Fault Segments Research Articles

New Evidence of Holocene Faulting Activity and Strike-Slip Rate of the Eastern Segment of the Sunan–Qilian Fault from UAV-Based Photogrammetry and Radiocarbon Dating, NE Tibetan Plateau

The eastern segment of the Sunan-Qilian Fault (ES-SQF) is located within the seismic gap between the 1927 M8.0 Gulang earthquake and the 1932 M7.6 Changma earthquake in China. It also aligns with the extension direction of the largest surface rupture zone associated with the 2022 Mw6.7 Menyuan earthquake. Understanding the activity parameters of this fault is essential for interpreting strain distribution patterns in the central–western segment of the Qilian–Haiyuan fault zone, located along the northeastern margin of the Tibetan Plateau, and for evaluating the seismic hazards in the region. High-resolution Google Earth satellite imagery and UAV (Unmanned Aerial Vehicle)-based photogrammetry provide favorable conditions for detailed mapping and the study of typical landforms along the ES-SQF. Combined with field geological surveys, the ES-SQF is identified as a continuous, singular-fault structure extending approximately 68 km in length. The fault trends in the WNW direction and along its trace, distinctive features, such as ridges, gullies, and terraces, show clear evidence of synchronous left lateral displacement. This study investigates the Qingsha River and the Dongzhong River. High-resolution digital elevation models (DEMs) derived from UAV imagery were used to conduct a detailed mapping of faulted landforms. An analysis of stripping trench profiles and radiocarbon dating of collected samples indicates that the most recent surface-rupturing seismic event in the area occurred between 3500 and 2328 y BP, pointing to the existence of an active fault from the Holocene epoch. Using the LaDiCaoz program to restore and measure displaced terraces at the study site, combined with geomorphological sample collection and testing, we estimated the fault’s slip rate since the Holocene to be approximately 2.0 ± 0.3 mm/y. Therefore, the ES-SQF plays a critical role in strain distribution across the central–western segment of the Qilian–Haiyuan fault zone. Together with the Tuolaishan fault, it accommodates and dissipates the left lateral shear deformation in this region. Based on the slip rate and the elapsed time since the last event, it is estimated that a seismic moment equivalent to Mw 7.5 has been accumulated on the ES-SQF. Additionally, with the significant Coulomb stress loading on the ES-SQF caused by the 2016 Mw 5.9 and 2022 Mw 6.7 Menyuan earthquakes, there is a potential for large earthquakes to occur in the future. Our results also indicate that high-resolution remote sensing imagery can facilitate detailed studies of active tectonics.

The Unusual Seismic Activity from 2021 to 2024 in Eastern Austria: Insights from Seismic Sequences Relocation and Moment Tensor Inversion

ABSTRACT From 2021 to 2024, unusually strong seismic activity, including multiple sequences with five 3.8≤ML≤4.6 shocks, struck the Vienna basin and Mur-Mürz fault systems in Eastern Austria. Three earthquakes (ML 4.6, 4.5, and 3.8) occurred between March and May 2021. These earthquakes were followed by an increase in seismic activity in March 2023, with an ML 4.3 earthquake to the southwest of the 2021 events. In February 2024, an ML 4.5 earthquake occurred 10 km southwest of the ML 4.3 in 2023. We investigate activated faults relying on nonlinear relocations of the seismic sequences and probabilistic moment tensor inversion. The seismic sequences’ temporal evolution indicates a migration of seismic activity and a decrease of focal depths from northeast to southwest, suggesting a potential reactivation of the major fault systems in the region. The joint interpretation of the results shows distinct clusters along fault segments and focal mechanisms that match the region’s complex tectonic activity.

Interpretation of a 3D Magnetotellurics Model of the Aceh and Seulimeum Segments of the Sumatran Fault Zone

The Sumatran Fault runs from the southeast (SE) to the northwest (NW) of Sumatra Island, with the highest slip rates reaching about 3.0 cm per year in the northwestern part. There is a seismic gap along this fault, including the northern Aceh domain, which consists of the Aceh and Seulimeum fault segments. Previous studies have used various methods to investigate the Sumatran Fault system, including seismic, geoelectric, gravity anomaly, and magnetotellurics (MT). The MT method has proven advantageous as it can non-destructively image a wide range of depths. However, previous studies using the two-dimensional (2D) MT inversion did not represent realistic information of the subsurface conditions. Therefore, a three-dimensional (3D) MT data inversion was used in this study to obtain more realistic information about the resistivity structure of the Aceh and Seulimeum segments. The results confirmed that the Sumatran Fault is a strike-slip fault, with a relatively northwest (NW)–southeast (SE) direction of conductivity strike with an angle of S 71.61° E from Groom–Bailey decomposition of MT data. The 3D resistivity distribution model from 33 stations showed that the Aceh Fault Segment is 20–30 km away, while the Seulimeum Fault Segment is 55–60 km away based on the MT data. The results also indicated a creeping zone at a depth of 2 km beneath the Aceh Fault Segment. Different rock formations were identified beneath the fault system, with the western part of the Aceh Segment dominated by high-resistivity metamorphic rocks (150–1000 Ωm) from the Triassic–Cretaceous age. The zone between the Aceh and Seulimeum fault segments exhibited low resistivity, characterized by volcanic rocks (1–15 Ωm) from the Lam Teuba Volcanic Formation and the Indrapuri Formation. Beneath the eastern part of the Seulimeum Fault Segment was found to consist of low-resistivity quaternary volcanic rocks (1–15 Ωm) and high-resistivity andesite rocks (4.5 × 104–1.7 × 105 Ωm). These findings correlated well with the geological map.

3D geological fine modeling and dynamic updating method of fault slope in open-pit coal mine

Combining the requirements for refined modeling and dynamic update of fault slope geological models in open-pit coal mines, we systematically elaborated on the elements and rules of slope 3D geological modeling and proposed a fine modeling and dynamic updating method based on digital elevation model (DEM) and half-edge boundary representation (B-Rep) data structures. Initially, the stratigraphic division of the study area is conducted based on borehole and section data, combined with the geological evolution history and the level set theory. Applying inverse distance weighting (IDW) for estimation, the geological interface triangular irregular network (TIN) is constructed using the TIN algorithm. Secondly, fine reconstruction of fault surfaces through Morphing transformation improves the accuracy and smoothness of fault surface shape reconstruction under sparse data conditions. Then, the intersection-cutting processing method of fault-fault and fault-stratum for slope geological ground state model modeling under complex fault segmentation is investigated. Finally, constructing 3D geological models of fault slopes and the integrated correction of surface and subsurface are realized through dynamic updating methods for 3D surfaces and strata. Taking the 3D geological modeling of a fault slope of an open-pit coal mine in China as an example, the feasibility of the technical method is verified.

The Inherited Crustal Structure and Lithospheric Thermal Field Beneath the Sea of Marmara (NW Türkiye): Observations From 3D Gravity Modeling and Seismic Tomography Analysis

AbstractThe North Anatolian Fault (NAF) extends for over 1,000 km across Türkiye and poses significant seismic hazard in the region. The Main Marmara Fault (MMF) segment of the NAF in the Sea of Marmara (NW Türkiye), exhibits along‐strike segmentation in its interseismic strain accumulation. Constraining the lithospheric configuration below the MMF is critical to understand its segmentation and assessing seismic hazard in the area. We present a new 3D lithospheric‐scale density of the Sea of Marmara, that combines gravity modeling and seismic tomography analysis. Using forward and inverse gravity modeling with free‐air gravity data and available constraints of geological units we derived the intra‐crustal density structure. Shear‐wave velocity tomography models provided insights into the temperature and density configuration of the uppermost mantle, and the geometry of the 1330°C isotherm. Our results highlight significant crustal density variations: lower‐density crust in the Sakarya Zone and Strandja Massif, and denser crust below the Istanbul Zone, which overlies a relatively hotter lithospheric mantle. This lithospheric configuration reflects both ongoing tectonic processes and inheritance from past geological events, including the drifting of the Istanbul Zone crustal block and the signature of past subduction events. The extent of the Istanbul Zone denser crust spatially correlates with the locked segment of the MMF. The bimaterial nature of the fault segment likely influences its interseismic and coseismic behavior. The denser, stiffer Istanbul Zone crust would promote interseismically locked conditions in contrast to the adjacent, more compliant crustal block and could result in asymmetric rupture with a preferred directivity.

Imaging the Devene fault system beneath the Iskar floodplain in Bulgaria through shallow electrical resistivity profiling

We studied the Nivyanin fault segment from the Devene fault system in NW Bulgaria, using shallow electrical resistivity profiling. The measured profile was located on the Iskar floodplain near Chomakovtsi village and crossed a lineament that connects the northernmost exposures of Upper Cretaceous limestone and Miocene conglomerate on elevated surfaces in which the Iskar valley was incised. The results suggest that the studied fault segment was a strike-slip fault that was active after the deposition during the Miocene. The fault zone is broad over 100 m and consists of sub-vertical fractures and breccia. Vertical fault displacement did not affect the Quaternary fluvial deposit.

Seismic and outcrop-based 3D characterization of fault damage zones in sandstones, Rio do Peixe Basin, Brazil

Fault damage zones, composed of sub-seismic deformation structures, are difficult to detect using seismic data. Still, they can be related to fault throw, which is widely measured in the subsurface. This research employs a multiscale approach that integrates outcrop studies with seismic reflection data to investigate the attributes of fault damage zones affecting porous sandstones. We provide an in-depth understanding of the 3D fault zone volume, investigating correlations between geometric attributes of faults (width of damage zone, frequency of subsidiaries structures, and fault termination) in the outcrop and the fault throw in the subsurface, with insights from deformation mechanisms within the fault zone. The methods encompass 1D scanlines to constrain the damage zone width on the outcrop. At the same time, the subsurface analysis uses 3D seismic data, seismic attributes, and deep learning neural network (DNN) fault volumes to interpret fault geometries and quantify fault throw at different depths. Results show that the area presents a complex fault zone with multiple fault sets, deformation bands, and fracture corridors trending mainly NE-SW, NW-SE, and E-W. The integration of surface and subsurface fault data enabled the identification of two portions in each fault set outcropping at the fault tip for E-W/ESE-WNW-striking faults and the central part of the fault for NE-SW-striking faults. Faults with the greatest length do not outcrop with the largest damage zone width since they are outcropping the tip of the fault. The parallel faults overlap their damage zones, increasing the deformation zones in the affected sandstones. Fault throw and damage zone width present a positive correlation. This relation is affected by fault segments and subsidiary faults.

Optimization of GGMplus Gravity Data to Identify Sumatran Faults Segments in Kaba Stratovolcano, Bengkulu, Revealed by FHD and SVD Techniques

Abstract Kaba volcano is a perilous and currently active volcanic site located near the Sumatran fault, specifically within the Kepahiyang region of Bengkulu. Given the intricate nature of its location, it is crucial to monitor the local fault zone activity in the vicinity of the Kaba volcano. meanwhile, these fault zones are typically associated with high permeability areas and are characterized by high-density contrasts. Therefore, we applied First Horizontal Derivative (FHD) and Second Vertical Derivative (SVD) methods to identify the presence of the Musi Kepahiyang segment in Bengkulu, using GGMplus high resolution gravity data. Based on the results of the FHD analysis, clear gravity anomalies are observed along the northwest (NW) and southeast (SE) regions of Kepahiyang, with Bouguer anomaly values reaching 800 mGal. The discernible patterns unveiled through FHD analysis distinctly delineate the Musi fault (NW), Kepahiyang fault (SW), and Garba fault, unveiling a rich tapestry of tectonic activity surrounding the Kepahiyang area in Bengkulu. Complementing these findings, SVD analysis reveals a consistent anomaly distribution, albeit with marginally diminished Bouguer anomaly values, affirming the robustness of the detected features. Through the fusion of FHD and SVD methodologies, our study offers an understanding of the structural complexities pervading the segment in Kaba Stratovolcano, shedding light on its dynamic geological evolution, and fortifying our comprehension of fault dynamics in the Sumatra region.

Injection-induced seismic moment in layered rock formations

Appropriate estimation of seismic moment release during fluid injection is critical to mitigate the risk of induced seismic hazards and to guide safe operation in the geo-energy industry. However, the present single-layer models overlook the contributions of fault slip in different rock layers to the seismic moment release. Here we report an analytical model incorporating a multiple-layer function to predict the injection-induced seismic moment in layered rock formations. This model is established based on fault slip triggered by aseismic motion, particularly considering the locations of aseismic slip front and fluid diffusion front on a seismogenic fault in relative to the layer interface. We compare the maximum seismic moment obtained from our model to those estimated from three single-layer models and those measured from eleven field cases. The results highlight possible underestimation of the maximum seismic moment using the single-layer models in the layered rock formations. We also emphasize that a proper selection of layer model is significant to reasonably assess the seismic moment release. Lastly, we apply both the single-layer and double-layer models to predict the maximum seismic moment of the 5 February 2019 Mw 4.0 earthquake in the Weiyuan Shale Gas Field in Sichuan, China. The engineering application further confirms the necessity of using the double-layer model in the layered rock formations. Additionally, the assessment of amplification factors for fault segments provides a better understanding of induced seismic hazards in different rock layers and possibly guides the safety threshold of injection rate during fluid injection.

Poroelastic effects on rupture propagation across fault stepovers

The role of poroelasticity in influencing the frequency of ruptures jumping through strike-slip stepovers remains unclear. To understand how poroelastic effects govern long-term rupture behavior in strike-slip fault systems with stepovers, we conduct earthquake sequence simulations incorporating undrained pore pressure responses across the full spectrum of Skempton's coefficient. Our findings reveal that Skempton's coefficient significantly affects the effective normal stress, which can either cause fault clamping or unclamping, and ultimately influences rupture propagation across fault stepovers. The likelihood of rupture jumping is predominantly determined by Skempton's coefficient and the width of the stepover, with Skempton's coefficient showing an approximately linear relationship to the critical jumpable step size. Specifically, a higher Skempton's coefficient facilitates rupture jumping across fault segments, even over larger stepover distances. Analytical solutions involving dislocation and Skempton's coefficient provide practical methods for evaluating pore pressure changes and associated seismic hazards near fault stepovers. Our statistical analysis identifies a critical jumpable width of 4.4–5.1 km due to static stress transfer, assuming a typical range of Skempton's coefficient for compressional stepovers, beyond which ruptures are unlikely to propagate. This study underscores the potential of using physics-based earthquake sequence models to reflect statistical fault rupture behaviors. Given that multi-segment earthquake ruptures present challenges in assessing maximum rupture lengths, our findings offer crucial insights into the role of poroelastic effects and the conditions that facilitate or limit rupture propagation across fault stepovers.

Contrasting landslides distribution patterns and seismic rupture processes of 2014 Jinggu and Ludian earthquakes, China

The destructiveness of earthquakes is often linked to their magnitude, but two similar-magnitude earthquakes in Yunnan, China in 2014 caused vastly different damage. The Ms 6.6 Jinggu earthquake triggered about 441 landslides, while the Ms 6.5 Ludian earthquake caused 10,559 landslides. In this paper, we focus on the correlations between distribution pattern of these landslides and seismic factors like seismogenic fault and epicenter. The results show that the landslides triggered by the Jinggu earthquake are mainly distributed along the blind northwest segment of the NW-striking Puwen fault, with more scattered and larger individual areas to the northwest of the epicenter, and more concentrated but smaller ones to the southeast. The distribution pattern of landslides triggered by the Ludian earthquake is predominantly controlled by the NW-striking Xiaohe-Baogunao fault. The landslides are concentrated within an area of 360 km2, with a greater number and larger size occurring in the southeastern section compared to the northwestern section. The distribution of coseismic landslides reveals key differences in rupture characteristics between the Jinggu and Ludian earthquakes. The Jinggu rupture mainly propagated deeper southeast, while the Ludian rupture in the southeast was shallower, reaching the surface. This was confirmed by aftershock data and field investigations. The larger rupture angle in the Ludian earthquake concentrated damage energy, causing more numerous and larger landslides. The contrasting rupture processes are a major factor behind the differing damage levels of these two similar-magnitude earthquakes.

Spatial landscape response to active tectonics along the Western Mae Chan Fault, Northern Thailand

Mae Chan Fault (MCF) is one of the active faults in northern Thailand, Lao PDR, and Myanmar. Despite fault activeness, the records of recent earthquakes show a lower frequency and magnitude of earthquakes across the MCF than those of the surrounding regions. The morphotectonic analysis evaluates relative tectonic activity based on the drainage systems’ adjustment and geomorphic expressions. We analyzed the Western Mae Chan Fault (WMCF) to explore the spatial response of landscape to relative rock uplift and long-term deformation. We delineated fault segment lines from the remote sensing and the topographic expressions from the digital elevation model. We analyzed the topographic variations from 70 low-order channels developed along the WMCF and evaluated the tectonic activity using geomorphic indices, including the hypsometric integral and hypsometric curve, the standardized stream length-gradient index, the elongation ratio, the drainage basin asymmetrical factor, the mountain front sinuosity, and the valley floor width-to-height ratio. Combining these geomorphic factors with field observations provides the relative indices of active tectonics (RIAT). Our findings revealed that lineament detection from the remote sensing technique indicated the ENE-WSW direction of fault segments. The overall tectonic activity is relatively moderate, but high tectonic activity is present along the western terrain and high topography in the east. The various tectonic activities are consistent with the spatial variations in lineament density and differential rock uplift, where the terrain experienced spatial variations in transtensional and transpressional regimes. This study highlights the potential long-term tectonic activity along the WMCF that dominantly sculpts the modern landscape.

Statistical and source characterization of the 2023 Kahramanmaraş Türkiye earthquake sequence

AbstractWe studied seismic features of the 2023 Kahramanmaraş Türkiye earthquake sequence by analyzing the spatiotemporal behavior of the b- and -p values and source characteristics of the mainshocks. We complemented our study by determining the regional stress field. The b-values in the region varied from 0.45 to 1.15. We observed a slight b-value decrease (Δb≈0.2) months before the two mainshocks. Our results exhibited complex b-value patterns on the fault planes and regular aftershock productivity rates (1.14 < p < 1.25). We compare static stress drop estimates derived from effective fault dimensions to those of finite-fault dimensions. Total effective stress drops (the sum of the stress drops of all fault segments) for the earthquake doublet were almost identical (~ 2.05 MPa), while those from finite-fault dimensions are somewhat lower (0.35 and 0.96 MPa). Based on a complete stress drop case, effective seismic efficiency was 0.65 and 0.43 for both mainshocks. The amount of partial stress drop was used to discriminate between different stress drop models. No clear model is discerned for the first mainshock, but a partial or even complete stress drop, assuming finite-fault dimensions, is supported by our measurements. Stress drop estimations derived from spectral analysis (25.46 and 34.39 MPa) agreed with global studies but larger than finite and effective fault estimates. Stress inversion results indicated that in the fault segments where the mainshocks occurred, the orientation of principal axes was consistent with a strike-slip regime. Conversely, the normal-faulting regime dominates adjacent areas of the main fault system.

The Seismic Surface Rupture Zone in the Western Segment of the Northern Margin Fault of the Hami Basin and Its Causal Interpretation, Eastern Tianshan

The Eastern Tianshan region, influenced by the far-field effect of northward compression and expansion of the Qinghai-Xizang block, features highly developed Late Quaternary active faults that exhibit significant neotectonic activity. Historically, the Barkol-Yiwu Basin, located to the north of the Eastern Tianshan, experienced two major earthquakes in 1842 and 1914, each with a magnitude of M71/2. In contrast, the Hami Basin on the southern margin of the Eastern Tianshan has no historical records of any major earthquakes, and its seismic potential, mechanisms, and future earthquake hazards remain unclear. Based on satellite image interpretation and field surveys, this study identified a relatively recent and well-preserved seismic surface rupture zone with good continuity in the Liushugou area of the western segment of the Northern Margin Fault of the Hami Basin (HMNF), which is the seismogenic structure responsible for the rupture. The surface rupture zone originates at Kekejin in the east, extends intermittently westward through Daipuseke Bulake and Liushugou, and terminates at Wuzun Bulake, with a total length of approximately 21 km. The rupture zone traverses the youngest geomorphic surface units, such as river beds or floodplains and first-order terraces (platforms), and is characterized by a series of single or multiple reverse fault scarps. The morphology of fault scarps is clear, presenting a light soil color with heights ranging from 0.15 m to 2.13 m and an average displacement of 0.56 m, suggesting that this surface rupture zone likely represents the most recent seismic event. Comparison with historical earthquake records in the Eastern Tianshan region suggests that the rupture zone may have been formed simultaneously with the Xiongkuer rupture zone by the 1842 M71/2 earthquake along the boundary faults on both sides of the Barkol Mountains, exhibiting a flower-like structural pattern. Alternatively, it might represent a separate, unrecorded seismic event occurring shortly after the 1842 earthquake. The estimated magnitude of the associated earthquake is about 6.6~6.9. Given that surface-rupturing earthquakes have already occurred in the western segment, the study indicates that the Erdaogou–Nanshankou section of the HMNF has surpassed the average recurrence interval for major earthquakes, indicating a potential future earthquake hazard.

Faulting by the 2023 great earthquakes of Türkiye and associated stress field and its effects on built environment

BackgroundFaulting induced by earthquakes and its analysis are of great importance for areas with high probability of earthquakes. However, there has been no particularly effective methods to evaluate the damage and applicable solutions for recovery. In this paper, the damage from Türkiye earthquakes is investigated by authors, which has important theoretical significance and practical value. On 6 February, 2023, two successive earthquakes with magnitudes (Mw) of 7.8 (called Pazarcık earthquake) and 7.6 (Ekinözü earthquake) occurred in the south-eastern part of Türkiye. The total length of the surface ruptures was more than 500 km long and resulted in striations reflecting the sinistral faulting and extensive ground deformations. In this study, the authors present the outcomes of the investigations on the surface ruptures, their characteristics, stress field associated with both earthquakes, shear strength property of fault segments and the damaging effects on various structures and made some recommendations with the purpose of how to build structures in active fault zones and decrease the negative effects of faulting on structures.ResultsBesides summarizing the main characteristics of the world-shaking Kahramanmaraş earthquake doublet in southeast Türkiye in 2023, this study described the main features of the surface ruptures, their relation to the inferred crustal stresses in Türkiye, and the damaging effects on the major engineering structures. The observations and inferences are of great significance for understanding the causes of earthquakes and future seismic risk assessment. Various laboratory experiments were performed on the samples of fault gouge gathered from the sites of surface ruptures and these experimental results provided very valuable quantitative information on the constitutive models of the fault zones. The observations clearly showed that it is almost impossible to prevent damage on structures due to surface ruptures, if certain engineering principles such as increasing higher ductility, lowering gravitational center and/or the implementations of raft foundations are followed.ConclusionsThe stress state inferences obtained from the striation of the fault surface ruptures as well as from the focal plane solutions are expected to be useful to evaluate the regional stress state of the earthquake region. Assessments indicated that the stress states in Arabian plate and Anadolu platelet are different from each other. However, in-situ direct stress measurement techniques would be quite useful to validate the stress state inferred in this study. The laboratory experiments on samples gathered from the fault outcrops of the earthquakes using direct shear tests, stick-slip tests as well as conventional tensile, compressive and triaxial tests provided the quantitative values for the parameters of constitutive laws for fault zones, which can be utilized in the numerical simulation of earthquakes. If a fault break happens to be just passing underneath the structures, it is almost impossible for mankind to prevent the damage to structures. However, the authors made some recommendations to reduce the negative effects of fault ruptures on structures. These recommendations are such that the structures should be built as ductile and redundant structures with lower center of gravity and raft foundations. Dam construction should be on active faults should be avoided. If they are to be built for whatever reason, they should be of rock-fill type. As tubular structures and tunnels would be generally subjected forced displacement field, it is recommended to utilize flexible joints, segmented and enlargement to deal such displacement fields.

Seismotectonic map of the Sinai Triple Junction

The geodynamic evolution of the Sinai Triple Junction, a highly deformed and seismically active area, is controlled by the Red Sea rift, Gulf of Suez and Aqaba-Dead Sea conjunctions. However, the driving forces for the focusing deformation at crustal depths beneath this area are still ambiguous. Here, we provide an updated seismotectonic map of the area relying on updated seismological and geodetic datasets. A homogenized earthquake catalog has been compiled from well-located earthquakes (> Mw 2.0) by the Egyptian Seismic Network and International Seismological Center in the period between 1990 and 2020. We calculated the average b-value along three seismogenic zones including Gulf of Aqaba, northern Red Sea and Gulf of Suez that amount to 1.1, 0.99 and 0.97, respectively. Additionally, we complied and updated a comprehensive P-wave-based database for the fault plane solutions in the area for events with Mw > 3.5 till 2023. Furthermore, a unified velocity field for the region as well as slip-rate and locking-depth at the active fault segments were estimated from a consistent geodetic dataset from peer-reviewed GPS velocities between 1999 and 2018. Results indicate a dominant NNE left-lateral strike-slip fault with normal component along the Gulf of Aqaba. Pure NW-SE to WNW-ESE dip-slip normal faulting, associated with a strike-slip component in some cases, is dominating the northern and central parts of the Gulf of Suez, whereas pure normal dip-slip movement with an NNE–SSW extension in a horizontal direction is observed in the southern part of the gulf. The estimated slip-rate and locking-depths at the Aqaba fault segments falls between 4.8 and 4.9 mm/yr and 8–12 km, respectively.

Characterization of a deep-water turbiditic reservoir under an active hydrodynamic regime (Niger delta)

The Egina Field, situated in the deep-water region of the eastern Niger Delta, is characterized by a NE-SW trending anticline that contains several Middle to Upper Miocene stacked reservoirs. Notably, one of the reservoirs in the field includes three sandy intervals, which have been identified as deep marine turbidite systems. The lowest level (Interval 1) comprises a lobe complex, while the uppermost Intervals 2 and 3 consist of erosive channel complexes. These three reservoir intervals exhibit both static and partial dynamic communication. The Egina Field is affected by a coherent network of extensional and oblique-slip faults, with current activity confined to the eastern sector of the structure. Large displacement faults (throw >30m) offset individual reservoir intervals, creating lateral disconnection and acting as barriers to fluid flow across faults. Minor NNE-SSW oriented faults can locally baffle the flow of fluids, as evidenced by well tests and 4D seismic data. Nonetheless, well interference tests suggest good communication within the reservoir. Vertical communication between the stacked sand bodies is facilitated through erosional contacts between reservoir intervals, while lateral communication occurs around segmented faults, via relay ramps and fault tips, in a NE-SW direction. Across the field, ten oil-water contacts have been identified, with a general trend of deepening towards the south. This pattern has been interpreted as a tilted contact caused by an active hydrodynamic regime, where aquifer overpressures gradually decrease from north to south (31 m of difference), aligning with the regional trend. This paper describes how the geological (structural and sedimentological) heterogeneities are paramount in deep-water turbiditic reservoirs, for both static and dynamic conditions. The work presented here shows how the geological analysis can impact reservoir management, driving strategic decisions on field development, as such as the identification of infill wells opportunities or the optimizing the placement of injectors wells, to effectively support producers.

Multidisciplinary high resolution Geophysical Imaging of Pantano Ripa Rossa Segment of the Irpinia Fault (Southern Italy)

The Irpinia Fault, also known as the Monte Marzano Fault System, located in the Southern Apennines (Italy), is one of the most seismically active structures in the Mediterranean. It is the source of the 1980, Ms 6.9, multi-segment rupture earthquake that caused significant damage and nearly 3,000 casualties. Paleoseismological surveys indicate that this structure has generated at least four Mw ~ 7 surface-rupturing earthquakes in the past 2 ka. This paper presents a comprehensive, high-resolution geophysical investigation focused on the southernmost fault segment of the Monte Marzano Fault System, i.e., the Pantano-Ripa Rossa Fault, outcropping within the Pantano di San Gregorio Magno intramontane basin. The project, named TEst Site IRpinia fAult (TESIRA), was supported by the University of Napoli Federico II to study the near-surface structure of this intra-basin fault splay that repeatedly ruptured co-seismically in the past thousands of years. Our imaging approach included 2D and 3D electrical and seismic surveys, gravimetry, 3D FullWaver electrical tomography, drone-borne GPR and magnetic surveys, and CO2 soil flux assessment across the surface rupture. This multidisciplinary investigation improved our understanding of the basin shallow structure, providing an image of a rather complex subsurface fault and basin geometry. Seismic data suggest that fault activity at the Pantano segment of MMFS is characterized by a near-surface cumulative displacement greater than previous estimations, calling into question earlier assumptions about the timing of its activation. Despite some challenges with our drone-mounted survey equipment, the integrated dataset provides a comprehensive and reliable image of the subsurface structure. This work demonstrates the utility of developing an integrated approach at high-resolution geophysical imaging and interpretation of fault zones with weak morphological expressions.

Exploring frictional properties of upper plate fault reactivation in subduction zones: The Atacama Fault System in northern Chile

The Maule 2010 and Tohoku-Oki 2011 earthquakes demonstrated how dormant upper plate faults can be reactivated as normal faults by plate margin relaxation following megathrust slip. However, the reactivation mechanisms of these types of faults are yet unexplored. To provide a better understanding of these mechanisms, we collected fault core samples from fault segments of the Atacama Fault System in northern Chile. The sampled fault segments have clear morphological evidence of Quaternary reactivation as normal fault. We performed laboratory experiments to measure the fault strength, stability and dynamic weakening. We consider low-velocity tests for exploring the frictional strength and velocity dependence of friction via a double-direct shear apparatus and ii) high-velocity tests for investigating the frictional properties at seismic velocities via a rotary shear apparatus. The experiments revealed that fault cores have low frictional strength, velocity-strengthening behaviour and strong dynamic weakening. Additionally, a novel experimental procedure that simulates stress relaxation by stepwise reducing of the normal stress on the sample assembly showed: 1. Accelerating creep towards dynamic weakening in chlorite-rich gouges and 2. oscillatory sliding in fault gouges enriched in illite. By extrapolating our experimental observations to natural conditions, we conclude that stable sliding is favoured during the interseismic phase of the subduction earthquake cycle, whereas unstable sliding is favoured during the coseismic and postseismic phases. The latter occurs via normal stress reduction during the shift from interseismic compression to co- and postseismic tension at the plate margin.

Slip History, Tectonic Evolution, and Fault Zone Structure Along the Southern Alpine Fault, New Zealand

AbstractThe study of active fault zones is fundamental to understanding both long‐term tectonics and short‐term earthquake behavior. Here, we integrate lidar‐enabled geomorphic‐geologic mapping and petrochronological analysis to reveal the slip‐history, tectonic evolution, and structure of the southern Alpine Fault in New Zealand. New petrographic, zircon U‐Pb and zircon trace‐element data from fault‐displaced basement units provides constraint on ∼70–90 km of right‐lateral displacement on the presently active strand of the southern Alpine Fault, which we infer is of Plio‐Quaternary age. This incremental displacement has accumulated while the offshore part of the fault has evolved within a distributed zone of plate boundary deformation. We hypothesize that pre‐existing faults in the continental crust of the Pacific Plate have been exploited as components of this distributed plate boundary system. Along the onshore southern Alpine Fault, detailed mapping of active fault traces reveals complexity in geomorphic fault expression. Our analysis suggests that the major geomorphic features of the southern Alpine Fault correspond to penetrative fault zone structures. We emphasize the region immediately south of the central‐southern section boundary, where a major extensional stepover and restraining bend are located along‐strike of each other. We infer that this geometry may reflect segmentation of the Alpine Fault between two distinct fault segments. The ends of these proposed segments meet near where several Holocene earthquake ruptures have terminated. Our new constraints on the evolution and structure of the southern Alpine Fault help contribute to improved characterization of the greatest onshore source of earthquake hazard in New Zealand.