Local Stress Orientation Research Articles

A 3D Full Stress Tensor Model for Oklahoma

AbstractThe stress tensor is an important property for upper crustal studies such as those that involve pore fluids and earthquake hazards. At tectonic plate scale, plate boundary forces and mantle convection are the primary drivers of the stress field. In many local settings (10–100 s of km and <10 km depth) in tectonic plate interiors, we can simplify by assuming a constant background stress field that is perturbed by local heterogeneity in density and elasticity. Local stress orientation and sometimes magnitude can be estimated from earthquake and borehole‐based observations when available. Modeling of the local stress field often involves interpolating sparse observations. We present a new method to estimate the 3D stress field in the upper crust and demonstrate it for Oklahoma. We created a 3D material model by inverting multiple types of geophysical observations simultaneously. Integrating surface‐wave dispersion, local travel times and gravity observations produces a model of P‐wave velocity, S‐wave velocity, and density. The stress field can then be modeled using finite element simulations. The simulations are performed using our simplified view of the local stress field as the sum of a constant background stress field that is perturbed by local density and elasticity heterogeneity and gravitational body forces. An orientation of N82°E, for the maximum compressive tectonic force, best agrees with previously observed stress orientations and faulting types in Oklahoma. The gravitational contribution of the horizontal stress field has a magnitude comparable to the tectonic contribution for the upper 5 km of the subsurface.

Characterizing Stress Orientations in Southern Kansas

ABSTRACT Induced seismicity predominantly occurs along faults that are optimally oriented to the local principal compressive stress direction, and the characterization of these stress orientations is an important component of understanding seismic hazards. The seismicity rate in southern Kansas rapidly increased in 2013 primarily due to the disposal of large volumes of wastewater into the Arbuckle Group. Previously, local stress orientations in this area were poorly constrained, which limited our understanding of the complex faulting and diverse earthquake mechanisms in this region. We use shear-wave splitting and focal mechanism inversion techniques to create multiple, independent estimates of maximum horizontal stress directions (SHmax) and their spatial variation across the study area. We then create an integrated model of stress orientations for southern Kansas and northern Oklahoma using our local results in conjunction with previous, regional stress orientation estimates. We find that SHmax in both southern Kansas and central Oklahoma exhibits an east-northeast (∼N78° E) orientation, and these regions bound a northeast (∼N59° E) rotation within a ∼20 km area in northern Oklahoma near the Nemaha ridge.

Tomographic imaging of the NW edge of the Hellenic volcanic arc

The broader South Aegean area is characterized by the convergence of the African and Eurasian lithospheric tectonic plates at a rate of approximately 30 mm/year, resulting in a complex and intense deformation of the Aegean (McClusky et al. Geophys. Res. 105: 5695–5719, 2000). The Saronikos Gulf, situated in the NW termination of the Hellenic volcanic arc (Central Aegean Sea), is an area of special geophysical interest due to the variety of the recorded seismicity, regarding both the origin (tectonic, volcanic) and the focal depths. The presence of Plio-Quaternary volcanic centers and the complex seismicity distribution urge the need of detailed research on the geophysical properties such as seismic velocities, revealing possible presence of magmatic bodies or geothermal fluids. In the present study, more than 3000 manually revised events were located using a local 1-D layered velocity model. The main spatiotemporal clusters are concentrated along the Leuces and Methana-Poros E-W striking neotectonic fault zones. A local earthquake tomography (LET) was performed, using body-wave (P and S) travel time data, to investigate small to medium scale (~ 10 km) anomalies that can be related to local neotectonic or volcanic patterns. Major shallow ~ E-W to NW-SE trending discontinuities between positive (to the north) and negative (to the south) VP perturbations, anticorrelated to the respective VS image, are identified south of Aigina Island. These results are mainly related to the principal local stress orientation and fault zones (~ E-W to NW-SE strike), such as the Moni, Anghistri, and Leuces, whereas the respective Vp/Vs ratio values imply the presence of possible magmatic features which can be linked to the Plio-Quaternary NW edge of the Hellenic volcanic arc and hydrothermal activity ENE of Poros Island.

Arcuate stress state in accretionary prisms from real-scale numerical sandbox experiments

The stress states in accretionary prisms are important for understanding the building and releasing of seismic energy. Numerous researchers have conducted sandbox experiments as a scaled physical analog model to understand the formation of accretionary prisms. However, measuring stress states in laboratory sandbox experiments is still practically infeasible. Here we performed real-scale numerical sandbox experiments using the discrete element method to understand the 3D stress state in the accretionary prism. Despite the nearly uniform initial conditions, macro-scale undulations of faults, which are similar to those observed in the trenches of an accretionary prism, appear. We reveal that these undulations are caused by the formation of stress arches. We show that the mechanism behind the arch formation is the discontinuous change in the stress orientation during the rearrangement of the stress chain. Furthermore, analyses demonstrate that the in-situ stress orientation from borehole data can be a signal of either the regional direction of plate convergence or the local stress orientation associated with the stress arch. The results may greatly enhance the outcome of long term monitoring in areas, such as the Nankai Trough.

Shear-Wave Splitting and Crustal Anisotropy in the Shillong–Mikir Plateau of Northeast India

Seismic anisotropy of crust beneath the Shillong–Mikir Plateau and the surrounding regions of northeast India have been investigated with the help of splitting analysis of S-wave of local earthquakes. We estimate a total 83 pairs of splitting parameters (Φ and δt) from 67 local shallow focus earthquakes (depth ≤ 32 km) recorded by the 10 broadband seismological stations operated in the study region. The results show delay times ranging from 0.02 to 0.2 s, which correspond to anisotropy up to 4%, suggesting significant strength of anisotropy in the study region. Fast polarization direction (Φ) in the Shillong Plateau shows mostly NW–SE trend in the western part and NE–SW trend in the northern part. Φs near Kopili fault (KF) follows NW–SE trend. Φ at most of the stations in the study region is consistent with the local stress orientation, suggesting that the anisotropy is mainly caused by preferentially aligned cracks responding to the stress field. On the other hand, anisotropy observed near the KF is due to aligned macroscopic fracture related to strike-slip movement in the fault zone.

Quantifying the heterogeneity of the tectonic stress field using borehole data

AbstractThe heterogeneity of the tectonic stress field is a fundamental property which influences earthquake dynamics and subsurface engineering. Self‐similar scaling of stress heterogeneities is frequently assumed to explain characteristics of earthquakes such as the magnitude‐frequency relation. However, observational evidence for such scaling of the stress field heterogeneity is scarce. We analyze the local stress orientations using image logs of two closely spaced boreholes in the Coso Geothermal Field with subvertical and deviated trajectories, respectively, each spanning about 2 km in depth. Both the mean and the standard deviation of stress orientation indicators (borehole breakouts, drilling‐induced fractures, and petal‐centerline fractures) determined from each borehole agree to the limit of the resolution of our method although measurements at specific depths may not. We find that the standard deviation in these boreholes strongly depends on the interval length analyzed, generally increasing up to a wellbore log length of about 600 m and constant for longer intervals. We find the same behavior in global data from the World Stress Map. This suggests that the standard deviation of stress indicators characterizes the heterogeneity of the tectonic stress field rather than the quality of the stress measurement. A large standard deviation of a stress measurement might be an expression of strong crustal heterogeneity rather than of an unreliable stress determination. Robust characterization of stress heterogeneity requires logs that sample stress indicators along a representative sample volume of at least 1 km.

Field-Scale Deformation Analysis of Cyclic Solvent Stimulation in Thin Unconsolidated Heavy-Oil Reservoirs With Developed Wormhole Network

Summary With the current interest in exploiting thin unconsolidated heavy-oil reservoirs, cold heavy-oil production with sand (CHOPS) can be considered to be a promising primary-recovery technique. However, it offers low recovery factors (~5-15%) and creates a complex network of high-permeability channels known as workholes, while simultaneously changing formation compressibility and in-situ stress conditions. Eventually, development of such a network would lead to a softer layer within the shallow unconsolidated reservoir, which can carry less of the overburden stress. Further development of such reservoirs is usually achieved through enhanced-oil-recovery (EOR) applications in the form of cyclic solvent stimulation within an economic framework. Cyclic loading and unloading of this EOR technique make it even more difficult to predict the recovery performance. In this study, a 3D geomechanical model was used to calculate the stress distribution in a history-matched field in Alberta with 15 producers. The fractal patterns generated from the diffusion-limited-aggregation (DLA) algorithm were used to represent the wormhole network with different strength properties. Foamy behaviour of heavy oil was modelled with the help of a set of kinetic reactions. The hydro-geomechanical model was then used for field-development planning, reservoir management, and assessment of near-wellbore regions during cyclic injection and production. The fieldwide deformation and stress changes were analyzed in deep overburden, caprock, and reservoir to show the influence of local stress orientations in soft and stiff layers. Next, we considered a sector model with a single well. The model introduced was used to assess EOR processes with different solvent streams. While light hydrocarbon components help to repressurize the formation, the heavier components seem more effective in heavy-oil dilution. This occurred while stress arching redistributed the cyclic injection/production-induced stresses to flow around soft inclusions. Although it is difficult to obtain data to calibrate the 3D hydro-geomechanical model, it allows for reliable investigation of reservoir performance and provides deeper insight than does the use of flow simulation alone. The assessment of the potential of geomechanics can ascertain whether a more-detailed modelling is necessary.

Upper crustal azimuthal anisotropy across the contiguous U.S. determined by Rayleigh wave ellipticity

AbstractConstraints on upper crustal seismic anisotropy provide insight into the local stress orientation and structural fabric, but such constraints are scarce except in areas with dense recordings of local seismicity. We investigate directionally dependent Rayleigh wave ellipticity, or Rayleigh wave H/V (horizontal to vertical) amplitude ratios, between 8 and 20 s period across USArray to infer azimuthal anisotropy in the upper crust across the contiguous U.S. To determine the H/V ratios, we use all available multicomponent ambient noise cross correlations between all USArray stations operating between 2007 and 2013. In many locations, the observed H/V ratios are clearly back azimuth dependent with a 180° periodicity, which allows the fast directions and amplitudes of upper crustal anisotropy to be determined. The observed patterns of anisotropy correlate well with both near‐surface geological features (e.g., the Intermountain Seismic Belt and Appalachian‐Ouachita collision belt) and a previous stress model.

Complex fault structure interactions of crustal shear zones revealed by seismic anisotropy: an example in the eastern betic cordillera (Spain)

Terra Nova, 25, 57–64, 2013AbstractCharacterizing the interaction of a fault with its surroundings is vital to fully understand the tectonic processes involved and predict future behaviour. Regional and local stress orientations affect different fracture length scales, manifested by numerous associated fault, fracture and crack structures. We use seismic anisotropy to constrain the dominant orientation of aligned rupture planes of various length scales. In particular, we study shear‐wave splitting of regional seismic events in Trans‐Alboran Shear Zone (TASZ), south‐east Spain. The TASZ consists of three major left‐lateral strike‐slip faults and numerous secondary strike‐slip and thrust faults. The observed orientations for S‐waves vary from roughly N–S in the northern segment of TASZ, to E–W in the centre, to NNW–SSE and NNE–SSW in the south. We show that the strikes of fast polarizations reflect both structural and lithological differences, indicating complex interactions of principal and secondary faults within the crust to accommodating tectonic stresses.

Comparison between curvature and 3D strain analysis methods for fracture predicting in the Gachsaran oil field (Iran)

AbstractMost carbonate fractured reservoirs display complex behaviour in the simulation and production stages of their development, and this complexity is thought to be the result of the different fracture distributions and intensities within the reservoir. Accurate fracture characterization is therefore essential and the two techniques most commonly used for fracture prediction are ‘strain analysis’ and ‘curvature analysis’. In this paper these two methods of fracture analysis are compared by applying them to the Gachsaran oil field in the Zagros folded belt and comparing the predictions of the two with the performance history of the reservoir. This reservoir is well suited for such a study as there is a large quantity of seismic data and over 350 wells have been drilled. Fracture intensity indicator maps have been produced using both methods and the results compared with production index data from the wells. The indicator map produced using the 3D strain analysis method in which special attention was given to the structural setting, structural evolution and the position of the fractures with respect to the local stress orientation, was found to be more compatible with the production index data than the map produced using the method of curvature analysis. In addition, the study also demonstrates that one of the great advantages of strain analysis compared to the curvature method is its ability to predict variations in the vertical direction and thus provide data related to a reservoir volume rather than simply to a surface.

Analysis and forward modeling of seismic anisotropy during the ongoing eruption of the Soufrière Hills Volcano, Montserrat, 1996–2007

[1] Volcanic stress field analysis has been lauded as a potentially powerful tool for midterm to long-term eruption forecasting. However, because tectonic processes can also produce localized stress field reorientations, evidence for a direct causal link between observed stress field reorientations and magmatic activity is of critical importance. In this study, we show that local stress field reorientations preceding changes in volcanic activity at the Soufrière Hills Volcano, Montserrat, are observable using two independent measures of crustal stress (local (volcano-tectonic) earthquake fault plane solutions and measurements of shear wave splitting in regional earthquakes). We further demonstrate that the local stress field orientation during a 6 month period preceding the onset of eruptive activity at Soufrière Hills in 1999 is highly localized and spatiotemporally variable and that the spatial pattern of precursory local stress orientations is consistent with numerically modeled patterns of stress resulting from pressurization of a vertical dike. These observations provide compelling evidence for a direct causal link between pressurization of midlevel volcanic conduit systems by ascending magma and precursory local stress field reorientations and demonstrate that seismological analysis can be used to detect subtle local changes in stress that herald eruptive activity.

Neogene to Quaternary stress field evolution in Lesser Caucasus and adjacent regions using fault kinematics analysis and volcanic cluster data

In the Great Caucasus, the Lesser Caucasus and Eastern Turkey, the distribution of Neogene to Quaternary volcanic cluster geometries, paleo-stress field data of the Lesser Caucasus area (Republic of Armenia) and the P axes of earthquakes focal mechanisms show the scale and time variability of the stress field since the beginning of the Arabia-Eurasian collision. In addition to the general N-S compression orientation, two other NW-SE and NE-SW secondary orientations are observed. Both orientations were successively significant for some period of tectonic activity. The first one was dominant between the Paleogene and the end of the Lower Miocene and the second one has prevailed between the Upper Miocene and the Quaternary. On a regional scale the principal stress axes orientations are mainly controlled by the Arabian-Eurasian plate convergence and have changed with time. Local stress orientations have been significantly influenced by secondary blocks motions and their geometries.

Pre-drilling prediction of the tectonic stress field with geomechanical models

Knowledge of the tectonic stress field in a reservoir is essential to optimize drilling and production. Borehole stability, orientations of natural and hydraulically induced fractures, fluid flow anisotropies, among others, all depend critically on the present-day stress distribution. Several techniques ranging from dipmeter analysis of borehole breakouts to anelastic strain recovery and shear acoustic anisotropy analysis of core samples (e.g., Yale, 2003; Sperner et al., 2003) can be used to determine the in-situ stress orientations and relative magnitudes, but obviously this valuable information will only become available after the well has already been drilled. However, there are also numerous cases where the stress orientation should be known prior to drilling. For example, if multiple fracs in a horizontal well are planned, the stress field orientation needs to be known beforehand because for optimal frac design the horizontal well path must be aligned parallel to the orientation of the least principal stress axis σ3. Similarly, the planning of well trajectories with respect to borehole stability as well as the design of secondary and tertiary recovery measures (e.g., water injection, hydraulic fracture treatments) are significantly improved by a pre-drilling knowledge of the subsurface stress field. Information on the regional stress orientations can be derived from large-scale data collections like, for example, the world stress map project (Zoback, 1992; Sperner et al., 2003). The orientation and magnitude of the stress field in sedimentary basins, however, can be highly variable and, particularly near faults, the local stress orientations can differ by up to 90° from the regional trend (e.g., Yale, 2003). In such cases, inference of reservoir-scale in-situ stress orientations from regional scale maps would inevitably lead to an incorrect pre-drilling prediction. This paper uses a numerical modelling approach to determine the magnitude and orientation of the tectonic stresses in a reservoir and, particularly, the local stress perturbations near faults. The model is based on reservoir and fault geometries taken from seismic data and boundary conditions representing the regional stress field. Thus, this tool is also applicable to cases where well data are absent. Following a brief outline of the modelling approach, a case study is presented to assess the practical value of such geomechanical models for the pre-drilling prediction of the tectonic stress field in fault-controlled reservoirs.

Control on seafloor spreading geometries by stress- and strain-induced lithospheric weakening

Seafloor spreading typically occurs along ridge segments oriented at right angles to plate motion and offset by orthogonal transform faults. Few regions exhibit different patterns, such as the East Pacific Rise (EPR), which additionally displays overlapping spreading centres (OSCs) and microplates. We introduce a dynamical model using two independent, scalar types of damage in an elastic plate that generates most observed spreading geometries as natural failure modes, suggesting that the dynamics of the underlying mantle have only a minor influence on accretionary plate margins. The elastic modulus that is affected by the damage determines the type of localized deformation. Damage reducing the bulk modulus tends to result in tensile fractures, while a reduction in shear modulus leads to shear fractures. The damage source determines the fracture orientation. Material weakening in tension results in fractures perpendicular to the most tensile principal stress, while shear weakening results in two conjugate fractures at 45° relative to the applied stress. Strain or energy-dependent damage results in propagating, localized fractures. Stress-dependent damage tends to evolve into diffuse regions that may eventually focus into narrow zones. Starting from small perturbations with reduced elastic moduli as nucleation points, all ridge geometries start with ridge propagation caused by tensile energy reducing both elastic moduli by a model of damage caused by tensile energy reducing both moduli. Orthogonal transform faults develop in regions between offset segments if there is an additional reduction in shear modulus due to shear stress. The orthogonality of the transform faults does not derive from the local stress orientation but from the dynamics of damage focusing which causes the fault to converge towards an optimal geometry that concentrates nearly all deformation into damaged zones. OSCs form when the shear damage leading to transform faults is suppressed, while microplate formation requires additional reduction of the shear modulus by tensile energy. Oblique spreading at 45°, recently discovered along ultraslow spreading ridges, occurs when both moduli are weakened by shear energy. A parameter regime exists in which ridge–transform patterns develop at low applied tension, while microplates form at higher stresses. These results indicate that at least three different micromechanical processes operate with different evolution rates. OSCs and oblique spreading require different material properties.

Spatial heterogeneity of tectonic stress and friction in the crust

The complex geometry of faults, seismicity, and diversity of earthquake mechanisms suggest that the stress and strength in Earth's crust are spatially heterogeneous. We investigated the degree of heterogeneity using the following two end‐member models. In one end‐member model, we assumed that the orientation of stress is uniform in the crust as is assumed in many stress inversion studies. In this model, the variability of earthquake mechanisms means that friction during faulting must vary for each event. We computed friction μ from the ratio of the resolved shear stress to the effective normal stress on the fault plane with the assumption of hydrostatic pore pressure. The values of μ vary over a large range from 0 to 1.5. In the other extreme model we assumed optimally oriented slip and a constant μ = 0.6, as is suggested by Byerlee's law, for all the earthquakes, and determined the local stress orientation for each earthquake. The orientation of the stress changes drastically from one earthquake to another, and the assumption of uniform stress field commonly used in stress inversion is not warranted. An important conclusion is that a regionally uniform stress field and constant friction on optimally oriented faults are mutually exclusive. The actual situation in the crust is most likely to be intermediate between these two end‐member models. From the existing data alone, we cannot determine the degree of heterogeneity uniquely, but both μ and the local stress field near earthquake faults are likely to vary substantially, and studies on earthquake rupture dynamics must take these heterogeneities into consideration.

Recent Tectonic Activity in the Imotsko Polje Area

Displacements of the Adriatic microplate, particularly of itssouthern part, are of crucial importance for the understanding ofrecent tectonic movements. Deformations of the structural fabric andthe resulting tectonic activity also encompass the studied area. Thereare four most active fault zones - Mosor-Biokovo, Zagvozd-Vrgorac-Metkoviae, Trilj-Tihaljina-Eapljina and Imotski-Meðugorje-Popovopolje. In the explored area, these zones delimit the Imotsko polje.The calculated regional stress is oriented in the range between10-190i and 350-170i. The relationship between the orientation ofstructural units and stress enables reverse displacements, most frequently in the direction of the south and south-east. The change instress orientation in the Mt. Biokovo hinterland makes the aforementioned fault zones surrounding the Imotsko polje favourably oriented in respect to the stress, thus enabling dextral horizontal tectonic transport of the structures in different fault blocks.In the two fault zones - Trilj-Tihaljina-Eapljina and Imotski-Meðugorje-Popovo polje, there are 98 outcrops suitable for the structuralgeology measurements. The obtained data on the local stress orientation and spatial displacement of structures are the most important.The character of faults and the most active fault sections are marked,as well as the local structures that are formed due to strong horizontalcomponent of structural displacement in the studied fault zones.Recent tectonic activity is confirmed by the occurrence of earthquakes.Spatial distribution of the earthquake epicentres depicts zonesof seismotectonic activity that are related to the aforementioned mostimportant fault zones. Two of the fault zones - Trilj-Tihaljina-Eapljinaand Imotski-Meðugorje-Popovo polje are especially well markedby earthquakes occurring at depths of between 3 and 15 km.

Stress analysis in the intraplate area of Gazli, Uzbekistan, from different sets of earthquake focal mechanisms

Occurrence of intraplate earthquakes have been related either to reactivation of preexisting zones of weakness in a uniformly oriented regional stress field or to local stress changes. More recently, buildup of pore pressure anomalies has been proposed to explain both the low recurrence rate and some misorientation of rupture planes in the prevailing regional stress field. In this study, we analyze local stress field orientation in the area of the three Ms = 7.0 Gazli (western Uzbekistan) earthquakes by using different sets of earthquakes focal mechanisms. We show that stress tensor inversions from both seven 4.2 ≤ Ms ≤ 7.0, forty‐five 0.1 ≤ ML ≤ 3.2 and twenty‐five 1.0 ≤ ML ≤ 3.2 earthquake focal mechanisms give constant maximum horizontal stress direction in agreement with the one deduced from the India‐Asia collision. The best model for the local stress tensor exhibits a significant (25°) discrepancy from vertical for one of the principal stresses. We note, moreover, that the common assumption that one principal stress axis is oriented vertically is not strictly supported by stress states derived from earthquake focal mechanisms available in the world stress database. By evaluating each hypothesis of stress inversion methods from earthquake focal mechanisms, it is shown that, especially in intraplate settings, the first motion focal solution should be the most appropriate one for a formal inversion and that the magnitude of the considered event may not be the only criterion to assess the quality of a stress inversion. This is particularly timely because more and more focal mechanisms are moment tensor solutions instead of first motion solutions. Concerning the Gazli sequence, the slip conditions (pore pressure and frictional coefficient) are evaluated for each of the three mainshocks in terms of an independently determined local stress field. Because variations in orientations between local and regional stress field are not detected, and because the three mainshock fault planes seem to be relatively well oriented for a frictional activation in the local stress field, it is concluded that this seismic sequence is localized on preexisting weakness zones and is not due to local stress orientation anomalies.

Seismicity following the 1985 eruption of Nevado del Ruiz, Colombia

Over 200 of the best-recorded earthquakes occurring near Nevado del Ruiz, Colombia, between December 1985 and May 1986 are examined for spatial patterns, information on local stress orientations, and evidence for seismogenic structures that may be related to magma-transport processes. These earthquakes occurred following the devastating eruption of November 13, 1985. Earthquake activity that is probably related to the volcanic system occurs over a large area, of the order of 50 km 2. A pattern of earthquakes beneath the southern half of Ruiz may be the expression of a conduit system, but evidence is inconclusive due to low spatial resolution of most hypocenters. Epicenter and first-motion plots suggest most earthquakes occur on small faults of varying orientation. Most seismicity during this time period was unrelated to mapped faults, including some with Holocene offsets, except for the Villamaria Fault near Ruiz's northeast sector. Composite focal mechanisms show a variation of nearly 90° in the strike of T-axes and indicate Ruiz is in a zone of local extension.

Seismic evidence for conjugate slip and block rotation within the San Andreas Fault System, southern California

The pattern of seismicity in southern California indicates that much of the activity is presently occurring on secondary structures, several of which are oriented nearly orthogonal to the strikes of the major through‐going faults. Slip along these secondary transverse features is predominantly left‐lateral and is consistent with the reactivation of conjugate faults by the current regional stress field. Near the intersection of the San Jacinto and San Andreas faults, however, these active left‐lateral faults appear to define a set of small crustal blocks, which in conjunction with both normal and reverse faulting earthquakes, suggests contemporary clockwise rotation as a result of regional right‐lateral shear. Other left‐lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data. Many of these structures predate the modern San Andreas system and may control the pattern of strain accumulation in southern California. Geodetic and paleomagnetic evidence confirm that block rotation by strike‐slip faulting is nearly ubiquitous, particularly in areas where shear is distributed, and that it accommodates both short‐term elastic and long‐term nonelastic strain. A rotating block model accounts for a number of structural styles characteristic of strike‐slip deformation in California, including: variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults; domains of evenly‐spaced antithetic faults that terminate against major fault boundaries; continued development of bends in faults with large lateral displacements; anomalous focal mechanisms; and differential uplift in areas otherwise expected to experience extension and subsidence. Since block rotation requires a detachment surface at depth to permit rotational movement, low‐angle structures like detachments, of either local or regional extent, may be involved in the contemporary strike‐slip deformation of southern California. A block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge‐bounding faults, but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction. This behavior, coupled with the presence of possible regional detachments, accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges. Although fault displacements along secondary structures associated with block rotations remain small, they may still influence the nucleation and the characteristic rupture length of large earthquakes. A more complete description of what these structures are, and how they interact, may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard.

A mechanism for the development of en-echelon gashes in kink zones

Consistent arrangement of en-echelon gashes occurring within kink zones was observed and measured at several localities in the Lubec—Belleisle structural zone of southwestern New Brunswick. The special geometric relationship of both kinks and en-echelon fracture systems allow an interpretation of the regional and local stress orientations necessary to produce such combined features. Mohr-circle stress analysis of a modified direct shear model is used to postulate a mechanism whereby varying orientations of en-echelon tension gashes and boundary shears within kink zones can be explained. Using this analysis, the observed gash orientations can be generated by reduction in the local normal stress acting on original kink-zone boundaries, coupled with a corresponding increase in the normal stress acting transverse to such boundaries. These stress adjustments could be due to deflection of regional stress trajectories around local zones of yielding (by kinking) rock.