Dip-slip Faults Research Articles

Fragility of Highway Embankments Exposed to Permanent Deformations Due to Underlying Fault Rupture

Seismic risk expresses the expected degree of damage and loss following a catastrophic event. An efficient tool for assessing the seismic risk of embankments is fragility curves. This research investigates the influence of embankment’s geometry, the depth of rupture occurrence, and the underlying sandy soil’s conditions on the embankment’s fragility. To achieve this, the response of three highway embankments resting on sandy soil was examined through quasi-static parametric numerical analyses. For the establishment of fragility curves, a cumulative lognormal probability distribution function was used. The maximum vertical displacement of the embankments’ external surface and the fault displacement were considered as the damage indicator and the intensity measure, respectively. Damage levels were categorized into three qualitative thresholds: minor, moderate, and extensive. All fragility curves were generated for normal and reverse faults, as well as the combination of those fault types (dip-slip fault). Finally, the proposed curves were verified via their comparison with those provided by HAZUS. It was concluded that embankment geometry and depth of fault rupture appearance do not significantly affect fragility, as exceedance probabilities show minimal differences (<4%). However, an embankment founded on dense sandy soil reveals slightly higher fragility compared to the one founded on loose sand. Differences regarding the probability of exceedance of a certain damage level are restricted by a maximum of 7%.

Microseismicity detection and spatial-temporal migration in the Xinfengjiang reservoir, Guangdong, China

There have been many earthquakes in the Xinfengjiang Reservoir (XFJR) in the past 60 years since the M6.1 earthquake that occurred in 1961. In the XFJR, seismicity has migrated from southeast to northwest; however, the mechanisms for this migration have not yet been fully investigated. In this study, we used six years of data from both permanent and temporary seismic networks in the XFJR to detect >23,500 earthquake events using the EQTransformer. The minimum magnitude of completeness of the earthquake catalog decreased to −0.1, and the spatial distribution of the microearthquakes showed clear high-angle faults in the area, which included a new fault within the reservoir. The focal mechanism inversion results showed that earthquakes in the northwestern cluster changed from strike-slip to dip-slip faults with time whereas those in the southeastern cluster remained a mixture of strike-slip and dip-slip faults. Further analysis showed that spatiotemporal variation in the northwestern reservoir was due to earthquake migration along different faults. Overall, we concluded that earthquake in the whole XFJR area were affected by water infiltration along fault zone; Coulomb stress transfer may also contribute to the migration of earthquakes from southeast to northwest direction.

Delineating petrological feature and geochemical characteristics of volcanic rocks from the Herson-Kilkis Area, Northern Greece

The geochemical data of this study confirm a previous suggestion that the Herson—Kilkis rhyolites are the crystallization products of a magma generated by the partial fusion of K-rich schists or para-gneisses and erupted along a dip-slip fault at the western boundary of the Serbo—Macedonian massif. High-K rhyolites occur in the area around Kilkis town along the western boundary of the Serbo-Macedonian massif. The chemistry of the rocks is characterized by high K2O contents (7–12 Wt %), high K2O/Na2O ratios (2–22), low CaO (0.04–0.16%) and Sr (0–83 mg Kg−1) contents and high Rb/Sr ratios (4–40). The trace elements (in mg Kg−1), Nb (Not Detected–41), Nd (5–87), Ce (16–119), La (9–67), Rb (Not Detected–358) and Y (10–87) present higher contents than those of rhyolites derived from a more basic magma by low-pressure fractional crystallization.

Application of seismic refraction and MASW methods for investigating the Spillway Fault trace along the western side of the Aswan High Dam, Egypt

AbstractAn earthquake of local magnitude ML = 4.6 occurred on November 7, 2010, 4.5 km northwest of the Aswan High Dam on the Spillway Fault. In the Aswan metropolitan region this earthquake was felt intensely. As no surface rupture was found, the focal mechanism and the distribution of seismic activity was one of the tools used for finding fault dimensions. The composite fault-plane solutions for the observed events on the Spillway Fault showed a left lateral strike-slip faulting with normal-fault component striking NNW-SSE. Also, remote sensing techniques were applied for the detection and identification of the geomorphology and geometry of the Spillway Fault. In this research, sub-surface layers and structures are delineated utilizing near-surface seismic techniques. Furthermore, the area’s supposed path and position of the Spillway Fault are also investigated. Two active seismic techniques, Seismic Refraction and Multi-Channel Analysis of Surface Waves (MASW), are utilized for recording near-surface seismic wave data at 9 sites. The seismic refraction profiles are conducted as a 2D cross-section on the trace of the detected Spillway Fault in the study area to evaluate the maximum depth of penetration of the P-wave for fault investigation. The constructed 2D seismic and structural sections from P-wave results show that the obtained average depth of about 30 m. In addition, the estimated P-wave velocities extend from 600 m/s to over 6500 m/s. Some lateral variation in the seismic wave velocities in all layers may represent fault zones. Moreover, the 1D MASW technique is conducted to estimate the velocities of the shear wave for the upper 30 m (Vs30) to provide the site classes and soil characteristics along both sides of the detected Spillway Fault trace in the study area. The calculated Vs30 values emphasized the idea of the existence of a normal dip-slip fault trace which divides the study area into two different lithological parts. The first part is located on the eastern side and characterized by almost class B (hard rock, according to NEHRP classification), while the other part is located to the west, and shows almost class type C (denoted as dense soil and soft rock soil).

Analysis of faults and pit chains in Noctis Labyrinthus: Implications for early extension and possible magmatic plumbing

Noctis Labyrinthus has been a region of disputed origin due to its complexity and poor understanding of how various processes and mechanisms may have combined to form it. The surface is an integrated record of intensive tectonic activity expressed by a multiple extended sets of dip-slip faults oriented in different directions, and thought to have acted on this region over its history. These faults are always coalescent to pits and pit chains displaying a complicated geological history in the region. To understand this geological history, we mapped the surface features in Noctis Labyrinthus using the High-Resolution Stereo Camera (HRSC) onboard Mars Express ND2 nadir channel basemaps, and we adapted the Digital Terrain Map (DTM) from the Mission Experiment Gridded Data Record (MEGDR) of Mars Orbiter Laser Altimeter (MOLA) onboard Mars Global Surveyor (MGS) for the topography. We have investigated the spatial distribution and trend of fault systems, the pit chains' morphology, and the correlation between these two types of features. Our results show three fault systems: i) NS and NNE-SSW, ii) EW and ENE-WSW, and iii) NNW-SSE and NW. The analysis of the faults trending, cross-cutting correlation and the superimposition led to identify multiple intersections between these faults that have been alongside with the reactivations of some inherited faults. We interpreted the first system of fault to be related to coeval lateral extension, generated by regional stress tensor, which is probably related to the slight bending of Valles Marineris within two phases of bidirectional extension. The second system of faults has been generated by the radial oblate stress tensor related to the formation of the small shield volcanoes in Syria Planum. However, the third system is likely related to the external driving process, probably in the Tharsis province. We classified pits in four evolutionary stages based on their morphometric attributes. We believe that the formation of the pit chains in Noctis Labyrinthus is related to a surface collapse after a pressure drop related to the magma chamber deflation associated with Syria Planum volcanic province. We propose a deformational model based on early extension and magmatic plumbing as driving processes for the formation of Noctis Labyrinthus.

A semi-analytical method for stress-strain analysis of tunnels under dip-slip faulting based on an elastic-plastic foundation beam model

Tunnels under faulting will be seriously damaged, and an accurate analytical method is an effective tool for tunnel response assessment in preliminary design. A series of mechanical analytical methods have been developed for tunnels under faulting. However, some unrealistic assumptions are still employed in the analytical method for tunnels under dip-slip faulting, including tunnel segmentation, constant stratum stress, and axial force in governing equations of tunnel displacement, which increases the solution complexity and decreases the accuracy. In this paper, a tunnel crossing an active fault is simplified as a large deformed continuous beam acting on an elastic-plastic foundation, and the tunnel-stratum interaction is treated as a series of axial and vertical elastic-plastic springs; the elastic-plastic tunnel-stratum interaction stresses and axial force are considered in the governing equations by using the finite difference method and a novel iterative method. In this way, the three assumptions employed in the existing analytical methods are removed. Later, the proposed method is verified by comparing it with the results from the model test, numerical simulation, and existing method. The comparisons show that the results of the proposed method agree excellently with those from the model test and numerical model, and the accuracy is higher than that of the existing analytical method. Finally, the failure scope and stress distribution of tunnels under the action of normal and reverse faulting are analyzed in detail.

Reconstruction and validation of ground motions across dip-slip faults: an application to response analysis of a long-span suspension bridge

Recent seismic events have unequivocally highlighted the susceptibility of fault-crossing bridges to the synergistic effects of ground surface vibrations on either side of the fault plane and the tectonic dislocations arising from fault-induced surface ruptures. This study delineates both seismic and parametric response analyses of fault-crossing suspension bridges, employing a straightforward yet efficacious method for simulating desired ground motions near fault-rupture zones. Herein, we introduce a user-friendly method to incorporate predicted fault-induced displacements, accounting for both fling-step and directivity effects, into processed ground motion chronologies, enabling the generation of dip-slip fault ground motions. The accuracy and efficacy of the proposed method are affirmed by juxtaposing the generated ground motions with the observed ones (MGM). An exhaustive parametric analysis, addressing factors like fault-crossing location, fault-crossing angle, and frequency components of fault-crossing ground motions, of a suspension bridge over a rupture fault, is executed using the fashionable ANSYS software. This study provides clear and specific guidelines for the seismic design of suspension bridges traversing rupture faults.

Trakya Havzasının Güneybatısında Fay Parametrelerinin Gravite Gradyent Verilerinden Kestirimi

Characterization of fault zones plays an important role for earthquake studies, geothermal and mineral resource exploration. This problem can be achieved by optimizing parameters of a defined synthetic model iteratively using gravity, gravity gradients or magnetic data. Curvature gradients indicate deviations of an equipotential surface from a spherical surface, therefore, reflecting deviations of mass distributions from a source point. In this study, differential curvature gradient is used as it identifies linear structures such as dip-slip faults and geological contacts. Curvature gradients are computed from gravity gradient observations which are derived from the high-resolution Earth Gravitational Model 2008. Estimating the fault parameters from gravity gradients is a complex nonlinear geophysical inverse problem that requires finding the minimum of a multi-variable cost function. A global optimization technique called simulated annealing (SA) is used to estimate location, dip angle and some depth parameters of the faults located in southwest side of the Thrace basin in Türkiye. In the optimization technique, a straight solution is inevitably applied to calculate the theoretical anomaly to be compared with the observed anomaly in each search step. In this case, the problem requires constructing a mathematical or geophysical interpretation model. Such a model is designed as a dip-slip fault model. The results show that dip angle estimates indicate high-angle faults which are found to be consistent with the seismic studies.

Asymmetrical Microfracture Density Across an Active Thrust Fault: Evidence from the Longmen Shan Fault, Eastern Tibet

Abstract Microfracture density in fault damage zones can reflect spatial variability that decays in intensity as a function of distance from the fault, which is crucial in understanding the mechanical, seismological, and fluid-flow properties of the fault system. However, few studies explored the characteristics of fracture density between the two sides of active dip-slip faults due to rare field observations. Here, we measured and modeled microfractures across an active thrust fault associated with the 2008 Mw 7.9 Wenchuan earthquake in the Longmen Shan, eastern Tibetan Plateau. The results showed that the microfracture density at the Qingping site developed more intensely in the hanging wall than in the footwall for an exposed thrust fault, indicating an asymmetrical pattern. The hidden thrust fault at the Jushui site showed that microfractures developed more intensely in vertical planes in the hanging wall than in the footwall, whereas microfractures developed similarly in horizontal planes within the two sides, indicating a quasiasymmetrical pattern. Comparing the data at the two sites with computational modeling, we suggest that fault geometry might exert a first-order control of the asymmetrical microfracture density pattern, which is helpful for revealing different deformational behaviors of rock masses in the fault damage zones and better understanding the hanging-wall effect for evaluating seismic hazards on active thrust faults.

Geometry, kinematics, and mechanics of the interaction between strike-slip and normal faults in active rifts: An example from the Quaternary-active Kane Springs Wash fault zone, Nevada, USA

Abstract The interactions among dip-slip and strike-slip faults are critical features in rift segmentation, including strain and slip transfer between faults of different rift segments. Here, we focused on the influence of factors such as fault and fracture geometries, kinematics, and local stress fields on the interaction and linkage of synchronous strike-slip and normal faults. Well-exposed faults along the tectonically active boundary between the central and northern Basin and Range provided for both reliable geometric data and consideration of rift segment development. We documented relative ages and distributions of Quaternary deposits, scarps, and geometries of three ~20–65-km-long Quaternary faults: the N-striking, normal Coyote Spring fault; the ENE-striking, left-lateral Kane Springs Wash fault; and the N-striking, normal Wildcat Wash fault. The normal faults bend to accommodate slip-type differences across linkage zones, with the strike-slip fault and local processes influencing interactions. Influenced by the local stress field of the Kane Springs Wash fault, the Coyote Spring fault bends SE as it approaches and links to the Kane Springs Wash fault. Influenced by the off-fault or process-zone fractures of the Kane Springs Wash fault, the Wildcat Wash fault bends NE and links with the Kane Springs Wash fault. The Kane Springs Wash fault continues beyond the normal fault terminations, suggesting slip transfer between them via the Kane Springs Wash fault. These relations and the ages of offset units suggest that activity on the faults was approximately synchronous despite slip-type differences. Consequently, in slip transfer, the local strike-slip stress environment and off-fault fractures influenced the geometry of the normal fault terminations; the strike-slip fault formed a boundary to dip-slip fault propagation; and this boundary facilitated kinematic and geodetic segmentation, forming a Basin and Range rift segment boundary.

Lithospheric plane strain deformation caused by a vertical dip-slip fault

Lithospheric plane strain deformation caused by a vertical dip-slip fault

Running safety analysis of a train-bridge coupled system under near-fault ground motions considering rupture directivity effects

The implementation of China’s high-speed railroad network and the “Bridges Instead of Roads” construction concept has raised concerns over earthquake safety. The complex mechanism of near-field ground motions poses greater risks than far-field ground motions due to the rupture directivity effect, and Hanging-wall/Footwall effects. The PEER earthquake database lacks earthquake records with fault distances less than 20 km, making it difficult to study train running safety (TRS) under near-fault ground motions. To address this issue, an empirical formulation method is adopted for near-fault three-dimensional seismic synthesis that considers source and site characteristics as well as rupture directivity effects using regression analysis. The method is verified with respect to directivity pulse and a train-bridge coupled system (TBCS) model is developed to verify its accuracy under earthquakes. Responses of the TBCS model under near-fault ground motions synthesized with different fault types, closest distances to the fault surface, and directivity parameters are analyzed by taking the average of the largest part of the response for comparison. Results show that dip-slip faults pose a greater danger than strike-slip faults, and the main reason for the hazard caused by rupture directivity effects is the generation of a forward directivity pulse, which makes the TBCS highly susceptible to derailment under near-fault pulse-like ground motions.

Overview of the tectonic and seismicity in Continental Borderland (offshore) Baja California, México

The seismic activity in the Continental Borderland, offshore Baja California, presented a unique opportunity to study a region that is not typically known for high seismic activity, in comparison to the nearby Mexicali Valley. In 2022 and the beginning of 2023, this region experienced unusual activity. Some events caused concern among the population, specifically, two earthquakes occurred near the city of Ensenada on April 15th and July 25th, 2022, with magnitudes of Mw 4.2 and 4.5, respectively. A third event, with Mw6.1, occurred near San Quintin, Baja California, on November 22nd, 2022, and is associated with the San Isidro Fault. This event, surpassing the typical magnitude observed in the region based on historical seismicity, was felt across a significant stretch of the Pacific coast, spanning from Guerrero Negro to Tijuana, México. In June of the same year, a magnitude 4.2 event occurred in the same fault. This fault is a relay for the Santa Catalina Fault in its southeastern sector. The hypocentral locations of the earthquakes varied in depth from 5 to 20 km, mostly occurring between 5 and 20 km. Magnitudes ranged from 1.5 to 6.1. Additionally, continuous offshore seismic activity was observed between Punta Colonet and the town of San Quintin in November–December 2022 and May 2023. The hypocentral locations of these events ranged from depths of 2–18 km, with the majority occurring between 3 and 10 km. Magnitudes ranged from 1.5 to 4.5. This activity appears to be in the form of swarms, as previously reported by other authors. However, the seismic activity observed in May 2023 displays a northeast-southeast trend, with focal mechanisms exhibiting similar strike angles. This leads us to suggest that this seismic activity is associated with the Colonet Fault, which can be observed in the continental region of the coast. Focal mechanisms of major earthquakes indicated strike-slip motion, aligning with the dominant deformation pattern controlled by major faults in the region. A concentration of activity near the coastline becomes apparent upon analyzing seismic activity recorded in various catalogs from agencies such as ISC, RESNOM, USGS, NCEDC, and SCEC. While no reports of moderate inverse mechanism earthquakes have been documented in the Baja California offshore region, they occur in California. Notably, no seismic activity was reported in the area where the Mw6.1 earthquake occurred between 1930 and 2021, suggesting the reactivation of old faults along the continental border, similar to what occurs in the sector of California. One possibility is that the recurrence intervals of these earthquakes are very long, which would explain the lack of historical records. Previous authors have highlighted the potential for large complex ruptures involving combinations of strike-slip and dip-slip faults, particularly exemplified by the 2010 Mw7.1 El Mayor- Cucapah event in northern Baja California. Seismic activity in the region concentrates along the El Descanso, Coronado-Banks, Santa Catalina, Bahía Soledad, and San Isidro faults. This observation is supported by gravimetric and magnetic anomalies found in various structures that have not been previously interpreted, including the newly reported La Chorera fault, based on bathymetry data. Most of the mentioned faults occur in the gravimetric and magnetic anomalies, however, many other structures have not yet been described.

Challenges of geomorphologic analysis of active tectonics in a slowly deforming karst landscape (W Slovenia and NE Italy)

Remote sensing-based tectonic geomorphology is widely applied to study neotectonic processes. This technique is relatively cheap and easy to apply, and it allows investigating large areas for tectonic activity. Existing literature comprises a variety of indices and techniques, seemingly able to work in almost all tectonic settings. In this paper we study a particularly challenging region in W Slovenia and NE Italy, characterised by slow deformation rates, high precipitation, dense vegetation, and intense modification of the landscape and drainage system due to karst processes. We apply a suite of well-established geomorphic indices including terrain ruggedness, surface roughness, hypsometric integral and analyses of drainage parameters, as well as several less commonly applied analyses, such as the assessment of doline density and doline geometry. The aim was to test the capabilities of the applied techniques at various spatial scales and to extract information about the active tectonics. We begin with broad, regional analyses to detect the morphological imprint of active faults in the landscape. Then, we perform detailed studies of selected sites and single faults. Finally, we analyse a polje located on the trace of an active fault in the highest detail. We then compare the different techniques and different digital elevation models regarding their usefulness for detecting tectonic influences on the karstic landscape. The results show that most of the well-established methods we tested are not suitable to detect even the largest, clearly active faults in the area. The resolution of the used DEMs also has a notable impact on the analyses. We conclude that it is almost impossible to identify active faults solely based on the used techniques. This is mainly due to the low deformation rates and to the intense karstification of the study area that leads to a disconnected surface drainage system. Additional problems arise from the strike-slip mechanism of active faulting. Lithological contrasts and inactive dip-slip faults can lead to false-positive signals of tectonic activity. However, some of our results also show a tectonic signal, indicating neotectonic strike-slip deformation with a notable dip-slip component. Remote sensing studies have demonstrated to be powerful tools for tectonic geomorphology studies, but low tectonic rate karstic landscape conditions as in W Slovenia/NE Italy push even the most trusted methods to their limits and highlight the importance of validation and ground-checks of the results.

How do inherited dip-slip faults affect the development of new extensional faults? Insights from wet clay analog models

The development of structurally controlled basins is frequently dominated by inherited geological and tectonic structures, especially when the affected region has undergone multiple tectonic phases. In this study we use physically scaled analog models to analyze the impact of inherited faults on the evolution of a new extensional fault system and its associated basin. In our experiments, we introduced inherited faults – bearing diverse geometries and orientations – cut through a homogeneous analog material (wet clay). After each experiment, we compare (a) how the inherited faults affected the inception and development of new faults and (b) the shape of the resulting basins, using a ‘reference model’ run without pre-existing faults. The results show that the orientation of pre-existing faults with respect to the extensional axis does affect the development of the new extensional structures. The main effects show up when the orientation of the pre-existing faults is closer to that expected for a fault that is optimally oriented (perpendicular) with respect to the direction of extension and has a dip close to an Andersonian extensional fault. Conversely, the impact on the resulting basin shape is more spatially complex, especially in the case of misoriented pre-existing faults. We also compare our experimental results with an analytical method based on the slip tendency theory. The application of our findings to selected natural cases demonstrates how one may interpret the occurrence, orientation, and activity of inherited faults by looking at the present-day geometry and wavelength of an extensional basin, particularly when newly formed extensional faults exhibit structurally unexpected trajectories.

Accuracy evaluation on five non-linear methods for fault parameter inversion of different-type fault earthquakes

Accurate inversion of seismic fault parameters has been a challenge in the studies of geophysical non-linear inversion problems. Many non-linear methods such as Simulated Annealing (SA), Genetic Algorithm (GA), Particle Swarm Optimization (PSO), and Multipeaks Particle Swarm Optimization (MPSO), have usually been applied to inverse the fault parameters from geodetic observation data. However, their accuracy and availability can vary from different-type fault earthquakes (pure strike-slip, pure dip-slip fault, oblique-slip fault earthquakes). In order to evaluate the accuracy and availability of these non-linear methods on inversion for fault parameters of different-type fault earthquakes, we applied the SA, GA, PSO, MPSO methods and a new non-linear method—Black Hole Particle Swarm Optimization (BHPSO), to inverse fault parameters of different-type earthquakes from synthetic and observed GPS and InSAR data. We found that the MPSO and BHPSO performed better than SA, GA, and PSO for inversion from both the synthetic and observed data. The synthetic data simulation results showed that the Root-Mean-Square Errors (RMSEs) of MPSO and BHPSO methods were 0.01–0.06 m, smaller than those of SA, GA and PSO. We then applied these five methods to inverse fault parameters of two real earthquakes—the 2020 Nevada Mw 6.4 earthquake and 2021 Maduo Mw 7.4 earthquake, from observed GPS and InSAR data. We found that the RMSEs of MPSO and BHPSO were 0.005–0.195 m, also smaller than those of SA, GA, and PSO, and the MPSO and BHPSO performed better than SA, GA, and PSO. The results in this study demonstrated that the MPSO and BHPSO, can hold high accuracy and availability for inversion of fault parameters of different-type fault earthquakes.

Numerical investigation on failure modes of bell-spigot jointed ductile iron pipelines subjected to dip-slip faults with different dip angles

Effective identification of failure modes for buried pipes subjected to earthquake-induced fault movements is crucial in the design and assessment of underground water supply networks. For bell-spigot jointed ductile iron (DI) pipes, the relationship between pipe failure mode and dip angle is still unclear. In this study, a three-dimensional finite element model of buried DI pipelines crossing dip-slip faults perpendicularly is established, based on which the joint kinematics and mechanical responses of DI pipes subjected to dip-slip faults with a dip angle between 50° and 130° are systematically evaluated. Four failure modes controlling the joint sealing performance and structural integrity of DI pipes are identified, i.e., joint pull-out failure, joint rotation failure, joint compressive failure, and excessive local buckling in the pipe segment. The results reveal that joint self-locking induced by the action of horizontal compression can reduce the joint rotational capacity greatly, rendering a segmented pipeline to behave like a continuous pipeline. Upon the occurrence of joint self-locking, the pipe mode changes from kinematic-controlled failure (joint pull-out or rotation failure) to mechanical-controlled failure (joint compressive failure or excessive local buckling). Finally, the relationship between failure mode and allowable dip angle is derived for use in design.

Evaluation of Natural Gas Hydrate Fault System: A Case from a Sag in Deep-Water Slope Area of the Northern South China Sea

AbstractThe fault system is one of the structural carrier systems of gas hydrate accumulation, which plays a vital role in controlling the distribution of natural gas hydrate (NGH) accumulation. The previous studies mainly focus on summarizing the vertical migration mode of high flux fluid along the fault with obvious geophysical response characteristics on the seismic profile, such as “fault with gas chimney,” “fault with mud diapir,” and “fault with submarine collapse”, but lack of evaluation methods for the fault carrier system. We use the X sag in the deep-water continental margin slope area of the northern South China Sea as an example to study the fault systems closely related to NGH. This paper puts to use attribute technologies, such as coherence, curvature, and fusion, to analyze the characteristics and combination of the fault systems. We discussed migration patterns and evaluation methods of dominant fault carrier systems. This research proves that the strike-slip fault system in the platform area can directly connect the gas source bed with high-quality hydrocarbon generation to the gas hydrate stability zone (GHSZ). The activity of this fault system is more conducive to the accumulation of hydrocarbon in the GHSZ. This area has a good site for pore-filling gas hydrate prospecting and a preferential favorable fault carrier system. The composite fault system, consisting of a normal dip-slip fault system and a polygonal fault system, in the slope area can jointly communicate the biogenic gas-rich reservoir. Its activity and well-migration performance are the main reasons for the submarine gas leakage and collapse. It is a secondary favorable fault carrier system in the study area. There may be massive and vein natural gas hydrate formation in fractures in the leakage passage, and pore-filled gas hydrate may exist in the submarine nonleakage area. In this work, a three-factor evaluation method of the fault carrier system is proposed for the first time. This method is of great significance for the evaluation and exploration of NGH reservoirs in the continental margin slope area of the northern South China Sea.

Characterising bed-parallel slip during gravity-driven deformation

Despite the recognition that bed-parallel slip (BPS) must operate during gravity-driven deformation of basinal sediments, there is a general paucity of detailed outcrop-based observations to characterise and detect such a process. We therefore present detailed timing relationships between BPS and steeper dip-slip faults that were both created during seismically-triggered downslope-directed movement of sediments. Using the late Pleistocene Lisan Formation that was deposited around the Dead Sea Basin as our case study, we show that ‘sub-seismic’ decametric scale BPS planes may pre-date, post-date, or operate coevally with steeper faults generated as sediments slip downslope towards the depocentre. Older BPS can be recognised by sediment injections and minor folds and fractures, whereas younger BPS displaces marker faults downslope towards the basin. BPS operating coevally with steeper faults results in complex overprinting and development of fault-bound lenses. BPS that forms along single surfaces in the footwall of normal faults becomes separated into two distinct planes in the downthrown hangingwall block, indicating broadly coeval development. Adjacent BPS planes that operate synchronously result in synthetic and antithetic faults that ‘hard-link’ and transfer displacement between BPS planes. Attenuated bedding between segments of BPS that overlap and terminate next to one another suggests that ‘soft-linkage’ also forms between coeval BPS planes. Displacement-length relationships of measured BPS planes plot in the same range as recorded for normal faults, although BPS with larger displacements have relatively ‘short’ lengths, suggesting that complete BPS planes are missing due to limitations of outcrop size. Although the lack of displaced bedding across BPS makes it largely invisible on seismic sections across large-scale gravity-driven systems, it does potentially contribute towards the apparent inbalance between net extension and contraction observed in many sections across mass transport deposits. In addition, the realisation that BPS interacts with dip-slip faults to create repeated and missing sections that are particularly focussed along earlier deformed horizons and turbidites, has implications for palaeoseismic studies that assume broadly continuous stratigraphy.

Relative importance of poroelastic effects and viscoelastic relaxation for postseismic velocity fields after normal and thrust earthquakes: Insights from 2D finite-element modelling

Earthquakes on faults in the brittle upper crust evoke sudden changes in pore fluid pressure as well as postseismic viscoelastic flow in the lower crust and lithospheric mantle but the relative importance of these processes during the postseismic phase has not been systematically studied. Here, we use two-dimensional finite-element models to investigate how pore fluid pressure changes and postseismic viscoelastic relaxation interact during the earthquake cycle of an intracontinental dip-slip fault. To isolate the effects from pore fluid flow and viscoelastic relaxation from each other, we performed experiments with and without pore fluid flow and viscoelastic relaxation, respectively. In different experiments, we further varied the permeability of the crust and the viscosity of lower crust or lithospheric mantle. Our model results show poroelastic effects dominate the velocity field in the first months after the earthquake. In models considering poroelastic effects, the surfaces of both hanging wall and footwall of the normal fault subside at different velocities, while they move upwards in the thrust fault model. Depending on the permeability and viscosity values, viscoelastic relaxation dominates the velocity field from about the second postseismic year onward although poroelastic effects may still occur if the permeability of the upper crust is sufficiently low. With respect to the spatial scales of poroelastic effects and viscoelastic relaxation, our results show that pore fluid pressure changes affect the velocity field mostly within 10–20 km around the fault, whereas the signal from viscoelastic relaxation is recognizable up to several tens of kilometres away from the fault. Our findings reveal that both poroelastic effects and viscoelastic relaxation may overlap earlier and over longer time periods than previously thought, which should be considered when interpreting aftershock distributions, postseismic Coulomb stress changes and surface displacements.