Vertical Fault Research Articles

Fault Slip Rates and Seismic Moment Deficits on Major Faults in Ordos Constrained by GPS Observation

The Ordos Block, surrounded by numerous active faults, is a relatively rigid but dangerous area with many strong historical earthquakes. We derive the block rotation velocity and fault slip rates in this area by using GPS data recorded from 1999 to 2007 and implementing an elastic block model. Instead of assuming vertical faults, as did most previous studies in and around Ordos, we use an improved method to invert for the fault dip angles and construct a closed 3-D fault system in our inversion. The predicted slip rates range from <1 mm/yr to ~ 10 mm/yr. Our results are roughly consistent with geological and other geodetic observations. Using the estimated slip rates, we also calculate the cumulative seismic moment due to fault locking and the released moment from historical earthquake catalogues. A comparison of the two quantities indicates that the Hetao Rift has an unreleased seismic moment equal to a Mw 7.9 earthquake, which is also indicated by frequent earthquakes above M6 after 1900.

Methane seepage inferred from pore water geochemistry in shallow sediments in the western slope of the Mid-Okinawa Trough

Abstract Widely distributed cold seeps are crucial sources of carbon to the seawater, whereas anaerobic oxidation of methane (AOM) and precipitation of authigenic carbonate might change the compositions of the methane-rich fluids and thus reduce the outputs of dissolved carbon to seawater. In this work, we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the Mid-Okinawa Trough. For the northern research area, active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes (sites C01 and C10), and strong emissions of gas and fluids may occur in the central mud volcanoes. In a submarine canyon at the south, C23 site exhibits rarely methane seepage, while high rate transportation of methane from deep sediments and associated AOM are identified at site C25 where small vertical faults and gas-bearing layers are developed at the surrounding. AOM at site C25 is indicated by (1) quasi-linearly decrease of sulfate concentrations with depths, (2) intense increases of dissolved inorganic carbon (DIC) concentrations and (3) significantly negative δ13C values (vary from −7.2 to −26.3‰) of DIC. Organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of NH4+, as well as negligible sulfate consumption at three of the four sites. Our data reveal that the cold systems are one of the DIC sources (the fluxes are ca. 11.2 mmol/m2/yr) to the seawater in the Okinawa Trough, which can be tracked by high depletion of 13C (δ13CDIC ranges from −7.2 to −26.3‰ VPDB).

Mainshocks/aftershocks study of the August 2012 earthquake doublet on Ahar-Varzaghan complex fault system (NW Iran)

Seismotectonic features of the Ahar-Varzaghan complex fault system are reviewed by the mainshocks/aftershocks study of the 2012 Ahar-Varzaghan earthquake doublet (Mw = 6.5 & Mw = 6.3). The early aftershocks were accurately monitored by a dense local seismological network. The mainshocks hypocenters were relocated using a new velocity model for the area. The regional displacement waveforms of the mainshocks and large aftershocks were jointly inverted for their moment tensors and centroids.Precisely located aftershocks are mostly distributed in three distinct clusters. These E-W trending clusters are situated to the north of the surface rupture over a ∼30 km long and 10 km wide zone, extending from the ground surface down to the depth of ∼15 km. Based on our results, the first mainshock (M1) with Mw = 6.5 nucleated 3 km east of the surface rupture at a depth of 10 km. The rupture evolved toward the west and shallower depths on an almost E-W striking right-lateral strike-slip vertical fault plane and produced ∼12 km surface rupture. Its main energy was released at about 5 km west of the hypocenter at an average depth of 5 km. About 11 min later, the second mainshock (M2) with Mw = 6.3 nucleated 5 km northwest of the M1 hypocenter and at a depth of 15 km. It occurred on an ENE-WSW striking, north-dipping (65° to 70°) fault plane with a dominant reverse mechanism and strike-slip component. The M2 related rupture also expanded to the west and to shallower depths and released most of its energy ∼5 km west of the respective hypocenter at an average depth of 11 km. The eastern aftershocks mainly show the right-lateral strike-slip mechanism on almost E-W trending fault, close to the location of the first main centroid (M1). The northern aftershocks mostly include reverse mechanisms with strike-slip component near the M2 centroid. A big gap of aftershock activity is observed close to the M1 centroid location, most likely associated with the area of maximum slip. We do not observe any N-S lineament of aftershocks neither near the M2 hypocenter and nor its centroid, thus making the suggestion of some previous investigators that M2 ruptured an N-S trending fault unlikely.

Imaging the Growth of Recent Faults: The Case of 2016–2017 Seismic Sequence Sea Bottom Deformation in the Alboran Sea (Western Mediterranean)

AbstractThe Eurasian‐African NW‐SE oblique plate convergence produces shortening and orthogonal extension in the Alboran Sea Basin (westernmost Mediterranean), located between the Betic and Rif Cordilleras. A NNE‐SSW broadband of deformation and seismicity affects the Alboran central part. After the 1993–1994 and 2004 seismic series, an earthquake sequence struck mainly its southern sector in 2016–2017 (main event Mw = 6.3, 25 January 2016). The near‐surface deformation is investigated using seismic profiles, multibeam bathymetry, gravity and seismicity data. Epicenters can be grouped into two main alignments. The northern WSW‐ENE alignment has reverse earthquake focal mechanisms, and in its epicentral region recent mass transport deposits occur. The southern alignment consists of a NNE‐SSW vertical sinistral deformation zone, with early epicenters of higher‐magnitude earthquakes located along a narrow band 5 to 10‐km offset westward of the Al Idrisi Fault. Here near‐surface deformation includes active NW‐SE vertical and normal faults, unmapped until now. Later, epicenters spread eastward, reaching the Al Idrisi Fault, characterized by discontinuous active NNE‐SSW vertical fractures. Seismicity and tectonic structures suggest a westward propagation of deformation and the growth at depth of incipient faults, comprising a NNE‐SSW sinistral fault zone in depth that is connected upward with NW‐SE vertical and normal faults. This recent fault zone is segmented and responsible for the seismicity in 1993–1994 in the coastal area, in 2004 onshore, and in 2016–2017 offshore. Insights for seismic hazard assessment point to the growth of recent faults that could produce potentially higher magnitude earthquakes than the already formed faults.

Consistency of stress state, locations and source mechanisms of events induced by hydraulic fracturing: downhole monitoring

ABSTRACTWe present results of processed microseismic events induced by hydraulic fracturing and detected using dual downhole monitoring arrays. The results provide valuable insight into hydraulic fracturing. For our study, we detected and located microseismic events and determined their magnitudes, source mechanisms and inverted stress field orientation. Event locations formed a distinct linear trend above the stimulated intervals. Source mechanisms were only computed for high‐quality events detected on a sufficient number of receivers. All the detected source mechanisms were dip‐slip mechanisms with steep and nearly horizontal nodal planes. The source mechanisms represented shear events and the non‐double‐couple components were very small. Such small, non‐double‐couple components are consistent with a noise level in the data and velocity model uncertainties. Strikes of inverted mechanisms corresponding to the nearly vertical fault plane are (within the error of measurements) identical with the strike of the location trend. Ambient principal stress directions were inverted from the source mechanisms. The least principal stress, σ3, was determined perpendicular to the strike of the trend of the locations, indicating that the hydraulic fracture propagated in the direction of maximum horizontal stress. Our analysis indicated that the source mechanisms observed using downhole instruments are consistent with the source mechanisms observed in microseismic monitoring arrays in other locations. Furthermore, the orientation of the inverted principal components of the ambient stress field is in agreement with the orientation of the known regional stress, implying that microseismic events induced by hydraulic fracturing are controlled by the regional stress field.

Geophysical investigations in the Gañuelas-Mazarrón Tertiary basin (SE Spain): A natural analogue of a geological CO2 storage affected by anthropogenic leakages

In the framework of a Spanish project focused on CCS technologies (PSE-CO2 Project), the Gañuelas-Mazarrón Tertiary Basin (SE Spain) was globally studied as a natural analogue of a CO2 reservoir affected by anthropogenic leakages. In this context, geophysical investigations were carried out in order to determine the location, morphology and extent of the CO2-enriched saline aquifer existing in the bottom of the basin, and evidenced by two core drills performed in the 1980s for geothermal purposes (La Ermita de El Saladillo and El Alto de El Reventón. The geophysical researches were performed by reprocessing the vertical electrical soundings also made in the 1980s, but also by means of new campaigns of electrical resistivity tomography, time-domain electromagnetic surveys and gravimetry. The main results from geoelectric and electromagnetic investigations have allowed identifying three main different geological materials in the basin based on their resistivity: metamorphic materials from the Nevado-Filábride Complex, and Tertiary sedimentary materials mainly consisted of sandy calc-marls and intruded by more or less altered Neogene volcanic rocks. Furthermore, it has been defined the basin as a graben limited by the Nevado-Filábride materials affected by very vertical faults and frequently filled by volcanic rocks, reaching a depth of about 500 m. However, the gravimetric research has allowed to better identifying the strong gradients in the borders of the basin, which is indicative of fairly net and vertical contacts, and also to establish a geological model of the basin. This model includes the SW and NE boundaries very vertical and a deeply sunken central zone with about 9 km wide. This area has been divided in turn into two sections, within which the one located at the SW (~5 km) has a depocenter that reaches a depth of about 900 m. Consequently, the possibility that the CO2 be in supercritical state in certain areas of the aquifer, which should be hosted at the contact between the Tertiary sediments/volcanic rocks and the Nevado-Filábride materials, has not to be discarded. Therefore the studied basin would represent an excellent natural analogue for a geological CO2 storage in a deep saline aquifer.

Strutting systems for deep excavations – technical challenges

Constant development carried out in town centres causes that many problems related to earthworks and tunnelling must be resolved. Investors are required to ensure respective number of parking bays, which in turn demands 2-3 storeys of underground car parks. This paper presents interesting case studies of deep excavations in Wrocław, Poland. As the geotechnical works in the old city centres may bring some risks to the surrounding area, proper strutting or anchoring systems must be provided. Apart from solving engineering problems, one needs to meet very stringent requirements of heritage conservator supervision. In order to ensure the stability of the excavation walls, it is necessary to examine the foundations of neighbouring structures, and if necessary, to strengthen them for the process of installing the excavation protection walls, progressing the excavations and constructing basement storeys. Another problem refers to constructing underground storeys below the level of groundwater. This requires efficient cutting-off or long-term lowering of water table inside the excavation with a possibly limited intervention in hydrological regime beyond the project in progress. In the typical case of old cities in river valleys such “hoarding off’ the excavation and cutting off groundwater leads to temporary or permanent disturbances of groundwater flow and possible local swellings. Contemporary technologies make it possible to protect vertical fault and simultaneously cut-off groundwater inflow by means of steel sheet pilings, diaphragm walls or secant pile walls.

Experimental investigation of reverse fault rupture propagation through cohesive granular soils

Naturally occurring dry cohesionless soil is rarely found in urban areas; however, previous studies on surface fault rupture propagation using physical modeling has usually concentrated on dry cohesionless soil. In this investigation, the effects of cohesion on fault rupture propagation through granular soil were studied. Physical models were developed in which inherent cohesion was produced by adding different percentages of clay to the sand. A dry test also was conducted for comparison. The results show that changes in cohesion affect all aspects of the behavior of faulting, including fault rupture propagation, fault scarp at surface and required displacement at bedrock for outcropping. It was found that the vertical fault displacement required for outcropping (h0∕H) increased as the percentage of clay increased.

Seismic source characteristics of the intraslab 2017 Chiapas-Mexico earthquake (Mw8.2)

Inversion of the parameters characterising the seismic source of the instraslab 2017 Chiapas Mexico earthquake (Mw 8.2) shows a simple rupture process with a unidirectional propagation and directivity towards the North-West and a duration of the rupture process around 75 s. The initial point source values of strike, dip and rake are 316°, 80° and −91° respectively. The focal mechanism indicates a normal fault type within the oceanic Cocos plate, with an almost vertical fault plane for a focal depth of 59 km. The seismic data was obtained from 51 seismic stations of the global seismic network IRIS for the epicentral distances between 30° and 90°. In the finite-fault inversion, 75 seismic signals between P and SH waves were used. The epicenter is on the southeast margin of the large slip zone which extends 75 km to the northwest, this large slip zone is located to the south of the city of Arriaga. The scalar seismic moment was estimated at 2.55×1021Nm, equivalent to a moment magnitude of Mw 8.2. The maximum dislocation or slip is 14.5 m. As a coseismic effect, a local tsunami was generated, recorded by several tidal gauges and offshore buoys. The deformation pattern shows a coastal uplift and subsidence.

Finite-fault source inversion using adjoint methods in 3-D heterogeneous media

Accounting for lateral heterogeneities in the 3-D velocity structure of the crust is known to improve earthquake source inversion, compared to results based on 1-D velocity models which are routinely assumed to derive finite-fault slip models. The conventional approach to include known 3-D heterogeneity in source inversion involves pre-computing 3-D Green’s functions, which requires a number of 3-D wave propagation simulations proportional to the number of stations or to the number of fault cells. The computational cost of such an approach is prohibitive for the dense data sets that could be provided by future earthquake observation systems. Here, we propose an adjoint-based optimization technique to invert for the spatio-temporal evolution of slip velocity. The approach does not require pre-computed Green’s functions. The adjoint method provides the gradient of the cost function, which is used to improve the model iteratively employing an iterative gradient-based minimization method. The adjoint approach is shown to be computationally more efficient than the conventional approach based on pre-computed Green’s functions in a broad range of situations. We consider data up to 1 Hz from a Haskell source scenario (a steady pulse-like rupture) on a vertical strike-slip fault embedded in an elastic 3-D heterogeneous velocity model. The velocity model comprises a uniform background and a 3-D stochastic perturbation with the von Karman correlation function. Source inversions based on the 3-D velocity model are performed for two different station configurations, a dense and a sparse network with 1 and 20 km station spacing, respectively. These reference inversions show that our inversion scheme adequately retrieves the rise time when the velocity model is exactly known, and illustrates how dense coverage improves the inference of peak-slip velocities. We investigate the effects of uncertainties in the velocity model by performing source inversions based on an incorrect, homogeneous velocity model. We find that, for velocity uncertainties that have standard deviation and correlation length typical of available 3-D crustal models, the inverted sources can be severely contaminated by spurious features even if the station density is high. When data from thousand or more receivers is used in source inversions in 3-D heterogeneous media, the computational cost of the method proposed in this work is at least two orders of magnitude lower than source inversion based on pre-computed Green’s functions.

Surface and shallow subsurface structure of the Middle Kedrovaya paleoseismic rupture zone in the Baikal Mountains from geomorphological and ground-penetrating radar investigations

The 30-km-long Middle Kedrovaya (MK) paleoseismic rupture zone, on the northwestern flank of Lake Baikal, is one of most impressive late Quaternary fault scarps in the Baikal rift. The aim of this study is to reconstruct the near-surface fault geometry of the MK fault zone to clarify its past movement history and seismic potential. We applied ground penetrating radar (GPR), along with high-resolution satellite imagery and field surface observations, to analyze the structure of the upper 12–13 m of the rupture zone and to characterize the complex structural environment in three dimensions. The GPR reveals that the shallow structure of the MK zone is defined by a combination of steep and listric normal faults, which form a surface pattern of subparallel ruptures trending predominantly N30°E. Individual surface scarps range from 5 m to 2.7 km in length and are separated by gaps of a few tens of meters to several kilometers apart. The greatest width of the scarp zone is 1.9 km. The vertical displacements measured independently from surface topographic profiles and from GPR radargrams are linearly related. The height of the main footwall (head) scarp is always 0.5–2 m greater than the throw measured on the radargrams, a result of the scarp face having broadened far up the talus slope after the latest displacement event. Maximum and average vertical normal fault displacements along the main fault trace, inferred from GPR data, are 8.3 and 4.98 m respectively. Paleoearthquake magnitude M estimated from empirical regressions ranges from 6.8 to 7.6, based on equations of M vs. surface rupture length, M vs. displacement, and M vs. landslide volume. Our study of the Middle Kedrovaya paleoseismic rupture zone confirms that listric normal faulting is widespread in the shallow (<13 m) subsurface of the western margin of the North Baikal basin. However, GPR data suggest that listric faulting is confined to the unconsolidated talus deposits and probably represents the effects of downhill gravitational sliding of the talus, and perhaps of underlying shallow bedrock. This downhill (lakeward) sliding of the hanging wall is partly responsible for the large heave component of movement on the MK rupture zone and its resulting unique geomorphic expression.This paper illustrates the practicality of investigating fault surface ruptures in complex environments by integrating data gathered by geophysics (GPR in this case) and by direct geomorphological observations. Because of the loose and mobile nature of the faulted talus deposit, the current surface morphology disguises the true number of subsurface faults, the lateral extent of faulting, and the unique structure types (all visible on radargrams) that developed to accommodate coseismic surface rupture with a large heave component.

Strain Partitioning and Present‐Day Fault Kinematics in NW Tibet From Envisat SAR Interferometry

AbstractAn 8 year archive of Envisat synthetic aperture radar (SAR) data over a 300 × 500 km2 wide area in northwestern Tibet is analyzed to construct a line‐of‐sight map of the current surface velocity field. The resulting velocity map reveals (1) a velocity gradient across the Altyn Tagh fault, (2) a sharp velocity change along a structure following the base of the alluvial fans in southern Tarim, and (3) a broad velocity gradient, following the Jinsha suture. The interferometric synthetic aperture radar velocity field is combined with published GPS data to constrain the geometry and slip rates of a fault model consisting of a vertical fault plane under the Altyn Tagh fault and a shallow flat décollement ending in a steeper ramp on the Tarim side. The solutions converge toward 0.7 mm/yr of pure thrusting on the décollement‐ramp system and 10.5 mm/yr of left‐lateral strike‐slip movement on the Altyn Tagh fault, below a 17 km locking depth. A simple elastic dislocation model across the Jinsha suture shows that data are consistent with 4–8 mm/yr of left‐lateral shear across this structure. Interferometric synthetic aperture radar processing steps include implementing a stepwise unwrapping method starting with high‐quality interferograms to assist in unwrapping noisier interferograms, iteratively estimating long‐wavelength spatial ramps, and referencing all interferograms to bedrock pixels surrounding sedimentary basins. A specific focus on atmospheric delay estimation using the ERA‐Interim model decreases the uncertainty on the velocity across the Tibet border by a factor of 2.

The role of fluid migration system in hydrocarbon accumulation in Maichen Sag, Beibuwan Basin

Fluid migration system is of great significance for hydrocarbon accumulation, including the primary migration and secondary migration. In this paper, the fluid migration system is analysed in Maichen Sag using seismic, well logging and core data. Results show that many factors control the hydrocarbon migration process, including hydrocarbon generation and expulsion period from source rocks, microfractures developed in the source rocks, the connected permeable sand bodies, the vertical faults cutting into/through the source rocks and related fault activity period. The spatial and temporal combination of these factors formed an effective network for hydrocarbon expulsion and accumulation, leading to the hydrocarbon reservoir distribution at present. Generally, a better understanding of the hydrocarbon migration system can explain the present status of hydrocarbon distribution, and help select future target zones for oil and gas exploration.

Preliminary analysis on the source properties and seismogenic structure of the 2017 Ms7.0 Jiuzhaigou earthquake

At GMT time 13:19, August 8, 2017, an M s7.0 earthquake struck the Jiuzhaigou region in Sichuan Province, China, causing severe damages and casualties. To investigate the source properties, seismogenic structures, and seismic hazards, we systematically analyzed the tectonic environment, crustal velocity structure in the source region, source parameters and rupture process, Coulomb failure stress changes, and 3-D features of the rupture plane of the Jiuzhaigou earthquake. Our results indicate the following: (1) The Jiuzhaigou earthquake occurred on an unmarked fault belonging to the transition zone of the east Kunlun fault system and is located northwest of the Huya fault. (2) Both the mainshock and aftershock rupture zones are located in a region where crustal seismic velocity changes dramatically. Southeast to the source region, shear wave velocity at the middle to lower crust is significantly low, but it rapidly increases northeastward and lies close to the background velocity across the rupture fault. (3) The aftershock zone is narrow and distributes along the northwest-southeast trend, and most aftershocks occur within a depth range of 5–20 km. (4) The focal mechanism of the Jiuzhaigou earthquake indicates a left-lateral strike-slip fault, with strike, dip, and rake angles of 152°, 74° and 8°, respectively. The hypocenter depth measures 20 km, whereas the centroid depth is about 6 km. The co-seismic rupture mainly concentrates at depths of 3–13 km, with a moment magnitude ( M w) of 6.5. (5) The co-seismic rupture also strengthens the Coulomb failure stress at the two ends of the rupture fault and the east segment of the Tazang fault. Aftershocks relocation results together with geological surveys indicate that the causative fault is a near vertical fault with notable spatial variations: dip angle varies within 66°–89° from northwest to southeast and the average dip angle measures ~84°. The results of this work are of fundamental importance for further studies on the source characteristics, tectonic environment, and seismic hazard evaluation of the Jiuzhaigou earthquake.

Evidence of extensional and strike-slip deformation in the offshore Gökova-Kos area affected by the July 2017 Mw6.6 Bodrum-Kos earthquake, eastern Aegean Sea

The interpretation of new multichannel seismic profiles and previously published high-resolution swath and seismic reflection data from the Gokova Gulf and southeast of Kos Island in the eastern Aegean Sea revealed new morphotectonic features related to the July 20, 2017 Mw6.6 Bodrum-Kos earthquake offshore between Kos Island and the Bodrum Peninsula. The seafloor morphology in the northern part of the gulf is characterized by south-dipping E–W-oriented listric normal faults. These faults bend to a ENE–WSW direction towards Kos Island, and then extend parallel to the southern coastline. A left-lateral SW–NE strike-slip fault zone is mapped with segments crossing the Gokova Gulf from its northern part to south of Kos Island. This fault zone intersects and displaces the deep basins in the gulf. The basins are thus interpreted as the youngest deformed features in the study area. The strike-slip faults also produce E–W-oriented ridges between the basin segments, and the ridge-related vertical faults are interpreted as reverse faults. This offshore study reveals that the normal and strike-slip faults are well correlated with the focal mechanism solutions of the recent earthquake and general seismicity of the Gokova Gulf. Although the complex morphotectonic features could suggest that the area is under a transtensional regime, kinematic elements normally associated with a transtensional system are missing. At present, the Gokova Gulf is experiencing strike-slip motion with dominant extensional deformation, rather than transtensional deformation.

The Pieniny Klippen Belt in Poland

The Pieniny Klippen Belt in Poland marks the Central Carpathian-North European plate suture zone. The strictly tectonic present-day confines of the Pieniny Klippen Belt are characterized as (sub)vertical faults and shear zones. A strong reduction in the space of the original sedimentary basins took place. The strike-slip-bounded tectonic blocks, thrust units, toe-thrusts and olistostromes are mixed together, resulting in the present-day mélange character where individual tectonic units are difficult to distinguish. The sedimentary rocks of the Pieniny Klippen Belt were deposited in the paleogeographic realm known as the Alpine Tethys that was divided into two basins separated by the Czorsztyn Ridge. The accretionary prism formed in front of the advancing Alcapa (Central Carpathians) terrane had overridden the Czorsztyn Ridge during the Late Cretaceous-Paleocene. The destruction of the Czorsztyn Ridge supplied huge amounts of coarse-clastic material, including olistoliths, into the Magura Basin during the Late Cretaceous-Paleocene. The rotation of the Alcapa caused the strike-slip motions which led to the deformation of the previously created nappes and development of the flower structure. Two parallel faults delineate the southern and northern limits of the Pieniny Klippen Belt. The stops in Sromowce (Macelowa and Sobczański Gorge area) allow the observation of the southern marginal zone of the Pieniny Klippen Belt, the overturned position of the rotated counterclockwise deformed rock as well as the diapiric uplift of the Pieniny rocks in a transpressional strike-slip regime. The Zawiasy (Krościenko area) stop is located in the northern marginal zone (Hulina Unit) of the Pieniny Klippen Belt along the major dextral strike-slip Dunajec Fault.

Extensional fault-related folding in the northwestern Red Sea, Egypt: segmented fault growth, fault linkages, corner folds and basin evolution

Abstract Segmented, planar, domino-style extensional fault arrays and their associated hanging wall fault-related folds form complex linked basins along the onshore margin of the northwestern Red Sea, Egypt. The extensional fault systems form half-graben basins with kilometre-scale, asymmetrical, doubly plunging longitudinal synclines and narrow, plunging transverse anticlines and synclines. The axial traces of the hanging wall longitudinal folds are curvilinear, sub-parallel to the half-graben Border faults, and bend or are offset at relay ramps and at fault linkage points. Transverse corner fold systems occur at the fault linkage points and fault jogs. The fold geometries, variations in fault displacement, and fault slip indicators indicate that the fold and fault systems are kinematically related and formed during the Late Oligocene–Miocene rifting of the northern Red Sea. The folds were controlled by vertical and lateral fault propagation and by the mechanical anisotropy of the pre-rift strata. The proposed model for these extensional folds is the initial formation of monoclinal flexures above reactivated blind basement faults. Increased displacement, propagation and segment linkage formed hanging wall longitudinal folds and transverse corner folds. The longitudinal folds grew progressively at the expense of the transverse folds and merged along-strike into long hanging wall synclinal basins.

“3D_Fault_Offsets,” a Matlab Code to Automatically Measure Lateral and Vertical Fault Offsets in Topographic Data: Application to San Andreas, Owens Valley, and Hope Faults

AbstractMeasuring fault offsets preserved at the ground surface is of primary importance to recover earthquake and long‐term slip distributions and understand fault mechanics. The recent explosion of high‐resolution topographic data, such as Lidar and photogrammetric digital elevation models, offers an unprecedented opportunity to measure dense collections of fault offsets. We have developed a new Matlab code, 3D_Fault_Offsets, to automate these measurements. In topographic data, 3D_Fault_Offsets mathematically identifies and represents nine of the most prominent geometric characteristics of common sublinear markers along faults (especially strike slip) in 3‐D, such as the streambed (minimum elevation), top, free face and base of channel banks or scarps (minimum Laplacian, maximum gradient, and maximum Laplacian), and ridges (maximum elevation). By calculating best fit lines through the nine point clouds on either side of the fault, the code computes the lateral and vertical offsets between the piercing points of these lines onto the fault plane, providing nine lateral and nine vertical offset measures per marker. Through a Monte Carlo approach, the code calculates the total uncertainty on each offset. It then provides tools to statistically analyze the dense collection of measures and to reconstruct the prefaulted marker geometry in the horizontal and vertical planes. We applied 3D_Fault_Offsets to remeasure previously published offsets across 88 markers on the San Andreas, Owens Valley, and Hope faults. We obtained 5,454 lateral and vertical offset measures. These automatic measures compare well to prior ones, field and remote, while their rich record provides new insights on the preservation of fault displacements in the morphology.

Porosity distribution in a heterogeneous clay-rich fault core by image processing of 14C-PMMA autoradiographs and Scanning Electron Microscopy

AbstractShale formations are considered by a number of countries as the most suitable media to dispose of high-level radioactive waste. This is mainly due to the impermeable, self-sealing, chemical reducing, and sorption properties that tend to retard radionuclide migration. However, shale formations can also contain highly connected fault zones with permeabilities that can differ of several orders of magnitudes with respect to the undeformed host rock. The objective of this work is to use the 14C-PMMA autoradiography method combined with SEM-EDS measurements to understand the porosity variations in and around fault gouges and to define their relationship to mechano-chemical processes. The studied samples were taken from a low permeability shale in a small-scale vertical strike-slip fault at the Tournemire underground research laboratory. Results display significant variations in porosity and mineralogy along the studied gouge zone due to polyphased tectonics and paleo-fluid circulations.

Impact of in situ stress and fault reactivation on seal integrity in the East Irish Sea Basin, UK

Abstract Despite having been affected by several stages of exhumation during the Cretaceous and Cenozoic, the contemporary stress state of the East Irish Sea Basin (EISB) is poorly characterised. As the basin is mature in terms of exploitation of hydrocarbons, future exploration beyond the conventional Sherwood Sandstone Group reservoir (Triassic) necessitates a greater understanding of the in situ stress field, while proposed natural gas storage and carbon sequestration schemes also require detailed stress field information. Using petroleum well data, the in situ stress field of the EISB has been characterised to assess the mechanical seal integrity. A strike-slip stress regime most-likely prevails in the basin, meaning the Maximum Horizontal Stress (SHmax) is the greatest of the principal stresses. Interpretation of stress orientation data suggests that SHmax is oriented 152° ± 12°, consistent with mean stress orientations across the wider region associated with plate boundary forces. Some degree of structural control appears to influence the orientation of SHmax, with orientations locally aligned sub-parallel to major Permo-Triassic basin-bounding faults. Fault reactivation risk is evaluated through modelling the pore pressure increase required to induce failure on pre-existing faults. Vertical faults striking 30° from SHmax are optimally-oriented to become reactivated under elevated pore pressure conditions. For any project relying on an element of fault seal for the containment of buoyant fluids at the average reservoir depth of 800 m, pore pressure increase should be less than 3.3 MPa to avoid reactivating pre-existing optimally-oriented faults. Higher pressure increases would be required to initiate reactivation of faults with other orientations. Vertical faults striking perpendicular to SHmax are least likely to become reactivated, and in the absence of halite, seal integrity would instead be limited by caprock strength and capillary-entry pressure. Major faults affecting the basin have been analysed for their slip tendency (ratio of shear to normal stress), which provides an indication of their susceptibility to become reactivated. Although the analysis is limited due to lack of an accurate 3D representation of the fault network, the results suggest that many of the fault orientations observed in the EISB exhibit high slip tendencies, including N–S striking faults to the north and west of the East Deemster Fault, where the SHmax orientation is NW–SE. Faults striking perpendicular to SHmax, such as the Lagman Fault, are least likely to become reactivated due to higher normal stresses that inhibit frictional sliding, while faults striking parallel or very close to SHmax also exhibit low slip tendency as they are not subjected to significant shear stresses.