Slip Vectors Research Articles

Strike‐slip fault terminations at seismogenic depths: The structure and kinematics of the Glacier Lakes fault, Sierra Nevada United States

Structural complexity is common at the terminations of earthquake surface ruptures; similar deformation may therefore be expected at the end zones of earthquake ruptures at depth. The 8.2 km long Glacier Lakes fault (GLF) in the Sierra Nevada is a left‐lateral strike‐slip fault with a maximum observed displacement of 125 m. Within the fault, pseudotachylytes crosscut cataclasites, showing that displacement on the GLF was accommodated at least partly by seismic slip. The western termination of the GLF is defined by a gradual decrease in the displacement on the main fault, accompanied by a 1.4 km wide zone of secondary faulting in the dilational quadrant of the GLF. The secondary faults splay counterclockwise from the main fault trace forming average angles of 39° with the main fault. Slip vectors defined by slickenlines plunge more steeply west for these splay faults than for the GLF. Static stress transfer modeling shows that the orientations of the splays, and the plunge of displacement on those splays, are consistent with displacement on the main fault. The GLF termination structure shows that structural complexity is present at the terminations of faults at seismogenic depths and therefore ruptures that propagate beyond fault terminations, or through step overs between two faults, will likely interact with complex secondary fault structures. Models of dynamic rupture propagation must account for the effect of preexisting structures on the elastic properties of the host rock. Additionally, aftershock distributions and focal mechanisms may be controlled by the geometry and kinematics of structures present at fault terminations.

Retrieval of source parameters of an event of the 2000 West Bohemia earthquake swarm assuming an anisotropic crust

We propose an inversion scheme for retrieval of characteristics of seismic point sources, which in contrast to common practice, takes into account anisotropy. If anisotropy is neglected during inversion, the moment tensors retrieved from seismic waves generated by sources situated in anisotropic media may be biased. Instead of the moment tensor, the geometry of the source is retrieved directly in our inversion; if necessary, the moment tensor can be then determined from the source geometry aposteriori. The source geometry is defined by the orientation of the slip vector and the fault normal as well as the strength of the event given by the size of the slip and the area of the fault. This approach allows direct interpretation of the source geometry in terms of shear and tensile faulting. It also makes possible to identify volumetric source changes that occur during rupturing. We apply the described algorithm to one event of the 2000 West Bohemia earthquake swarm episode. For inversion we use information of the direct P waves. The structure is approximated by three different models determined from travel-time observations. The models are inhomogeneous isotropic, inhomogeneous anisotropic, and homogeneous anisotropic. For these models we obtain seismic moments MT = 3.2 − 3.8 × 10 14 Nm and left-lateral near-vertical oblique normal faulting on a N-S trending rupture surface. The orientation of the rupture surface is consistent with fault-plane solutions of earlier studies and with the spatial distribution of other events during this swarm. The studied event seems to be accompanied by a small amount of crack opening. The amount of crack opening is slightly reduced when the inhomogeneous anisotropic model is assumed, but it persists. These results and additional independent observations seem to indicate that tensile faulting occurs as a result of high fluid pressure.

The Neogene Veracruz fault: evidences for left-lateral slip along the southern Mexico block

AbstractStructural data combined with analysis of satellite images and seismic profiles show that a major left-lateral strike-slip fault affects the Veracruz basin and post-5 Ma volcanic rocks of the Los Tuxtlas volcanic field (LTVF). The main volcanic alignment of the LTVF is located along this fault. Additional structural data collected in the Trans-Mexican volcanic belt (areas of Xalapa, Teziutlán and Huauchinango) show that the shear zone affects Pliocene Trans-Mexican volcanic rocks. Low seismicity associated to faulted Quaternary markers such as alluvial fans, alluvial terraces and volcanoes argue for active faulting in this area. Plio-Quaternary strike-slip faulting in the Veracruz basin and in the eastern Trans-Mexican volcanic belt is important because it connects two important structural provinces: the left-lateral strike-slip faults province to the south and the left-lateral transtensive faulting that affects the central part of the Trans-Mexican volcanic belt. These three active deformation zones constitute the boundary between the southern Mexico block and the North American plate. It is generally assumed that strike-slip faulting along the Trans-Mexican and Central America volcanic arcs is the result of oblique subduction of the Cocos plate under the North American and Caribbean plates. However slip vectors along the Middle America trench are almost perpendicular to the trench. This Neogene sinistral strike-slip motion could be partially driven by the eastward motion of the Caribbean plate rather than by strain partitioning along the oblique Middle America trench subduction zone.

A graphical–algebraic method for analysing shear zone displacements from observations on arbitrarily oriented outcrop surfaces

We present a practical graphical–algebraic method that enables one to achieve the true displacement and shape of deformed geological markers in fault or shear zones as observed on arbitrarily oriented outcrop surfaces. We use as our natural example deformed quartz veins that have been sheared across brittle–ductile faults in the Southern Alps of New Zealand. The technique is based on the assumption that simple shear has dominated the shear zone formation. For input data we require the strikes and dips of the outcrop surfaces, the offset markers, and the shear zones as well as the pitch of the simple shear vector in the plane of the shear zone. The paper develops a set of algebraic and graphical operations that allow one to convert photographs of faulted or sheared planar markers observed on an arbitrary outcrop surface into an equivalent view that is coincident with the movement ( m) plane of the fault or shear zone. This view displays the true displacement of the offset marker and delineates its deformed shape as seen in a section that is parallel to the slip vector.

A kinematic model for the East African Rift

The kinematics of the East African Rift (EAR) is the least well‐known of all major plate boundaries. Here, we show that present‐day data (a GPS+DORIS geodetic solution and earthquake slip vectors) are consistent with 3.2 Myr‐average spreading rates and transform‐fault azimuths along the Southwest Indian Ridge and support a kinematic model that includes three subplates (Victoria, Rovuma, and Lwandle) between Nubia and Somalia. Continental rifting in the EAR appears to involve localized strain along narrow rift structures that isolate large lithospheric blocks.

A Vision-Based Technique for Vehicle Slip and Velocity Estimation

This paper proposes a novel technique to estimate slips and velocities of an unmanned ground vehicle (UGV). A visual odometry sensor looking down the terrain surface is employed to measure the motion of the UGV, by tracking features selected from the terrain surface. The visual odometry sensor can provide motion information even when the terrain surface contains no distinctive features. A sliding mode observer (SMO) based-on a kinematic model is designed to deal with noise and uncertainty of the measurements from the visual odometry sensor, and simultaneously estimate the slip and velocity vectors of the UGV. The non-GPS slip and velocity estimation technique is independent of terrain parameters and robust to noise and uncertainty. Experimental results are given to show that the technique has good potential for vehicle slip and velocity estimation.

Quaternary temporal stability of a regional strike-slip and rift fault intersection

Intersecting strike-slip and normal fault systems form either two or three plate configurations. In circumstances where they intersect to form a triple junction, internal block deformation produces a quasi-stable 3-D configuration permitting maintenance of both the regional geometry and kinematics of the intersection. This paper examines the temporal stability of such a triple junction in New Zealand over a range of timescales, from individual earthquakes to millions of years, and discusses the factors that impact its stability. Using seismic-reflection, gravity, drill-hole and outcrop data, the accumulation of vertical displacements is examined, both on individual faults and across the intersection zone between the strike-slip North Island Fault System (NIFS) and the Taupo Rift. Fault-throws and throw rates for the NIFS and Taupo Rift suggest that despite a three-fold increase in throw rates in both fault systems at about 0.3 Ma, which may have modified the orientation of the slip vectors and therefore the contemporary kinematics of the triple junction, the intersection of the two fault systems has functioned coherently for the last 0.6–1.5 Myr accumulating vertical displacements interdependently since the early stages of rifting. The apparent arrest of a large magnitude earthquake on the rift-bounding fault adjacent to the intersection, suggests that the short-term dynamic behaviour of the system is relatively complex compared to the more coherent long-term kinematics.

Trishear in 3D. Algorithms, implementation, and limitations

The algorithms and implementation of pseudo-3D and true-3D trishear models are explained, including a strategy to model lateral fault propagation. I show that the pseudo-3D algorithm is adequate and sufficient to model trishear in three-dimensions. Although ad-hoc, the pseudo-3D algorithm preserves volume in simulations without and with lateral fault propagation. A disadvantage of the pseudo-3D algorithm is that it produces very simple, and perhaps not realistic hanging wall geometries, specially in simulations in which the fault slip varies along strike. The true-3D algorithm has a more elaborate and richer kinematics that produces more realistic hanging wall geometries. However, the true-3D algorithm contains mathematical inconsistencies that result in considerable volume changes when the slip gradients along the tip line are high and the tip line is highly oblique to the slip vector and/or the fault strike. The volume changes occur to a large extent in the hanging wall, and to a minor extent in the forelimb and footwall areas.

Along-strike variations of the slip direction on normal faults: Insights from three-dimensional finite-element models

Normal faults in nature exhibit a systematic variation of the slip direction along strike, with pure dip-slip at their centres that changes gradually to oblique slip near their tips. Here we evaluate the variation of the slip direction along normal faults by three-dimensional finite-element modelling. The model results reveal a nearly linear increase of the strike-slip component over two-thirds of the distance from the fault centre to its tips, which results in an increasing obliqueness of the slip vector along strike. Excluding the fault tips, the relationship between lateral slip and fault length can be approximated by a power-law function, which shows that the strike-slip component is proportional to the dip-slip and to the fault dip. The development of the sinistral and dextral slip components is caused by the orientation of the minimum principal stress, which is deflected toward the fault centre in both footwall and hanging wall. A comparison with two faults in the Central Apennines, Italy, shows good agreement between modelled and measured slip vectors. The occurrence of converging slip patterns implies that caution is advised when inferring oblique extension, regional stress trajectories and palaeostress fields from slip vectors measured away from the fault centre.

Scotia arc kinematics from GPS geodesy

GPS crustal velocity data from the Scotia and South Sandwich plates, transform azimuths, spreading data, and an updated earthquake slip vector catalog provide the first Scotia and South Sandwich plate Euler vector estimates not dependent on closure as the GPS data tie them to the global plate circuit. Neither the GPS data, which sample limited portions of the plates, nor the geologic data, which are not tied to the global spreading circuit, are sufficient individually to define the Euler vectors. As Scotia plate GPS measurements do not sample the stable plate interior, plate boundary deformation field modeling is necessary for Euler vector estimation. Our South America‐Antarctic and Scotia‐South Sandwich Euler pole estimates agree with previous estimates from either GPS or geologic data. Our South America‐Scotia Euler vector, however, is significantly different and near the South America‐Antarctic Euler vector producing an approximately coaxial motion of Scotia between South America and Antarctica.

Parameter Inversion of the 1997 Mani Earthquake from Insar Co‐Seismic Deformation Field Based on Linear Elastic Dislocation Model–I. Uniform Slip Inversion

AbstractThe Nov. 8, 1997 Mani (Xizang) Mw7.5 Earthquake is an important event since Synthetic Aperture Radar Interferometry was used for earthquake studies. Peltzer et al. argued that the deformation field indicated the nonlinear elastic response of crust with a ratio of 2 between compressive and tensile elastic moduli. No other similar case appeared in co‐seismic deformation field. Generally, linear elastic theory can give satisfactory explanation to the observation. Considering their excessively simplified fault model and the pure strike‐slip assumption, we deem that nonlinear elasticity explanation is far‐fetched. By using the widely‐used Okada linear elastic dislocation model and the fault location on the surface from satellite observations, removing the zero dip‐slip constraint, we seek best‐fit fault geometry parameters and uniform slip vectors from radar observation based on nonlinear optimization inversion. The result shows that linear elastic model can satisfactorily explain the observation. The maximum depth of the fault reaches 16.5 km at the segment near the east side of Chaoyang Lake. The depth decreases gradually when the fault extending to the east and the west; inverted fault dipping direction and dip‐slip are consistent with field investigation; the maximum strike‐slip component reaches 4.44 m, and the dip‐slip component also reaches its maximum 1.1 m at this segment, which can not be ignored in the inversion; the total seismic moment is 2.08×1020 N·m from inversion, which equals to a Mw7.5 earthquake. The seismic moment is close to the 2.23×1020 N·m of Harvard solution and the 1.8×1020 N·m of Velasco's inversion.

Fault locking, block rotation and crustal deformation in the Pacific Northwest

SummaryWe interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4–1.0 ° Myr−1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10–15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material—the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca—North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr−1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.

The 2004 May 28 Baladeh earthquake (Mw6.2) in the Alborz, Iran: overthrusting the South Caspian Basin margin, partitioning of oblique convergence and the seismic hazard of Tehran

We use teleseismic waveform analysis and locally recorded aftershock data to investigate the source processes of the 2004 Baladeh earthquake, which is the only substantial earthquake to have occurred in the central Alborz mountains of Iran in the modern instrumental era. The earthquake involved slip at 10–30 km depth, with a south-dipping aftershock zone also restricted to the range 10–30 km, which is unusually deep for Iran. These observations are consistent with co-seismic slip on a south-dipping thrust that projects to the surface at the sharp topographic front on the north side of the Alborz. This line is often called the Khazar Fault, and is assumed to be a south-dipping thrust which bounds the north side of the Alborz range and the south side of the South Caspian Basin, though its actual structure and significance are not well understood. The lack of shallower aftershocks may be due to the thick pile of saturated, overpressured sediments in the South Caspian basin that are being overthrust by the Alborz. A well-determined earthquake slip vector, in a direction different from the overall shortening direction across the range determined by GPS, confirms a spatial separation ('partitioning') of left-lateral strike-slip and thrust faulting in the Alborz. These strike-slip and thrust fault systems do not intersect within the seismogenic layer on the north side, though they may do so on the south. The earthquake affected the capital, Tehran, and reveals a seismic threat posed by earthquakes north of the Alborz, located on south-dipping thrusts, as well as by earthquakes on the south side of the range, closer to the city.

Magnetic fabrics and microstructures of the Jurassic Shah-Kuh granite pluton (Lut Block, Eastern Iran) and geodynamic inference

The Jurassic Shah-Kuh granite pluton was emplaced in the northeastern part of the Lut Block (Eastern Iran) while this block was a part of the active margin under which the Tethys ocean, that separated Arabia from Central Iran, was subducting. Since this time, the Lut Block has rotated and migrated northward up to its present position. This structural study of the Shah-Kuh aims at strengthening the evidence of its original geodynamical location. Thanks to a systematic collection of oriented samples in the field, well-defined fabric and microstructural patterns were deduced from magnetic fabric measurements and optical microscopy observations. The overall magnetic fabric of the pluton yields vertical, north–south striking foliations and shallow lineations plunging to the north. The corresponding microstructures attest to their formation in the magmatic stage, i.e. during emplacement. Subsequent deformation, characterized by (sub)mylonitic microstructures, has modified the original fabric into a northwest and locally a west–northwest trend inside a two kilometre-wide corridor. This corridor reflects the trace of a sinistral shear zone that offsets the previous north–south magmatic pattern of the pluton. The latter magmatic pattern is proposed to result from the shear component of strain that was parallel to the ancient active margin during pluton emplacement, as a result of strain partitioning, a situation frequently documented at convergent margins. Sinistral shear along an ∼ E–W directed south-facing active margin, assuming a northeastward slip vector for the Tethys ocean during the Jurassic, and accepting a ∼ 150° counterclockwise rotation of the Lut Block after emplacement of the Shah-Kuh pluton, best explains both the magmatic and the solid-state lineation patterns.

The source motion of 2003 Bam (Iran) earthquake constrained by satellite and ground-based geodetic data

SUMMARY The interpretation of coseismic surface deformation measurements through inversion techniques is of major importance to understand the mechanical behaviour of a seismic fault. Dense geodetic data sets in the vicinity of the ruptured fault provide unique constraints on detailed fault geometry and slip distribution at depth, making them complementary to seismological data. Bam earthquake (Mw 6.6, 2003 December 26) induced surface deformation has been precisely mapped by Envisat ASAR interferometry and by subpixel correlation techniques applied to Spot-5 and ASAR amplitude images. These oblique and horizontal estimations of deformation have been completed with one levelling profile along the main road crossing the rupture from west to east. We process these data (separately and jointly) in a two-step inversion technique, within the elastic half-space theory framework. Our objective is to determine the dislocation model at depth that satisfies simultaneously all the geodetic constraints. Also, we estimate the relative contribution of each geodetic data set to this inversion process. We first use a stochastic direct approach called neighbourhood algorithm in order to estimate the average characteristics of the rupture, and their relative uncertainty. Constraining in this way the geometry of the ruptured fault, we then linearize the inverse problem and compute the slip distribution on the fault using a standard weighted least-square technique, assuming the solution is smooth to some degree. At each step, we discuss the optimal models, their stability as well as the relative influence of each data set on the derived models parameters. Our preferred model reveals a shallow dislocation on a quasi-vertical fault, slightly dipping towards east. The slip vector has a strike-slip component as high as 2 m, while the dip-slip component seems negligible. However, the estimation of the resolution matrices emphasizes the fact that the details of deep fault slip distribution remain out of the scope of this ill-conditioned inverse problem. Yet, our preferred model suggests a main dislocation limited at depth between 1 and 6 km. By contrast, the aftershocks observed in the months following the earthquake are located just beneath the estimated main shock.

Comment on ‘Deformation of the NE Basin and Range Province: the response of the lithosphere to the Yellowstone plume?’ by R. Westaway

(i) Westaway assumes that a general horizontal deformation in the crustal layer, accommodated on a single set of parallel faults with parallel slip vectors, can closely correspond with that in an underlying fluid which is deforming with a horizontal uniaxial strain rate tensor. This is incorrect. The bulk horizontal deformation in the brittle crust, accommodated on a single set of straight and parallel faults with parallel slip vectors, always has a biaxial strain rate tensor, except in the special case when there is no strike-slip component on the faults. This is easy to show, because such deformation can always be divided into a component of simple shear parallel to the faults, with a biaxial strain rate tensor, uniaxial extension or compression perpendicular to the faults, and a rigid-body rotation about a vertical axis of the whole fault system. Therefore, except when there is no strike-slip motion on the faults, the bulk deformation must have a biaxial strain rate tensor. Unfortunately, Westaway’s theory specifically deals with the case when there is strike-slip motion. (ii) Westaway derives a general relation between the orientation of fault slip vectors and the asymmetrical component of the velocity gradient tensor [equation (2.19) in Westaway (1989)]. This is incorrect. The relative motion between parallel fault blocks can only give information about internal changes in shape, which are described by the strain rate tensor (symmetrical component of velocity gradient tensor). Information about rotation of the whole fault system about a vertical axis is contained in the asymmetrical part of the velocity gradient tensor and can only be determined relative to an external frame of reference. Jackson & McKenzie (1988) have dealt with this important point in some detail. However, if it is assumed that a material line in a particular direction in the deforming mne does not rotate with respect to some frame of reference, then it is possible to determine the asymmetrical component in this frame of reference. Thus, McKenzie & Jackson (1983) assumed that a line parallel to the length of the zone of deformation did not rotate relative to the margins, and calculated the asymmetrical component of the velocity gradient tensor in this frame of reference. Westaway makes no such assumptions. His relation between the vorticity and extensional strain rate is invalid in all frames of reference except one where the vorticity is zero. In this case, his relation predicts correctly that there will be no strike-slip component on the faults. (iii) Westaway asserts that a set of long fault blocks will always rotate about vertical axes at a rate equal to half the vorticity of the velocity gradient tensor of the underlying continuously deforming fluid. This is incorrect. There are always two possible cases when the motions of long straight fault blocks will correspond closely with those of an underlying fluid deforming with constant velocity gradients: (a) when the faults are parallel to the length of a zone of deformation with no velocity gradients along its length, relative to the margins, in which case the fault blocks will not rotate about vertical axes but translate relative to the margins of the zone; and (b) when the faults are perpendicular to the azimuth of the relative velocity vectors in the deforming zone, in which case the blocks will translate relative to h c h other and rotate about vertical axes relative to the margins of the zone at a rate equal to the vorticity of the velocity gradient tensor.

Discussion of and correction to: Deformation of the NE Basin and Range Province: the response of the lithosphere to the Yellowstone plume?

This article corrects and further develops theory that is necessary in order to relate slip sense on faults in the upper crustal brittle layer to continuous plastic deformation of the underlying uppermost mantle. A reasonable starting point in order to do this, but which some previous theoretical models for continental deformation fail to satisfy, requires that (in the absence of external forces acting on the brittle layer or the underlying plastically deforming material) velocity in both layers must match overall as closely as possible. In a region extending by pure shear only, fault slip vector azimuth gives the extension direction beneath the brittle layer, which equals the azimuth of the extensional eigenvector of the strain rate tensor beneath the brittle layer. In a region that takes up distributed simple shear on parallel faults that do not rotate around vertical axes, and extends by pure shear in the direction perpendicular to the simple shear, neither the eigenvectors of the strain rate tensor nor the slip vector azimuth on the faults gives the true extension direction. However, individual faults in these two types of region may appear the same, and unless one knows or infers independently the overall deformation sense in a region one cannot unambiguously make use of fault slip sense information from it. Earlier conclusions that tectonic deformation in the NE Basin and Range province of the western United States is driven by flow in the Yellowstone upwelling plume are confirmed by these additional investigations.

Impact of riser reconstructions on estimation of secular variation in rates of strike–slip faulting: Revisiting the Cherchen River site along the Altyn Tagh Fault, NW China

Slip rates are heavily contested for many of the first-order strike–slip faults within the Indo–Asian collision zone. Rates determined geodetically are generally lower than those reported using reconstructions of offset landforms, and it is unclear if this discrepancy reflects true secular variation in slip history, systematic errors in interpretation, or both. Here I examine the methods used to derive slip rates from reconstructions of displaced fluvial risers, and show that such rates are subject to potentially important uncertainties that have largely been overlooked. Review of published data demonstrates that the slip rate can vary by a factor of 1.2 to 5 for the same site, depending on whether the reconstruction uses the age of the upper or lower terrace for the initiation of riser offset. To reduce this epistemic uncertainty, I have developed six geomorphic indices that can be used to identify the most accurate end-member reconstruction: (1) comparison of riser offset with inset channel width; (2) similarity of riser and tread displacements; (3) morphological analysis of the scarp profile; (4) riser deflection in plan view; (5) diachroneity of terrace abandonment; and (6) whether riser crests or bases yield the slip vector. Without the use of these geomorphic observations the epistemic uncertainty associated with current methods for determining slip rate from offset fluvial risers is likely to be so large that there is no resolvable discrepancy between the GPS and geologic rates within the Indo–Asian collision zone. Application of three of these indices to new field mapping of the Cherchen He (River) Site (∼ 86.4°E) along the central Altyn Tagh Fault yields a revised slip rate of 9.4 ± 2.3 mm/yr, equivalent within error to the rates previously derived from GPS and paleoseismic studies. Although reinterpretation of published data from the Sulamu Tagh site (∼ 87.4°) is consistent with this revised rate, additional work is needed to reconcile this result with some of the rates reported near Aksay (∼ 94°E).

Terminations of large strike-slip faults: an alternative model from New Zealand

Abstract The 500-km-long strike-slip North Island Fault System (NIFS) intersects and terminates against the Taupo Rift. Both fault systems are active, with strike-slip displacement transferred into the rift without displacing normal faults along the rift margin. Data from displaced landforms, fault-trenching, gravity and seismic-reflection profiles, and aerial photograph analysis suggest that within 150 km of the northern termination of the NIFS, the main faults in the strike-slip fault system bend through 25°, splay into five principal strands and decrease their mean dip. These changes in fault geometry are accompanied by a gradual steepening of the pitch of the slip vectors, and by an anticlockwise swing (up to 50°) in the azimuth of slip on the faults in the NIFS. As a consequence of the bending of the strike-slip faults and the changes in their slip vectors, near their intersection, the slip vectors on the two component fault systems become subparallel to each other and to their mutual line of intersection. This subparallelism facilitates the transfer of displacement from one fault system to the other, accounting for a significant amount of the NE increase of extension along the rift, whilst maintaining the overall coherence of the strike-slip termination. Changes in the slip vectors of the strike-slip faults arise from the superimposition of rift-orthogonal differential extension outside the rift margin, resulting in differential motion of the footwall and hanging-wall blocks of each fault in the NIFS. The combination of rift-orthogonal heterogeneous extension (dip-slip) and strike-slip, results in a steepening of the pitch of the slip vectors on the terminating fault system. Slip vectors on each splay close to their terminations are, therefore, the sum of strike-slip and dip-slip components, with the total angle through which the pitch of the slip vectors steepens being dependent on the relative values of both these two component vectors. In circumstances where interaction of the velocity fields for the intersecting fault systems cannot resolve to a slip vector that is boundary-coherent, either rotation about vertical axes of the terminating fault relative to the through-going fault system may take place to accommodate the termination of the strike-slip fault system, or the rift may be offset by the strike-slip fault system rather than terminating into it. At the termination of the NIFS, an earlier phase of such rotations may have produced the 25° anticlockwise bend in fault strike and contributed up to about one-third of the anticlockwise deflection in slip azimuth. On the terminating strike-slip NIFS, therefore, rotational and non-rotational termination mechanisms have both played a role, but at different times in its evolution, as the thermal structure, the rheology and the thickness of the crust in the rift intersection region have changed.

Fault orientation variations along the strike of active normal fault systems in Italy and Greece: Implications for predicting the orientations of subseismic-resolution faults in hydrocarbon reservoirs

The orientations of subseismic-scale faults vary systematically along the strike of basin-bounding, active normal faults in Italy and Greece. A total of 16,272 outcrop measurements of the strike and dip of fault planes and their associated slip vectors indicate oblique slip near the lateral fault tips of the normal faults; subseismic faults vary in orientation to accommodate this slip. If viewed in map view from the footwall looking toward the hanging wall, right-lateral slip occurs at the left end of the faults, with left-lateral slip at the right end. Overall, slip vectors converge toward the hanging wall, with intersection lines for faults accommodating this slip parallel to the slip vector. If the same faulting style applies elsewhere, the empirical relationships presented herein can be used to calibrate results from mechanical fault models and provide a pragmatic guide to the orientations of normal-fault-bound compartments of rock in subsurface hydrocarbon reservoirs, given that the offsets across such faults may be below seismic resolution and not amenable to direct measurement.