Striking Faults Research Articles

Integration of 2D and 3D electrical resistivity tomography and 2D gravity modeling to evaluate groundwater investigations in the Burka Uke catchment area, western Ethiopia

The increasing demand for water resources due to the expanding infrastructure and growing population of Uke town, West Ethiopia, necessitates a comprehensive understanding of groundwater potential within the Burka Uke catchment. The aims and objectives of this study are to identify a map of the aquifer by characterizing the subsurface earth, basement rock depth, resistivity and density of geological layers, their thicknesses, and structural features. The investigation integrates electrical resistivity tomography and gravity modeling to delineate potential groundwater zones and their structural controls. Electrical resistivity tomography profiles (1, 4, and 6) revealed low resistivity zones indicative of potential aquifers. The significant resistivity contrast (378 to 1412 Ω.m) suggests a high fracture density, potentially enhancing groundwater storage. The Bouguer gravity anomaly map, separated into regional and residual components, displayed values ranging from 51.79 to 41.59 mGal, highlighting density variations in the subsurface. As a result, analysis of the residual gravity anomaly map identifies north–south striking fault elements, indicating their influence on groundwater flow.The combined 2D and 3D ERT data also revealed a 2D gravity model of the aquifer from which the concentration extended throughout the study area. Data validation with previously drilled wells has confirmed the effectiveness of the geophysical methods used in identifying potential groundwater zones. This study provided critical information for the sustainable management of existing groundwater and demonstrated the effectiveness of integrating geophysical techniques to delineate and characterize aquifers in water-stressed areas.

Surface geochemistry as a key to understanding the petroleum system in regions of complex geology - Polish Outer Carpathians

The goal of the study is to investigate the relationships between soil gas composition and geological structures in a structurally complex area of the Carpathian fold and thrust belt. Surface geochemical surveys, aimed at determining the molecular and isotopic composition of soil gas samples, were carried out in the central part of the Polish Outer Carpathians in two major tectonic units, the Magura Unit and the Fore-Magura Group of units, which outcrop at the front of the former and within it as a tectonic window (the so-called Mszana Dolna Tectonic Window, MDTW). The tectonic structure of the area has numerous deformations, such as folds, thrusts, and strike-slip faults, that are accompanied by broad zones of mélange. In all 572 soil gas samples, the concentrations of light alkanes, gaseous alkenes as well as non-hydrocarbon (e.g. hydrogen) components were determined, and part of the samples were analysed to test the carbon isotope composition of methane and carbon dioxide.The results of the surface geochemical survey showed significant differences in the recorded microconcentrations of hydrocarbons within the tectonic units of the Outer Carpathians. Significantly higher concentrations of total alkanes C2-C5 were mainly recorded in the MDTW, although the distribution of total alkane values within the Window shows strong connections with the arrangement of strata and the strike of faults and thrusts. Generally, the sealing is insufficient within the MDTW to support the existence of conventional economically viable hydrocarbon accumulations. Our data confirms the sealing role of the Magura Unit in the Carpathian oil system. Therefore, hydrocarbon sources (i.e. potential hydrocarbon accumulations) may be expected within the Fore-Magura Group beneath the cover of the Magura Unit.Our results are also of key importance for the interpretation of surface geochemistry data in other oil and gas provinces with a complex geological structure.

Morphostructural evidence of crustal-scale, active along-strike segmentation of the Umbria-Marche Apennines, Italy

This paper discusses the response of topography and river networks to non-uniform lithology and tectonic forcing in the Umbria-Marche sector of the Apennines fold and thrust belt. We ruled out the role of variable erosion of rock types and interpret channel steepness data in terms of rock uplift, discovering a southward increase in the total amount of uplift. Such a trend appears as the large-scale response to uneven vertical motions of different sectors of the mountain ridge and foothills. The general coincidence between sector boundaries and transversal, NE-SW striking faults mapped by seismic interpretation in the outer zone of the fold and thrust belt, suggests that such faults extend to the SW, beneath the allochthonous thrust sheets of the mountainous area. Therefore, it may be inferred that such transversal faults represent long-lived, deeply rooted basement structures compartmentalizing both the axial and the outer zones of the fold and thrust belt. We suggest that differential uplift was essentially controlled by variable amounts of basement thrust displacement characterizing the compartmentalized belt. This interpretation deviates from a more conventional view that uplift of the central Apennines, particularly prominent in the south, is dynamically supported. Our results, besides shedding new light into the active tectonic behavior of a large portion of the Italian peninsula, also provide general insights into the surface response to the differential behavior of crustal blocks produced by along-strike segmentation of active mountain belts.

Structure and Evolution of Multi-Trend Faults in BZ19-6 Buried Hill of the Bohai Bay Basin, Eastern China

Defining the structure and evolution of multi-trend faults is critical for analyzing the accumulation of hydrocarbons in buried hills. Based on high-resolution seismic and drilling data, the structural characteristics and evolutionary mechanism of multi-trend faults were investigated in detail through the structural analysis theory and quantitative calculations of fault activity, allowing us to determine the implication that fault evolution exerts on hydrocarbon accumulation in the BZ19-6 buried hill. There are four kinds of strike faults developed on the buried hill: SN-, NNE-, NE–ENE-, and nearly EW-trending, which experienced the Mesozoic Indosinian, Yanshan, and Cenozoic Himalayan tectonic movements. During the Indosinian, the BZ19-6 was in a SN-oriented compressional setting, with active faults composed of SN-trending strike-slip faults (west branch of the Tanlu fault zone) and near EW-trending thrust faults (Zhang-peng fault zone). During the Yanshanian, the NNE-trending normal faults were formed under the WNW–ESE tensile stress field. Since the Himalayan period, the BZ19-6 buried hill has evolved into the rifting stage. In rifting stage Ⅰ, all of the multi-trend pre-existing faults were reactivated, and the EW-trending thrust faults became normal faults due to negative inversion. In rifting stage II, a large number of NE–ENE-trending normal faults were newly formed in the NW–SE-oriented extensional setting, which made the structure pattern more complicated. In rifting stage III, the buried hill entered the post-rift stage, with only part of the NNE- and NE–ENE-trending faults continuously active. Multi-trend faults are the result of the combination of various multi-phase stress fields and pre-existing structures, which have great influence on the formation of tectonic fractures and then control the distribution of high-quality reservoirs in buried hills. The fractures controlled by the NNE- and EW-trending faults have higher density and scale, and fractures controlled by NE–ENE trending faults have stronger connectivity and effectiveness. The superposition of multi-trend faults is the favorable distribution of high-quality reservoirs and the favorable accumulation area of hydrocarbon.

Present-day incipient fault coalescence at a relay zone (Jiloca extensional basin, Spain): Evidence from instrumental seismicity

The relay zones between NW-SE to NNW-SSE striking faults of the Jiloca graben (Iberian Chain) mostly show distributed along-strike fault and fracture patterns. The latter are chiefly controlled by the Late Pliocene-Quaternary regional stress field, and secondarily respond to local controls from inherited structures. Such fracture patterns contrast with the classical models of transverse connecting faults controlled by relay kinematics. North of the Concud fault trace, at the relay zone with the Sierra Palomera fault, an unusually high seismic activity has been noticed since 2014, with magnitudes up to M = 3.5. Upgrading of the National Seismic Network allowed obtaining such new detailed records, while the installation of a new seismometer by the IGN within the study area has improved the reliability of focal depth data since 2017. A high-precision absolute relocation of seismicity from 01/01/2000 to 30/05/2022 has been carried out. The results show that (i) the epicentres are significantly clustered along a nearly N-S trending band, and (ii) the focal depths range from 0 to 14 km, in good agreement with the thickness of the brittle crust. This 3D spatial distribution of seismicity is interpreted as a consequence of activation of either a single fault or a fault zone, nearly vertical and N-S striking. Such structural setting is consistent with the surficial fracture patterns observed at both map and outcrop scale: NNW-SSE and NNE-SSW oriented faults and fractures, orthogonal to the ENE-WSW to ESE-WNW regional σ3 trajectories, together with NW-SE trending ones controlled by inherited contractive faults. The present-day seismic activity suggests that along-strike, incipient fault propagation at the relay zone between the Concud and Sierra Palomera faults is currently operating under the control of the remote stress field.

Normal fault architecture, evolution, and deformation mechanisms in basalts, Húsavik, Iceland: Impact on fluid flow in geothermal reservoirs and seismicity

Faults within layered basaltic sequences significantly influence hydrothermal fluid flow in shallow geothermal reservoirs and potentially during CO2 sequestration and storage. Nevertheless, their characterization regarding fault zone architecture, fluid flow, deformation mechanisms, and seismic potential remains underdeveloped. This study addresses this gap by integrating structural and microstructural observations with X-ray diffraction analyses of exposed normal-transtensional faults associated with the seismically active Húsavík-Flatey Fault in the Tjörnes Fracture Zone, Northern Iceland. Our findings demonstrate that the evolution of basalt-hosted normal-transtensional faults progresses through distinct stages: (1) low-displacement fault propagation from pre-existing cooling joints; (2) fault linkage via dilational jogs; (3) damage zone/fault core growth through brecciation and cataclastic processes; (4) shear localization along sharp slip surfaces; and (5) smearing of volcaniclastic interbeds along the principal fault plane. Evidence of shear localization, truncated clasts, and hydrothermal breccias/veins suggests repeated seismic slip events facilitated by overpressured fluids. Conversely, the presence of clay-rich foliated cataclasite indicates aseismic slips during interseismic periods. Slip along fault jogs, bends, geometric irregularities, and orientation changes causes the dilatant opening of the fault planes and extensional horsetail fractures at fault tips. These structures create main tabular zones for lateral movement of hydrothermal fluids parallel to the fault strike in shallow geothermal reservoirs situated in active extensional-transtensional tectonic settings. In addition, the dilational jogs and the intersection of horsetail veins with the hosting faults may define linear zones of high structural permeability and intense localized fluid flow parallel to the σ2 paleostress orientation and finally mineral precipitation. The results of this study can be utilized to improve models of geothermal fluid flow for enhanced recovery in basaltic reservoirs and assess seismic risk in basaltic faults.

A new example of polyphase basin formation in western Anatolia: Plio-Quaternary Tavşanlı Graben, Kütahya

ABSTRACT The southwestern part of Turkey is called the ‘western Anatolia Extensional Province‘. Today, there is an approximately N-S-oriented extension in this area. The seismically active region is characterized by E-W and N-S-trending grabens. The tectonism and evolution of Tavşanlı Graben, one of these grabens, during the Plio-Quaternary period are the main subjects of this study. The Tavşanlı Graben, about which there is no information in the literature, has been defined for the first time in this study, and the geometrical and kinematic properties, relative ages, and interrelationships of some Neogene-aged faults, especially Quaternary-aged faults, have been presented in the manuscript. The data obtained indicate strike-slip faulting in the late Pliocene in the region. These E-W striking faults and the basin they formed are the first in western Anatolia with these features. In the Quaternary, E-W-oriented normal faults were formed in the region. The asymmetric topography of the graben area is consistent with the other E-W-oriented grabens in western Anatolia. Late Cretaceous basement rocks and Miocene-aged units are observed at the margins of the graben, which are fragmented by faults. The fill of the graben is Quaternary-aged fan deposits and alluvium. In this study, it was determined that the Tavşanlı Basin, which started to form with a transtensional mechanism at the end of the Pliocene, was transformed into the Tavşanlı Graben in the Pleistocene. The basin formation, which started with directional faulting in the late Pliocene, evolved into a graben in the Quaternary, and reached its present form by completing its development in two phases, supports the views that adopt the multi-stage deformation of western Anatolia in the late Cenozoic. In addition to contributing to the literature on this subject, the article will also serve as a basis for future paleoseismological studies.

Slope stability modeling using limit equilibrium and finite element methods: A case study of the Adama City, Northern Main Ethiopian Rift

Slope failure is a prominent and recurring geohazard in numerous parts of Ethiopia, including Adama City, which is located in the Northern Main Ethiopian Rift (NMER). The city is surrounded by two ridges oriented in the NNE-SSW direction, which are susceptible to slope instability. Thus, this study is aimed at modeling slope stability along these two ridges using Finite Element Method (FEM) and Limit Equilibrium Method (LEM). The modeling was carried out on slopes of multifaceted geometry composed of eluvium soil, pumice, and moderately to highly-weathered ignimbrites. Critical slope sections were identified using satellite imagery and field manifestations such as slope toe condition and slope face tilting. Their geometries were then inferred from detailed geological cross-sections based on field data. Input parameters for the modeling, such as cohesion, friction angle, and elastic modulus, were calculated via Back Analysis using the Hoek-Brown criterion while unit weight and Poisson ratio were determined from empirical equations. For soil formations, the parameters were determined via standard laboratory experiments. The modeling was then carried out under different conditions, including dry, saturated, static, and dynamic conditions. Results from both LEM and FEM models revealed that three of the four analyzed slope segments were unstable under dynamic and saturated conditions, highlighting the influence and importance of precipitation and seismicity as triggering variables. Results from both methods tend to agree when the critical slip surface passes through a single geological material in both models. However, notable differences arise when the slip surface involves multiple geological materials. Under such conditions, LEM tends to yield higher FOS values compared to FEM. The results also showed that all unstable slopes were associated with the NNE-SSW striking fault of the study area, as inferred from failure surfaces generated from both models and field data. The study concluded that unstable slopes pose a serious risk to nearby residents and infrastructure, and as a remedy, it designed and recommended coupled benching and slope flattening.

Comment on: Structural analysis of the central Tunisian Atlas: Implications for salt-related structures and conjugate strike-slip faults by Haji et al. (2024)

Based on limited geophysical data (seismic lines and gravity maps), Haji et al. (2024) presented a study on the relationship between deep-seated faults, their strike-slip kinematics and salt tectonism in the Central Tunisian Atlas. The geological structures presented in this paper have been already extensively studied in detail through fieldwork, hydrogeological studies, geophysical and structural analyses (Kadri, 1988; Ben Ayed, 1993; Chihi and Ben Ayed 1991; Khazri and Gabtni, 2015; Creusot and Ouali, 1989; Chihi, 1995; Zouaghi, 2008; Philip et al., 1986; Boukadi and Bedir, 1996; Azaiez et al., 2011; Zidi et al., 2024). The central Tunisian Atlas is mainly characterized by NE-trending folds, E-W striking faults, NW-SE to E-W grabens and some Triassic salt outcrops interpreted as diapirs.After a careful reading of this paper, we find that it is flawed in parts, and has problems of both form and content. Furthermore, this study has several significant scientific problems, including the misuse and misinterpretation of geophysical data, inaccuracies in several sentences and paragraphs, a lack of field data, and lack of proper discussion of previous results. Therefore, our main aim in this comment is to highlight these scientific problems. We briefly discuss some formal and substantive issues that justify our main objections to the paper by Haji et al. (2024) and their inadequate approaches.

Block Rotations in NW Iran in Response to the Arabia‐Eurasia Collision Constrained by Paleomagnetism

AbstractNorthwest Iran is a seismically active region dominated by NW‐SE trending strike‐slip faults, such as the North Tabriz and Qosha Dagh faults, and smaller NNE‐SSW striking faults. The Bozgush Mountains are shaped by these faults and divided into two domains that show a difference in strike. To quantify rotational tectonic deformation in NW Iran, we performed a paleomagnetic study along three transects of the Bozgush and Qosha Dagh Mountains with 127 sites. Our large new paleomagnetic data set shows that the Bozgush Mountains did not rotate as a single rigid block. In the western domain of the Bozgush Mountains, we find evidence for clockwise vertical axis rotations of ∼40°, while the eastern domain has rotated up to ∼80° clockwise. Declinations of the western Bozgush domain fit well with observed declinations in the Qosha Dagh Mountains. Fault patterns show that the eastern domain of the Bozgush Mountains is divided by a set of NNE‐SSW striking sinistral strike‐slip faults, which created domino‐style blocks that accommodated the additional 40° of rotation. We estimate that these extra rotations have resulted in around 4 km of N‐S shortening and more than 1.5 km of differential uplift.

Spatial analysis of gold occurrences in the Zuru Schist Belt, northwest Nigeria

ABSTRACT The Zuru Schist Belt in Northwest Nigeria is one of the auriferous schist belts occurring in the western half of Nigeria. The gold mineralization largely hosted within Neoproterozoic metasedimentary rocks is considered orogenic. In an attempt to understand the pattern of occurrence of gold mineralization, their spatial distribution and association with regional structures, spatial analysis including point pattern, fractal, Fry, and distance distribution analyses were carried out to determine and quantify spatial pattern, spatial association, and regional structural control. Point pattern analysis reveals that the gold occurrences in the Zuru Schist Belt have a clustered pattern. Fractal analysis shows that the structures that controlled the distribution of gold occurrences can be divided into two categories: those that operated at a spatial scale of below 8 km, and those that operated at around 8 km and more. Fry analysis shows that the gold occurrences in the Zuru Schist Belt are aligned in a NNE-NE direction. This is parallel with the NNE-NE striking faults, establishing these faults as the main controlling structures. Fry plot of orientations of translated pairs within an 8 km distance shows that in addition to NNE-NE striking faults, folds with ESE trending axial trace and NW striking faults also exert some control on these known gold occurrences. The influence of these structures on the gold occurrences is further established by the result of distance distribution analysis. The results of spatial analysis using point pattern, fractal, Fry, and distance distribution analyses established the NNE-NE striking faults as the major structures that controlled the distribution of gold occurrences in the belt, and NW striking faults as important structures at a local scale. In addition to NW striking faults, this study further identified folds with axial trace oriented in ESE as additional secondary control structures for gold mineralization in the Zuru Schist Belt.

Rupture Model of the 5 April 2024 Tewksbury, New Jersey, Earthquake Based on Regional Lg-Wave Data

Abstract On 5 April 2024, an earthquake of magnitude 4.8 occurred in Tewksbury, New Jersey. It was the largest instrumentally recorded event since 1900 in New Jersey and southern New York. Millions of people around New York City, ∼65 km east-northeast of Tewksbury, felt the shaking from the mainshock, but the epicentral area experienced no known significant property damages. We determine the focal mechanism, which is oblique faulting, and retrieve the Lg-wave relative source time functions (RSTFs) from the stations at regional distances to understand rupture processes and ground motions. Our fault-slip models well explain azimuthal variations of the RSTFs. The models show the rupture propagating toward the east-northeast (∼50° to 60°), not along the fault strike. The slip distribution on the nodal plane striking north and dipping to the east shows a slip area of 1.1 km radius with the rupture propagating down-dip. The down-dip rupture may account for the observed lack of strong shaking in the epicentral area.

Using Edge Detection Techniques on Aeromagnetic Data to Delineate Structures Controlling Orogenic Gold Deposits in the Sefwi Greenstone Belt, Ghana

ABSTRACT The orogenic-style gold deposits in the Sefwi greenstone belt are associated with quartz veins and occur along shear zones. Edge detection techniques (i.e., analytic signal, vertical, and tilt derivatives) were employed to delineate regional planar fabrics (e.g., shear zones, faults) using aeromagnetic data. The enhancing techniques aided the delineation of NE-SW striking lineaments associated with the Kenyase-Yamfo, Hwidiem, Ketesso, Bibiani, and Afema shear zones in the study area. These shear zones are within the Sefwi belt and at the contact zones between the belt and the adjacent metamorphosed volcano-sedimentary rocks of the Sunyani-Comoé and Kumasi-Afema basins. Low-displacement N-S to NNE-SSW trends which are oblique to the NE-SW shear zones, were also delineated. The local bend and offset along the NE-SW striking faults created by the cross-cutting N-S to NNE-SSW faults serve as favourable trap zones to focus fluids migrating along these shear zones. Fry analysis of known orogenic gold occurrences in the study areas shows that the NE-SW lineaments, with an azimuth of 40°-50°, are predominant in controlling the distribution of deposits. There is also a significant structural control in the N-S to NNE-SSW direction with azimuths 0°-10° and 10°-20°.

Coseismic deformation and seismogenic structure of the 2024 Hualien Earthquake measured by InSAR and GNSS

On April 3, 2024, an M7.3 earthquake occurred in the offshore area of Hualien County, Taiwan, China. The seismogenic structure at the epicentral location was highly complex, and studying this earthquake is paramount for understanding regional fault activity. In this study, we employed ascending and descending orbit Sentinel-1 Synthetic Aperture Radar (SAR) data and utilized differential interferometry (InSAR) technique to obtain the co-seismic deformation field of this event. The line-of-sight deformation field revealed that the main deformation caused by this earthquake was predominantly uplift, with maximum uplift values of approximately 38.8 ​cm and 46.1 ​cm for the ascending and descending orbits, respectively. By integrating the three-dimensional GNSS co-seismic deformation field, we identified the seismogenic fault located in the offshore thrust zone east of Hualien, trending towards the northwest. The fault geometry parameters, obtained through the inversion of an elastic half-space homogeneous model, indicated an optimal fault strike of 196°, a dip angle of 30.9°, and an average strike-slip of 0.4 ​m and dip-slip of −2.6 ​m. This suggests that the predominant motion along the seismogenic fault is thrusting. The distribution of post-seismic Coulomb stress changes revealed that aftershocks mainly occurred in stress-loaded regions. However, stress loading was observed along the northern segment of the Longitudinal Valley Fault, with fewer aftershocks. This highlights the importance of closely monitoring the seismic hazard associated with this fault segment.

High-Resolution 3D Shallow S-Wave Velocity Structure Revealed by Ambient-Noise Double Beamforming with a Dense Array in Guangzhou Urban Area, China

Abstract Shallow velocity structure surveys are very important for urban seismic hazard monitoring and risk assessment. Ambient-noise tomography provides an ideal way to obtain urban fine structure. In this study, we obtained a high-resolution 3D VS model of the metropolitan areas of the Pearl River Delta (PRD) using the ambient-noise double-beamforming method with a dense nodal array. The new model reveals shallow structures that correlate well with surface geological features, with low-velocity anomalies in fault depressions and high-velocity anomalies in fault uplifts. Our findings reveal detailed fault geometries and basin characteristics of the PRD. The Guangzhou–Conghua fault emerges as a prominent velocity boundary, playing a significant role in controlling the development and subsidence of the Longgui basin. The Xinhui–Shiqiao fault and Shougouling fault are identified as major faults that control the formation and evolution of depressions in the PRD. The basin structures in the PRD are classified as semigraben basins controlled by synsedimentary faults. The long axes of the sub-basins align with the strike of the major faults, and the deposit centers are located in close proximity to these faults. Furthermore, our investigation reveals low-velocity anomalies along the faults, suggesting the existence of pre-existing faults facilitating heat transfer and fluid/melt migration from the deep crust. Our results provide new constraints on the geometric structure of the sedimentary basins and fault systems in the PRD area, thereby contributing to urban seismic hazard assessment and offering valuable insights into potential geothermal resources.

Research on the Deformation Mechanism of Railway Subgrade under Buried Strike–Slip Fault Dislocation

With the gradual densification of China’s railway network, more and more railways are inevitably crossing active fault zones. Creep dislocation and stick–slip dislocation of the fault zones can lead to uneven deformation and cracks in the railway subgrade, threatening the sustainability of the railway. Therefore, a model test and numerical simulation were used to analyze the deformation mechanism and sustainability of the subgrade and foundation induced by a strike–slip fault dislocation in a flat site under different angles between the subgrade and fault strikes. The test and simulation results show that (1) under the influence of strike–slip fault dislocation, the horizontal direction of the subgrade appeared to have an S–shaped displacement. The top surface of the subgrade appeared to have a vertical settlement. Further, tension–shear cracks and compression crush zones appeared on the subgrade’s surface. (2) When the angle between the fault and subgrade strikes increased, the subgrade’s uneven deformations induced by fault dislocation first decreased and then increased. The angle between the main tension–shear crack on the top surface of the subgrade and the fault strike showed a trend of increasing and then decreasing. In addition, the compressive stress along the subgrade strike decreased, while the tensile stress along the subgrade strike gradually increased. (3) The tension–shear cracks in the subgrade and foundation were spiral and converged to the basement strike–slip fault along the depth direction. This study provides a reference for the safe operation and sustainability of railways near faults.

Investigation on dynamic mechanism of fault slip and casing deformation during multi-fracturing in shale gas wells

Fracturing horizontal well casing deformation has become very prominent, particularly in tectonic stress-concentrated shale gas fields, limiting the efficient development progress of shale gas. The main failure mode of casing shearing deformation had been attributed to fault slip caused by multi-fracturing. The current research did not provide a clear picture of the dynamic evolution relationship between hydraulic fracturing, fault slip, and casing deformation. In this paper, the dynamic model of fault slip induced by formation pressure change is established, incorporating the effects of stress drop, physical change of friction, and casing and cement-sheath resistance loads. The discontinuous displacement approach and explicit/implicit coupling iteration methods are used to reveal the relationships between the effective normal stress, shear stress, friction coefficient, and sliding velocity during the fault slip process. Furthermore, the microscopic process of casing deformation sheared by fault slip is investigated using static equilibrium theory, and a characterization method for determining the amount casing deformation caused by real-scale fault slip is proposed. The results show that three stages exist in the process of casing deformation sheared by fault slip, including trigger activation stage, accelerated slip stage, and deceleration slip stage. Fault slip is clearly influenced by fault strike. To reduce the amount of fault slip, the fault direction with the maximum in-situ stress should be avoided as much as possible. Serious casing deformation still occurs for large-scale activated faults even though the optimization measure of wellbore structure has been well taken. To fundamentally reduce the possibility of casing shear deformation, it is necessary to prevent fault slip through optimizing the design of hydraulic fracturing. This study lays the theoretical groundwork for the casing deformation control method in shale gas wells.

Directional amplification across the San Jacinto fault zone, CA

SUMMARY The amplitude, frequency and polarization of ground motion at the surface can be affected by the local geology. While low-velocity sediments and fill can amplify ground motions in certain frequency ranges, the low velocities found in fault zones can also produce prominent wavelets. In this paper, we provide further evidence that polarization of ground motion can be affected by the geological fabric in fault zones that have sustained significant brittle deformation. Aside from the well-known effect of fault-trapped waves in the low-velocity zone with polarization azimuths parallel to the fault strike, the effect of stiffness anisotropy was recently recognized with polarization azimuths at high-angle to the fault strike and orthogonal to the locally predominant fracture field in the fault damage zone. To clarify further such features, we investigate directional amplification effects across the San Jacinto fault zone in Southern California using seismic data recorded by permanent seismic stations and dense across-fault arrays. We observe three main polarization trends. The first trend parallel to the fault strike is ascribed to fault-trapped waves along the low-velocity zone, in agreement with several studies in the last decade in the same region. The second and third trends are orthogonal to the orientation of R and T Riedel planes, respectively. They are related to the stiffness anisotropy in densely fractured rocks in the damage zone, which are more compliant orthogonal to their fractures. At some locations the two effects are superimposed, occurring in different and distinct frequency ranges. Directional amplification at rock sites can be important for expected ground motion and seismic hazard. However, in seismic engineering the current prescriptions of seismic codes do not account for amplification effects at rock sites at frequencies of engineering interest.

Quantifying fault interpretation uncertainties and their impact on fault seal and seismic hazard analysis

Fault-horizon cut-off data extracted from seismic reflection datasets are used to study normal fault geometry, displacement distribution, and growth history. We assess the influence of three seismic interpretation factors (repeatability, measurement obliquity, and fault cut-off type) on fault parameter uncertainty. Two repeat interpretations resulted in mean differences of 5–15% for throw, 11–42% for heave, 9–31% for displacement, and 7–27% for dip across faults. Measurement obliquity, where faults are interpreted using non-perpendicular transects to fault strike, show increasing uncertainty with increasing obliquity. Uncertainty in throw is 14–24% at obliquities >20° and 6–13% where obliquities <20°. Continuous cut-offs, including non-discrete deformation, generally exhibit greater uncertainties compared to discontinuous (discrete) cut-offs. We consider the effect of interpretation factors on fault parameters used in seismic hazard assessment (SHA) and fault seal, using the established Shale Gouge Ratio (SGR). Even modest measurement obliquities and repeatability errors can affect inputs for SHA, causing large differences in throw- or slip-rate and inferred fault length. Measurement obliquity and repeatability have a variable impact on SGR calculations, highlighting the additional importance of sedimentary layer thickness and distribution. Our findings raise questions about the optimum workflow used to interpret faults and how uncertainties in fault interpretation are constrained and reported.

Evaluating the Relation of Cave Passage Formation to Stress-Field: Spatio-Temporal Correlation of Speleogenesis with Active Tectonics in Asprorema Cave (Mt. Pinovo, Greece)

Caves serve as time capsules, preserving significant markers of tectonic activity and offering insights into geological history. Fault geometries and past activations found in caves can be correlated with known deformational events in the broader area, temporally delimiting the speleogenesis. More specifically, cave passage formation is suggested to be affected by the regional stress-field. The Asprorema Cave in Northern Greece is a typical example of a fracture guided cave, with passage geometry influenced by relative sidewall movements, revealing these discontinuities as faults. This study constructs the timeframe and conceptual model of speleogenesis in relation to tectonic events, geomorphological evolution and hydrological zones, and verifies its relation to the stress-field. Active tectonics, mineralogy and cave geomorphology are investigated. Results suggest syntectonic speleogenesis under phreatic and epiphreatic conditions. The absence of corrosion on fault slip surfaces implies recent activations post cave’s shift to the vadose zone. Structural analysis identifies three main neotectonic phases: NNW-SSE striking faults (oldest group of structures), NE-SW striking faults with dextral strike-slip movement (post-middle Miocene), and NE-SW striking normal faults indicating extensional stress-regime (Quartenary). The last two phases affect cave passage shape causing wall displacement, highlighting passage formation along discontinuities perpendicular to the horizontal minimum stress axis.