic record. Offset of terraces and other landforms indicates that the Santa Cruz Island fault is predominantly left lateral, having a horizontal slip rate of not more than 1.1 mm/yr and probably about 0.8 mm/yr. The fault also has a smaller reverse component, slipping at a rate of between 0.1 and 0.2 mm/yr. Combined with measurements of slip per event, this information suggests a long-term average recurrence interval of at least 2.7 k.y. and probably 4–5 k.y., and average earthquake magnitudes of Mw 7.2–7.5. Sense of slip, recurrence interval, and earthquake magnitudes calculated here for the Santa Cruz Island fault are very similar to recent results for other faults along the southern margin of the western Transverse Range, including the Malibu Coast fault, the Santa Monica fault, the Hollywood fault, and the Raymond fault, supporting the contention that these faults constitute a continuous and linked fault system, which is characterized by large but relatively infrequent earthquakes.

The repetition of large-earthquake ruptures.

This survey of well-documented repeated fault rupture confirms that some faults have exhibited a "characteristic" behavior during repeated large earthquakes--that is, the magnitude, distribution, and style of slip on the fault has repeated during two or more consecutive events. In two cases faults exhibit slip functions that vary little from earthquake to earthquake. In one other well-documented case, however, fault lengths contrast markedly for two consecutive ruptures, but the amount of offset at individual sites was similar. Adjacent individual patches, 10 km or more in length, failed singly during one event and in tandem during the other. More complex cases of repetition may also represent the failure of several distinct patches. The faults of the 1992 Landers earthquake provide an instructive example of such complexity. Together, these examples suggest that large earthquakes commonly result from the failure of one or more patches, each characterized by a slip function that is roughly invariant through consecutive earthquake cycles. The persistence of these slip-patches through two or more large earthquakes indicates that some quasi-invariant physical property controls the pattern and magnitude of slip. These data seem incompatible with theoretical models that produce slip distributions that are highly variable in consecutive large events.

Style and rate of Holocene slip, Palos Verdes fault, southern California

Recent studies of the Palos Verdes fault in the Los Angeles Outer Harbor characterize the fault's Holocene activity. The studies included areal and local geophysical surveys, as well as drilling, sampling, and dating of the subsurface materials. The studies identify the fault's location in the shallow subsurface, document evidence of Holocene strike‐slip, quantify dip‐slip versus sediment age, and quantify the net slip where an early Holocene paleochannel crosses the fault. The slip rate for the Palos Verdes fault is constrained to between 2.7 and 3 mm/yr for the past 7.8 to 8 ka. Because the new evidence shows that the Palos Verdes fault has one of the highest slip rates in the Los Angeles basin, the studies provide significant new insight into the regional tectonics and the seismic risk of the Los Angeles region. Using this rate of slip and segmentation models for the fault, the coastal Los Angeles region can expect an MW7 to 7.2 event about every 400 to 900 years from this potential seismic source.

Structure and late quaternary activity of the northern Owens valley fault zone, Owens valley, California

The Fish Springs fault is a primary strand in the northern end of the Owens Valley fault zone (OVFZ). The Fish Springs fault is the northwest strand in a 3-km-wide left echelon step of the OVFZ which bounds the Poverty Hills bedrock high. The Fish Springs fault strikes approximately north-south, dips steeply to the east, and is marked by a prominent east-facing scarp. No other faults in the OVFZ have prominent east-facing scarps at the latitude of Fish Springs, which indicates that the Fish Springs fault has accommodated virtually all of the local late Quaternary vertical displacement on the OVFZ. The Fish Springs fault exhibits normal dip slip with no measurable lateral slip. Vertical displacements of a Late Pleistocene (0.314 ± 0.036 Ma, 2σ) cinder cone and of an overlying Tahoe-age (0.065–0.195 m.y.) alluvial fan are 76±8 m and 31±3 m, respectively. The maximum vertical 3.3. m. Two nearly equal vertical displacements of the active stream channel in the Tioga-age fan total 2.2. m. Vertical displacement of a stream terrace incised into the cinder cone is 1.2 ± 0.3 m. The minute amount of incision into that terrace indicates that uplift of the terrace probably occurred during the 1872 Owens Valley earthquake. Three displacements of 1.1 ± 0.2 m each apparently have occurred at the Tioga-age fan since the midpoint of the Tioga interval, allowing an average recurrence interval of 3500 to 9000 years. Based on the age and displacement of the cinder cone, the average late Quaternary vertical displacement rate is 0.24 ± 0.04 mm/yr (2σ). At this rate, and assuming an average vertical displacement of 1.1 ± 0.2 m per event, the average recurrence interval would be 4600 ± 1100 years (2σ). The recurrence interval for the Fish Springs fault is similar to that for a strand in the southern part of the OVFZ which also ruptured in 1872. Right-lateral, normal oblique slip characterizes the OVFZ. The location of the Poverty Hills bedrock high at a left step in the north-northwest-striking fault zone is consistent with the style of slip of the zone. The pure normal slip on the north-striking Fish Springs fault and the alignment of local cinder cones along north-striking normal faults indicate that the late Quaternary maximum horizontal compression has been oriented north-south at the north end of the OVFZ. Data from southern Owens Valley indicate a similar stress regime there. Late Quaternary slip on the OVFZ is consistent with north-south maximum horizontal compression.

Cypriote Imitations of Tell el-Yahudiyeh Ware from Toumba tou Skourou

A series of Tell el-Yahudiyeh juglets found at Toumba tou Skourou in northwest Cyprus are to be dated, according to their context, to the initial phase of Late Cypriote I. Juglets of this non-Cypriote type have been previously recorded from other sites in Cyprus, mainly in the southeastern part of the island. Archaeological evidence indicates that Tell el-Yahudiyeh juglets were manufactured in the Levant and Egypt during the Second Intermediate Period and therefore the imported vessels found in Cyprus can hardly extend beyond the Middle Bronze Age. Typological study of the Toumba tou Skourou juglets together with related specimens discovered in the neighboring cemeteries of Stephania and Pendayia reveals that shape and decoration of imported Tell el-Yahudiyeh Ware were locally adapted to the style and technique of Black Slip III and other Cypriote wares. These imitations took place in Cyprus at the opening of the Late Cypriote period because the Syro-Palestinian and Egyptian vessels ceased to be imported into the island at the close of the Middle Cypriote period.