Horizontal Σ1 Research Articles

Structural expression of the frontal thrust of an active fold-and-thrust belt: The Holocene 123-km-long Kur fault, Greater Caucasus, Azerbaijan

Here we present the main features of the frontal structure, known as Kur Fault, of the Plio-Quaternary Kura fold-and-thrust belt in the Greater Caucasus (Azerbaijan). The Kur Fault has been analysed thanks to geological-structural and geomorphological surveys of its whole length, integrated by a relocation of instrumental seismicity, data on historical seismicity, new focal mechanism solutions, and ambient vibration measurements across the fault trace. The in-depth study of the frontal structure can: i) provide insights into the shallow propagation of a regional reverse fault, ii) contribute to a better understanding of the earlier stage of development of a young continent-continent collision, and iii) have implications for seismic hazard assessment because the area is seismically active and hosts the most important infrastructure for energy production in the country. The results show that the fault deforms the surface for a total length of 123 km. The shallow expression is given by four main scarp segments, with a right-stepping arrangement, which have different structural significance; they are represented by an alternation of fault-propagation folds, folds with offset frontal limbs, and shallow faulting. Analyses of the age of deformed deposits and landforms suggest activity from Mid-Late Miocene times to the Holocene. The fault attitude and pure reverse kinematics are coherent with the Holocene and present-day state of stress, characterized by a N–S to NNE-SSW horizontal σ1, suggesting the capability for seismic reactivation. Calculation of potential Mw indicates values in the range 7.5–7.9 if we consider its entire fault length, 6.1–7.2 if we consider the single segments.

Paleostress evolution of the West Africa passive margin: New insights from calcite twinning paleopiezometry in the deeply buried syn-rift TOCA formation (Lower Congo basin)

Accurate characterization of hydrocarbon reservoirs requires a good understanding of the paleostress history of their host sedimentary basin. This study aims at reconstructing the paleoburial and paleostress history of the syn-rift Barremian TOCA Fm. in the Lower Congo basin (West African margin). Two oriented borehole cores drilled offshore Congo were investigated using stress inversion techniques applied to calcite mechanical twins to constrain paleostress orientations and magnitudes. The inversion of calcite twins was performed on a widespread early diagenetic cement (~129 to ~113 Ma) and revealed that the TOCA Fm. has undergone a complex polyphase stress history, including (1) extensional stress regimes with vertical σ1 and horizontal σ3 trending NE-SW and WNW-ESE (N100) associated with the rifting episode preceding the opening of the South Atlantic (130 to 112 Ma), and with mean differential stresses of ~47 MPa for (σ1-σ3) and ~ 20 MPa for (σ2-σ3). The early N100 extension is associated with the development of normal faults striking N-S that likely reactivated inherited basement structures. The direction of extension evolved during the Barremian into the main NE-SW extension marked by the dominant normal faults striking NW-SE. (2) Compressional and strike-slip stress regimes with horizontal σ1 oriented ~N-S possibly related to the far-field intraplate transmission of orogenic stresses generated at the distant Africa-Eurasia plate boundary at ~67–60 Ma, and with mean differential stresses of 40 MPa for (σ1-σ3) and 28 MPa for (σ2-σ3). (3) Compressional and strike-slip stress regimes with horizontal σ1 oriented ENE-WSW to ~E-W that we tentatively relate to the mid-Atlantic ridge push and prevailing since ~15 Ma onwards, associated with mean differential stresses of ~45 MPa for (σ1-σ3) and ~ 18 MPa for (σ2-σ3). Our results reveal that the TOCA Fm. recorded mainly far-field stress effects that could be related to interactions between the African plate and surrounding plates, and that divergent passive margins may experience a complex tectonic history including both extensional and compressional events.

Seismicity in a weak crust: the transtensional tectonics of the Brawley Seismic Zone section of the Pacific–North America Plate Boundary in Southern California, USA

SUMMARYThe Brawley Seismic Zone (BSZ) is a ∼58-km-long section of the Pacific–North America Plate boundary that connects the southernmost San Andreas Fault (SAF) and the Imperial Fault in southern California. We analyse the BSZ as two segments: a north segment, dominated by SAF tectonics, and a south segment that accommodates a higher level of seismicity. The south segment includes a ∼6 km wide right lateral step-over that includes the Salton Sea geothermal field (SSGF) and Holocene subaerial lava domes at the south end of the Salton Sea, called Salton Buttes. In general, the 40 yr (1981–2021) of BSZ seismicity is characterized by transitory sequences that often are accommodated simultaneously or in quick succession on intricate ladder-like faults. These sequences rarely reactivate faults associated with previous seismicity alignments but rather activate adjacent faults, sometimes located within less than 1 km. They can include several events of similar size as the mainshock, followed by bursts of aftershocks sometimes located away from the mainshock rupture. The seismicity rate and the spatial geometrical complexity varies between the BSZ-north and BSZ-south segments. The 24-km-long BSZ-north accommodates a ∼12-km-long linear trend of seismicity that extends from the SAF terminus into the Salton Sea, where moderate-sized northeast striking sequences form the rungs in a ladder-structure in a weak crust. The seismicity in this area is most likely influenced by the stress state of the SAF. In contrast, the 34 km long BSZ-south segment, which also has a weak crust, has accommodated larger sequences that illuminate irregular ladder-type faulting, with aftershocks defining linear distributions striking either north or northeast. The focal mechanisms exhibit a mostly strike-slip style of faulting with minor dip-slip faulting in the south Salton Sea area as well as the Mesquite basin to the south. The state of stress, as determined from focal mechanisms, consists of almost horizontal σ1, and σ3 with vertical σ2. The activation angle between the trend of σ1 and the preferred nodal plane of the largest nearby event decreases systematically from north to south along the long-axis of the BSZ. In the step-over zone, the Holocene volcanism and the frequent seismicity sequences suggest crustal extension as well as associated reduced crustal strength as compared to other parts of the BSZ. The presence of weak, thin, hot crust and distributed ladder-like or conjugate predominantly strike-slip faulting suggests that the whole BSZ acts as a several kilometre wide fault zone where no prominent principal slip surfaces are required to accommodate moderate-sized (M6+) earthquakes.

Evolution of the stress field at the junction of Talesh – Alborz – Central Iran during the past 5 Ma: Implications for the tectonics of NW Iran

Tectonic stresses governed by the Arabia – Central Iran collision are transferred throughout the Iranian Plateau. The orientation of the maximum horizontal stress (σ1) significantly changes (~90°) in the Persian block of NW Iran. However, the exact spatial extent and boundary conditions for these changes are poorly defined. This study investigates the late Cenozoic stress regime in a key region in NW Iran including the Mahneshan – Mianeh Cenozoic Basin (MMCB), the Garmachay Fault Zone (GFZ), and the southern flank of the Bozqush Mountains (SBM). We mapped folds and fault zones in detail using high-resolution satellite imageries and field surveys. We have measured fault-slip data at 49 localities distributed over the study area. The results of fault-slip inversion analysis indicate significant temporal and spatial changes in both the modern and paleostress fields. In the MMCB and GFZ, the orientation of the horizontal σ1 axis changed from N148°E to N041°E, during the Pliocene and Quaternary times. During the same period of time, it changes from NE-SW to N138°E in the SBM. Our study reveals that the southern boundary of the Persian block corresponds to the deep-seated North Tabriz fault (NTF), which is spread out over several splays at the southern flank of the Bozqush Mountains. The NTF acts as a boundary condition in both the stress and strain fields of the region. The GFZ defines the southeastern border of the Persian block, where the NW-trending maximum compression encounters the remote stress field in the hinterland of the Zagros collision. Our results indicate that independent movements of crustal blocks transfer crustal stresses beyond the collision zone and produce large-scale stress perturbations by discarding compatibility in regional strain/stress fields.

Interaction between the far-field stress and crustal boundary faults in the kinematics of active deformation around the Lut Block, Eastern Iran

We investigate the effects of far-field stresses and major crustal boundary faults in the deviation of local stresses around the Lut Block of the southeastern Iranian plateau in the light of the 2010–2011 Rigan earthquakes. Dextral 2010 (Mw 6.5) and sinistral 2011 (Mw 6.2) Rigan earthquakes occurred in the southern edge of the Lut Block, where there is no report of moderate to large magnitude historical and instrumental seismicity. To address the style and distribution pattern of local stresses, we apply the inversion method to focal mechanism solutions of 74 earthquake events (1933–2018, Mw ≥ 4.8). The data were separated into six clusters including the Tabas – Dasht-e-Bayaz, East Lut, Kahurak, Rigan, Shahdad – Bam, and West Lut – Kuhbanan zones. Moving toward the Rigan area, the results indicate a gradual deflection in the direction of the horizontal σ1 axis along the East Lut fault system (i.e., from north to south, Dasht-e-Bayaz: N044°E, East Lut: N051°E, Kahurak: N066°E, Rigan: N077°E), while it reveals a sharp change along the West Lut fault system (i.e., from north to south, Tabas: N044°E, West Lut – Kuhbanan: N010–13°E, Shahdad – Bam: N013–26°E, Rigan: N077°E). The deviation in the long-lasting strike-slip/transpressional stress regimes along the eastern and western boundaries of the Lut Block is mainly controlled by the geometry and kinematics of the East Lut fault system, and the direction of the Arabia – Eurasia convergence, respectively. The geodynamic interaction of the East and West Lut fault systems which bound the crustal block on both sides has a significant effect on the evolution of local stresses in the southern edge of the Lut Block, which is bounded by second-order faults that are the termination of the major faults. The results may apply to other crustal blocks limited in some boundaries by the second-order splays of crustal faults with different geodynamic implications.

Seismicity, Stress State, and Style of Faulting of the Ridgecrest-Coso Region from the 1930s to 2019: Seismotectonics of an Evolving Plate Boundary Segment

ABSTRACTDecadal scale variations in the seismicity rate in the Ridgecrest-Coso region, part of the Eastern California Shear Zone, included seismic quiescence from the 1930s to the early 1980s, followed by increased seismicity until the 2019 Mw 6.4 and 7.1 Ridgecrest sequence. This sequence exhibited complex rupture on almost orthogonal faults and triggered aftershocks over an area of ∼90 km long by ∼5–10 km wide, which is a fraction of the area of the previously seismically active Indian Wells Valley and Coso range region. During the last 40 yr, the seismicity has been predominantly the result of strike-slip motion, extending north from the Garlock fault, along the Little Lake and Airport Lake fault zones, and approaching the southernmost Owens Valley fault to the north. The Coso range forms an extensional stepover between these two strike-slip fault systems. This evolution of a plate boundary zone is driven by the northwestward motion of the Sierra Nevada, and crustal extension along the southwestern edge of the Basin and Range Province. Stress inversion of focal mechanisms shows that the postseismic stress state consists of almost horizontal σ1 and vertical σ2. The σ1 is spatially rotated across the Coso range stepover with σ1-trending ∼N17° E to the north, whereas, along the Mw 7.1 mainshock rupture, the trend is ∼N6° E. The friction angles as measured between fault strikes and the σ1 trends correspond to a frictional coefficient of 0.75, suggesting average fault strength. In comparison, the mature Garlock fault has a smaller frictional coefficient of 0.28, similar to weak faults like the San Andreas fault. Thus, it appears that the heterogeneously oriented and spatially distributed but strong Ridgecrest-Coso faults accommodate seismicity at seemingly random places and times within the region and are in the process of self-organizing to form a major throughgoing plate-boundary segment.

Active stress field and fault kinematics of the Greater Caucasus

This work contributes to depict the current seismicity, fault kinematics, and state of stress in the Greater Caucasus (territories of Georgia, Azerbaijan and Russia). We merged and homogenized data from different earthquake catalogues, relocated ~1000 seismic events, created a database of 366 selected focal mechanism solutions, 239 of which are new, and performed a formal stress inversion. Preferential alignments of crustal earthquake foci indicate that most seismic areas are located along the southern margin of the belt and in the north-eastern sector. This is consistent with the presence of dominant active WNW-ESE faults, parallel to the mountain range. In the entire Greater Caucasus, a dominant NNE-SSW-oriented greatest principal stress (σ1) controls the over-all occurrence of earthquakes of minor and major magnitude. Main earthquakes are characterized by a vertical least principal stress (σ3), corresponding to reverse kinematics. Reverse slip is more common along the southwestern and north-eastern foothills of the Greater Caucasus, although in these areas there are also scattered strike-slip events. This suggests the presence of local stress fields with horizontal σ1 and σ3. In the central-southern part of the mountain belt, in correspondence of the local collision between the Lesser and the Greater Caucasus, σ1 rotates to NNW-SSE. The strike-slip events, instead, dominate along the southern flank of the central-eastern mountain range; this is interpreted as the effect of the collision that promotes eastward escape of the tectonic blocks located to the east.

Cenozoic Shift From Compression to Strike‐Slip Stress Regime in the High Andes at 30°S, During the Shallowing of the Slab: Implications for the El Indio/Tambo Mineral District

AbstractIn the High Andes of central Chile, above the flat‐slab segment, analysis of more than 1,000 fault slip data from Miocene outcrops provides evidence for a change of the regional tectonic regime from compressional to strike slip. This shift in tectonic regime occurred during the waning stages of arc volcanism between 14 and 11 Ma, as a result of the shallowing of the Nazca plate, in conjunction with the migration of deformation to the Precordillera. During the early to middle Miocene, a compressive regime with horizontal σ1 axis (N86°E) was responsible for reverse slip along NNE to N‐striking faults. During the late Miocene, a shift to strike‐slip tectonics took place due to an increase in the absolute magnitude of the vertical stress component as the crust thickened and the gravitational potential energy increase. We argue that instead of the previously accepted highly compressional setting in the arc region during the slab flattening, the change to a strike‐slip regime was the main control on mineralization. Mineralization was controlled by the promotion of fluid expulsion from the magma chambers along active, subvertical strike‐slip fault systems with a high slip tendency, and focusing of fluids in localized areas undergoing extension. Under this strike‐slip regime, the El Indio, Tambo, and La Despensa fault systems formed as dextral strike‐slip systems. The tips and jogsites along these faults experienced local extensional stress fields, forming the El Indio and Tambo mineral districts.

Architecture of the intracontinental Jaibaras Rift, Brazil, based on geophysical data

Qualitative and quantitative integration and interpretation of magnetic, gravity and magnetotelluric data help to determine the internal architecture of the Jaibaras rift, and allow assessing the evolution of the Jaibaras Rift within the Precambrian crystalline basement of Borborema Province, NE Brazil. This was achieved by 2D joint modeling of magnetic and gravity data in five sections across the main axis of the Jaibaras Rift. Surface data, rock density measurements, depth constraints from 2D Euler deconvolution and geophysical information from previous work in the area were integrated to constrain the modeling. The magnetic and gravity profiles of the Jaibaras Rift indicate estimated source bodies at depths up to 2.5 km, showing complex configuration for the structural framework, with a set of asymmetric grabens and horsts. The 2D magnetotelluric inversion shows that the Jaibaras Rift is marked by low resistivity values, and maximum thickness of the sedimentary package up to approximately 3 km. Shallow dipping conductive material may represent either a suture zone between the Ceará Central and Médio Coreaú domains or a set of fractures due to horizontal σ1 stress in the Ceará Central Domain. The Jaibaras Rift displays a very complex internal structure, with discontinuous sequences of grabens and horsts, and a significant volume of surface and subsurface volcanic rocks. The sedimentary packages with volcanic rift sequences have variable thicknesses, from 1 to 3 km. These rock units are controlled by normal faults that developed from older discontinuities, such as the Transbrasiliano lineament.

Using regional moment tensors to constrain the kinematics and stress evolution of the 2010–2013 Canterbury earthquake sequence, South Island, New Zealand

On September 3, 2010, a MW 7.0 (U.S. Geological Survey moment magnitude) earthquake ruptured across the Canterbury Plains in South Island, New Zealand. Since then, New Zealand GNS Science has recorded over 10,000 aftershocks ML 2.0 and larger, including three destructive ~MW 6.0 earthquakes near Christchurch. We treat the Canterbury earthquake sequence as an intraplate earthquake sequence, and compare its kinematics to an Andersonian model for fault slip in a uniform stress field. We determined moment magnitudes and double couple solutions for 150 earthquakes having MW 3.7 and larger through the use of a waveform inversion technique using data from broadband seismic stations on South Island, New Zealand. The majority (126) of these double couple solutions have strike-slip focal mechanisms, with right-lateral slip on ENE fault planes or equivalently left-lateral slip on SSE fault planes. The remaining focal mechanisms indicate reverse faulting, except for two normal faulting events. The strike-slip segments have compatible orientations for slip in a stress field with a horizontal σ1 oriented ~N115°E, and horizontal σ3. The preference for right lateral strike-slip earthquakes suggests that these structures are inherited from previous stages of deformation. Reverse slip is interpreted to have occurred on previously existing structures in regions with an absence of existing structures optimally oriented for strike-slip deformation. Despite the variations in slip direction and faulting style, most aftershocks had nearly the same P-axis orientation, consistent with the regional σ1. There is no evidence for significant changes in these stress orientations throughout the Canterbury earthquake sequence.

Recent spatial and temporal changes in the stress regime along the southern Tunisian Atlas front and the Gulf of Gabes: New insights from fault kinematics analysis and seismic profiles

Tectonic deformations in the southern Atlassic front of Tunisia (SAFT) and the Gulf of Gabes result from the Nubia-Eurasia convergence. This study, based on the inversion of geologically determined fault slip vectors, presents evidences for spatial and temporal changes in the stress state. Fault kinematics analyses reveal a temporal change in states of stress during the Late Cenozoic. A paleostress (Miocene–Pliocene) state is characterized by a regional compressional tectonic regime with a mean N134 ± 09°E trending horizontal maximum stress axis (σ1). A modern (Quaternary to present-day) state of stress also corresponds to compressional tectonic regime with a regionally mean N05 ± 10°E trending horizontal σ1. The SAFT corresponds to two distinct domains: a far-foreland Atlassic front related to contractional deformation associated with southward propagate thrusting, and eastward, the Gulf of Gabes characterized by normal component NW to WNW trending faults at the crustal margin scale probably related with a transtensional tectonic regime. We propose that the spatial and temporal changes in the stress during the Miocene–Pliocene and Quaternary may result from the geodynamic evolution of the Tunisian Atlas in relation to the rotation of Corsica–Sardinia blocks and the separation of the Sicilian continental lithosphere from the Ionian oceanic lithosphere. These temporal and spatial stress changes along the SAFT and the Gulf of Gabes are probably the engine behind the moderate seismicity, leading at the same time to a reactivation of the inherited major structures created during the evolution of the southern Tethyan margin.

State of stress in the northern Tabas block, east-central Iran, as inferred from focal mechanisms of the 1978 Tabas earthquake sequence

Abstract In this paper, the state of stress in the northern Tabas block in east-central Iran is analyzed based on the systematic inversion of aftershock focal mechanisms from the 1978.09.16 Tabas earthquake, to characterise the stress regime that controls most earthquakes in this area. Here, stress inversions of double-couple focal mechanisms of earthquakes recorded during the 30 days following the main shock have been carried out. The calculated average stress regime indicates dominant major 226° to 237° trending compression for the Tabas region. The dominating regime in east-central Iran is thrusting with a minimum stress axis, σ 3, close to vertical. The reconstruction of the main seismotectonic stress in east-central Iran with a NE-SW compression is consistent with independent information of the active plate convergence related to Arabia-Eurasia convergence. Most earthquakes in the mentioned area occur near or around concealed Quaternary thrust faults with their activity being controlled by the NE-SW compression. Where ϕ, the ratio of principal stress differences, is 0.5, a small difference between σ 2; σ 3 and σ 1 and small amounts of deviatoric stress is indicated. Therefore, for small deviatoric horizontal σ 1 it is not possible to increase and reactivate small sections of basement thrust faults and create secondary basement aftershocks. Reconstructed stress regimes in this study for sedimentary cover (237) and basement (226) of Tabas are similar. Therefore, it seems that the basement and cover were coupled together, possibly along the 2–4 km of upper Precambrian low-grade metamorphic rocks. Then these segments of the fold-and-thrust belt were involved in similar seismic activity under a similar stress regime.

Stress state reconstruction of oblique collision and evolution of deformation partitioning in W-Zagros (Iran, Kermanshah)

The W-Zagros of Iran is characterized by a deformation partitioning pattern with a right-lateral strike-slip motion along the Main Recent Fault and a shortening component across the Zagros Simple Fold Belt under the late Cenozoic N–S oblique collision between the Arabian and Iranian continental plates. In this study, we examined the brittle structures that developed during the mountain building process in the High Zagros Belt of Kermanshah Province, which is structurally located between the two partitioned components of deformation, to decipher the history of polyphase deformation and variations in stress fields within the sedimentary cover since the onset of collision. The earthquake focal mechanisms of W-Zagros were analysed to evaluate the recent stress fields within the basement. Analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors. Brittle tectonic analyses were carried out to reconstruct possible geometrical relationships between different structures and to establish relative chronologies of the corresponding stress tensor, with reference to folding. Our stress reconstruction indicates a major change in stress state from a Miocene–Pliocene syn-folding compressional regime to a Pliocene–Recent post-folding strike-slip system, with a significant anticlockwise reorientation of the horizontal σ1 stress axis from NE–SW to N–S. The results of the recent stress states indicate that the stress trends are similar throughout the basement and sedimentary cover. The differences between stress and strain axes suggest different possible degrees in partitioning of seismic and aseismic deformation, as revealed by earthquake focal mechanisms in the basement and geodetic surveys at the surface of the sedimentary cover. The integrated palaeostress results on the regional tectonic framework suggest that the late Cenozoic brittle tectonic evolution of the W-Zagros was achieved through the transition from thin- to thick-skinned deformation under the oblique plate convergence, highlighting a southwestward shift of the deformation partitioning during the final Pliocene collisional reorganization.

Crustal velocity model along the southern Cuban margin: implications for the tectonic regime at an active plate boundary

SUMMARY A new 1-D velocity model along the southern Cuban margin has been determined using local earthquake data, which are the result of the merged Cuban and Jamaican catalogues. Simultaneous inversion using joint-hypocentre determination was applied to solve the coupled hypocentre‐velocity model problem. We obtained a seven-layer model with an average Moho interface at 20 km. The average velocity was found to be 7.6 km s −1 on the top of the crust‐ mantle transition zone and 6.9 km s −1 in the basaltic layer of the crust. The improvement in the earthquake locations allowed us for the first time to use local seismicity to characterize the activity on local faults and the stress regime in the area. For this purpose, 34 earthquake focal mechanisms were determined along the eastern segments of the Oriente Fault. These solutions are consistent with the known left-lateral strike-slip motion along this major structure as well as with the stress regime of two local structures: (1) the Cabo Cruz Basin and (2) the Santiago deformed belt. The first structure is dominated by normal faults with minor strikeslip components and the second by reverse faults. The shallow seismicity in the Cabo Cruz Basin is associated with fault planes trending N55 ◦ ‐58 ◦ E and dipping 38 ◦ ‐45 ◦ to the north. The Santiago deformed belt, on the other hand, exhibits diverse fault plane orientations. These local structures account for most of the earthquake activity along the southern Cuban margin. Deep seismicity observed in the Santiago deformed belt, supported by focal mechanisms, suggests underthrusting of the Gonave Microplate beneath the Cuban Block in this area. The principal stress orientations obtained from stress inversion of earthquake focal mechanisms suggest a thrust faulting regime along the Southern Cuban margin. We obtained a nearly horizontal σ1 and nearly vertical σ3, which indicates active compressional deformation along the major Oriente transcurrent fault in agreement with the dominant structural trend associated with the Santiago deformed belt.

Rupture planes of the Gazli earthquakes deduced from local stress tensor calculation and geodetic data inversion: Geotectonic implications

Three major earthquakes (with Ms ≥ 7) were clustered in space and time (1976–1984) near Gazli, Uzbekistan, without reported evidence for fault scarps. To constrain the kinematics of the seismic sequence, we invert both focal mechanism solutions of the mainshocks and major aftershocks and vertical surface displacements associated with the three major shocks. For seven M > 4 earthquakes that occurred in the 1976–1984 period, we find that all the focal mechanisms agree with a single stress tensor. One of the mechanical implications of this mathematical solution is the choice of the seismic fault plane. The 1976 earthquake fracture planes are well constrained by the stress tensor to be striking E‐W and ENE‐WSW and dipping to the south and the NNW respectively. From stress tensor inversion alone, it is impossible to determine if the fracture plane of the 1984 event dips to the SE or to the NW. Coseismic vertical surface displacements allow us to infer possible fault plane orientations of the 1976 and 1984 events. The best fit to the surface displacement for the 1976 earthquakes is obtained from the geometries that are derived from the homogeneous stress tensor inversion. The inversion of vertical surface displacements for the 1984 event constrains the associated fault plane to correspond to a NE‐WS striking and SE dipping plane. The inversions of complementary geophysical data argue for the complexity of the orientations of the faults activated by the 1976–1984 seismic sequence. The 1984 earthquake fault plane is well orientated to allow failure to be activated by a horizontal σ1 under a Mohr‐Coulomb failure criterion. Under the same failure criterion and stress conditions, unlike the 1984 fracture plane, the 1976 fault planes are misorientated and likely to correspond to preexisting discontinuities. Concerning the principle stress orientations, the Gazli region seems to be under control of the regional tectonic setting. From a regional point of view, the WNW‐ESE strike of σ1 and the NNE‐SSW strike of σ2 deduced from this study for the Kyzylkum Desert can be explained at once by the interaction of the regional plate boundary motions.

Stress deflection in a tectonic compressional field: A model for the northwestern Iberian Chain, Spain

By means of fault population analysis (using the right dihedra, y‐R diagram and Etchecopar's methods), the states of stress in a number of sites of the Cameras Massif (Northwestern Iberian Chain, Spain) and nearby areas of the Ebro Basin have been determined. These states of stress have been used as a framework in order to infer a model of the actual stress field in that area during the main stage of the Alpine compression (Oligocene‐lower Miocene). The three main compression directions obtained (NW‐SE, NNE, NE‐SW) can be explained by a single compression field with horizontal σ1 trending NNE and vertical σ2, whose primary trajectories are deflected due to (1) large‐scale strike‐slip faults or lateral ramps of thrusts or (2) frontal ramps of thrusts oblique to the main compressive axis. The reliability of this scheme has been checked by means of a two‐dimensional mathematical model based upon the finite element method and applied on a sheet whose rheological properties are similar to those of a standard limestone. This model simulates the main features of the regional stress field, and it was constructed after determining the average parameters of the possible external or “primary” field and simulating the major faults that could deflect the stress trajectories.