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The Humbly Grove Field has, for the UK, a unique development history. It was discovered as an oilfield in May 1980 and produced as an oilfield until 2000 along with small satellite fields Herriard (developed) and Hester9s Copse (not developed). Peak production of 2219 bopd was achieved during July 1986 but, by October 1988, the rate had fallen to around 1000 bopd, a rate that was more or less maintained until October 1995 after which the production fell rapidly. At this point the decision was taken to reconfigure the field as a gas storage facility. Significant renewed pressure depletion occurred between 2000 and 2005, following which first cushion and then storage gas was injected into two reservoirs: the Middle Jurassic, Great Oolite Group and the uppermost Triassic, Rhaetian Westbury Formation. Gas storage operations commenced in 2005 and the reservoirs have undergone cyclical gas injection and gas withdrawal since that date. The cyclical injection of gas and re-pressuring of the Great Oolite reservoir causes mobile oil to be swept towards dedicated oil production wells. This operates effectively as an enhanced oil recovery scheme. The co-produced liquid hydrocarbons provide a valuable secondary income stream for the field.

Our knowledge of subsurface structures often derives from seismic velocities that are measured during seismic acquisition surveys. These velocities can greatly change due to lithological, fracture frequencies and/or effective pressure/temperature variations. However, the influence of such intrinsic lithological properties and environmental conditions at the large scale is poorly understood due to the lack of comprehensive datasets. Here, we analyze 43 borehole-derived velocity datasets of 3 end-member tight carbonate sequences from Central Italy, including massive pure limestone (Calcare Massiccio, CM), thick-layered (20–50 cm) pure limestone (Maiolica, MA), and thin-layered (2–20 cm) marly limestone (Calcareous Scaglia, CS). Our results show that the main rock parameters and environmental conditions driving large scale velocity variations are bedding and paleostresses, while mineralogical composition and current tectonic stress also play a role. For each of the 3 end-members, measured VP values vary differently with depth, as the thin-layered CS units show a clear increase in Vp, while velocity slightly increases and remains constant for the thick-layered MA and massive CM units, respectively. Such observations show that velocities are affected by specific characteristics of lithological discontinuities, such as the thickness of bedding. Counterintuitively, larger Vp values were recorded in the deformed mountain range than in the undeformed foreland suggesting that higher paleo-stresses increase velocity values by enhancing diagenesis and healing of discontinuities. Our results thus demonstrate that large scale velocity variations are strictly related to variation of lithological properties and to the geological and tectonic history of an area. We suggest that such lithological and environmental controls should be taken into account when developing velocity and mechanical models for tectonically active regions of the Mediterranean Area, where earthquakes mostly nucleate and propagate through carbonate formations, and for resource exploration in fractured carbonate reservoirs.

Serpentines are common minerals in several major tectonic faults in a variety of geodynamic settings and have variable frictional strength and complex deformation processes. Here we present friction experiments carried out on a suite of serpentine samples that include veins of antigorite, lizardite and fibrous serpentine (chrysotile and polygonal serpentine) together with massive samples of retrograde (lizardite and chrysotile rich) and prograde (antigorite‐rich) serpentinites. These samples were characterized from the hand specimen down to the nanoscale to precisely constrain their mineralogical composition and are interpreted to represent typical fault rocks and host rocks in serpentine‐bearing shear zones, respectively. Experiments were performed at effective normal stress from 5 to 120 MPa, at temperatures of 25 °C and 170 °C and water‐saturated, i.e. under the faulting conditions of the brittle upper lithosphere. Friction of antigorite samples, either massive or vein, is relatively high μ = 0.53. Retrograde, massive serpentinites, constituted primarily of lizardite and fibrous serpentines are frictionally weak, μ = 0.30. End‐members lizardite and fibrous serpentines are even weaker, 0.15 < μ < 0.19, and this weakness is unchanged at high temperature. We document deformation of lizardite and fibrous serpentines occurring predominantly via mode II cracking, crystal/fiber folding, and frictional sliding, which account for the observed mechanical weakness. When combined with frictional reactivation analysis, our data provide mechanical evidence for fault weakness inferred from earthquake dip distributions at oceanic outer rises and low‐angle normal faults beneath rifted continental margins and at slow/ultraslow spreading mid‐ocean ridges.

The Calabria Arc presents the highest probability of occurrence of major earthquakes in the Italian peninsula. Several destructive historical earthquakes (i.e. 1638, 1659, 1783, 1905 and 1908) affected, in particular, the Catanzaro Trough and its neighbouring areas. These events have been tentatively related to the activity of NE-SW trending normal faults. Some of these earthquakes have been followed by tsunamis, which caused further damages along the Tyrrhenian coast. In this paper, we reconstruct the Quaternary evolution of the Catanzaro Trough by combining field geo-structural and marine geophysical data. The results have been compared with existing database of earthquake focal mechanisms, updated with 8 new focal solutions performed in the present work. Analysis of faults offsetting the Lower-Middle Pleistocene deposits shows that the Catanzaro Trough experienced transcurrent and extensional phases of deformation. In particular, conjugate systems with NW-SE right-lateral and NE-SW left lateral faults were observed to displace the Lower Pleistocene deposits. Whereas NE-SW and N-S oriented normal faults have been identified as the main fault systems acting during late Pleistocene-Holocene phase. The interpretation of on-land structural datasets has been supported by geophysical data (multichannel and Chirp profiles) acquired in the offshore and onshore of the study area. Multidisciplinary approach has allowed to define NE-SW elongated sedimentary basins, as the Lamezia Basin, bordered on the one hand by Sant'Eufemia Fault that may extend up to 30 km-length, on the other hand by two overstepping faults, Vibo Valentia and San Pietro Lametino Faults. These findings carry some relevant implications in terms of seismic hazard, as they suggest that the longer fault segment, the greater its energetic seismic event. Finally, these data fit perfectly with the observed late Pleistocene-Holocene WNW-ESE extensional stress regime derived from existing and new database of earthquake focal mechanisms. This is in agreement with the orientation of the most seismically active grabens of the Calabrian Arc (the Crati, the Mesima and the Gioia Tauro Basins). Amongst these structural lineaments, the NE-SW and N-S trending normal faults play surely a relevant role as part of recent seismotectonics processes controlling the Late Quaternary geodynamics of the central Calabrian Arc, representing the source of the main destructive earthquakes occurred in the region.

To better understand fault zone architecture and fluid flow in mesoscale fault zones, we studied normal faults in chalks with displacements up to 20 m, at two representative localities in Flamborough Head (UK). At the first locality, chalk contains cm-thick, interlayered marl horizons, whereas at the second locality marl horizons were largely absent. Cm-scale displacement faults at both localities display ramp-flat geometries. Mesoscale fault patterns in the marl-free chalk, including a larger displacement fault (20 m) containing multiple fault strands, show widespread evidence of hydraulically-brecciated rocks, whereas clays smears along fault planes, and injected into open fractures, and a simpler fault zone architecture is observed where marl horizons are present. Hydraulic brecciation and veins observed in the marl-free chalk units suggest that mesoscale fault patterns acted as localized fault conduit allowing for widespread fluid flow. On the other hand, mesoscale fault patterns developed in highly fractured chalk, which contains interlayered marl horizons can act as localized barriers to fluid flow, due to the sealing effect of clays smears along fault planes and introduced into open fractures in the damage zone. To support our field observations, quantitative analyses carried out on the large faults suggest a simple fault zone in the chalk with marl units with fracture density/connectivity decreasing towards the protolith. Where marls are absent, density is high throughout the fault zone, while connectivity is high only in domains nearest the fault core. We suggest that fluid flow in fractured chalk is especially influenced by the presence of marls. When present, it can smear onto fault planes, forming localised barriers. Fluid flow along relatively large displacement faults is additionally controlled by the complexity of the fault zone, especially the size/geometry of weakly and intensely connected damage zone domains.

The integration of seismic reflection profiles with well-located earthquakes shows that the mainshocks of the 1997–1998 Umbria–Marche seismic sequence nucleated at a depth of ∼ 6 km on normal faults within the Triassic Evaporites. In the same lithology, made of interbeds of dolomites and anhydrites, two deep boreholes (∼ 4–5 km), drilled northwest of the epicentral area, encountered CO2 at near lithostatic pressure. In order to investigate the deformation processes operating at depth in the source region, we have combined field studies on a major evaporite-bearing normal fault (the Roccastrada fault) with mechanical and permeability data from rock deformation experiments on anhydrite rocks. The Roccastrada fault is a mature normal fault (inferred displacement > 100 m and exhumation from depths > 1 km) that dips in the range 40°–45°. The fault zone structure is characterized by a 5–6 m thick fault core, which appears to be zoned. The inner fault core is made of fine-grained fault rocks (∼ 1 m thick), with deformation localized along continuous and straight slip surfaces associated with a dolomite-rich cataclasite (brittle deformation). The outer fault core is mainly characterized by distributed deformation accommodated by a fault parallel fabric consisting of interbeds of cataclastic dolostones and foliated Ca-sulphate rocks. The damage zone consists of foliated Ca-sulphate rocks (foliation almost perpendicular to the fault zone) and heavily fractured and boudinaged dolostones. Mechanical data obtained from triaxial loading tests on borehole-recovered anhydrites with different grain size and mesoscopic fabric, show that the transition from localized to distributed deformation occurs at effective pressures of about 20 MPa. The permeability measured under hydrostatic stress conditions, before loading, is generally low and ranges between 10− 21 ≤ k ≤ 10− 19 m2. During sample loading, the permeability increases up to 3 (prior to brittle localized failure) and 2 (prior to distributed ductile failure) orders of magnitude, with measured k values of 10− 17 m2 and 10− 18 m2, respectively. The integration of field observations with mechanical data allows us to propose a fault evolution model where fault initiation occurs as distributed deformation within the anhydrites, along fault zones dipping at ∼ 45°. With increasing displacement, dolostones are concentrated into the inner fault core. The ultimate result is a fault zone structure dipping at ∼ 45° , made of an inner fault core affected by brittle processes surrounded by a macroscopically ductile outer fault core. This fault zone evolution can explain the low rupture dip (38°–48°) of the Umbria–Marche 1997–1998 mainshocks. The combination of field observations and permeability measurements suggests a fault zone permeability low enough for fluid overpressures to develop. Therefore in the active area of the Umbria–Marche Apennines deep-seated CO2-rich fluids can be trapped at seismogenic depths within evaporite-bearing faults and can potentially promote earthquake nucleation.

The Catanzaro Trough is a Neogene-Quaternary basin developed in the central Calabrian Arc, between the Serre and the Sila Massifs, and filled by up to 2000 m of continental to marine deposits. It extends from the Sant’Eufemia Basin (SE Tyrrhenian Sea), offshore, to the Catanzaro Basin, onshore. Here, onshore structural data have been integrated with structural features interpreted using marine geophysical data to infer the main tectonic processes that have controlled the geodynamic evolution of the western portion of the Catanzaro Trough, since Upper Miocene to present. The data show a complex tectonostratigraphic architecture of the basin, which is mainly controlled by the activity of NW–SE and NE–SW trending fault systems. In particular, during late Miocene, the NW-SE oriented faults system was characterized by left lateral kinematics. The same structural regime produces secondary fault systems represented by E-W and NE-SW oriented faults. The ca. E-W lineaments show extensional kinematics, which may have played an important role during the opening of the WNW–ESE paleo-strait; whereas the NE-SW oriented system represents the conjugate faults of the NW-SE oriented structural system, showing a right lateral component of motion. During the Piacenzian-Lower Pleistocene, structural field and geophysical data show a switch from left-lateral to right-lateral kinematics of the NW-SE oriented faults, due to a change of the stress field. This new structural regime influenced the kinematics of the NE-SW faults system, which registered left lateral movement. Since Middle Pleistocene, the study area experienced an extensional phase, WNW-ESE oriented, controlled mainly by NE-SW and, subordinately, N-S oriented normal faults. This type of faulting splits obliquely the western Catanzaro Trough, producing up-faulted and down-faulted blocks, arranged as graben-type system (i.e Lamezia Basin). The multidisciplinary approach adopted, allowed us to constrain the structural setting of the central Calabria segment. The joined onshore with offshore structural data analysis allowed us to image a more faithful geodynamic evolution of the Calabrian Arc, included in the wider geodynamic framework of the Mediterranean region Moreover, our results show the close correlation between the NE-SW and N-S normal fault systems and evidence of deformed Quaternary deposits. These findings are relevant to seismic hazard understanding in an area which is historically considered at the highest risk of earthquake and tsunami and where are present important infrastructures and cities.

We propose a theoretical model, supported by a field study, to describe the patterns of fault/fracture meshes formed within dilational stepovers developed along faults accommodating regional scale wrench-dominated transtension. The geometry and kinematics of the faulting in the dilational stepovers is related to the angle of divergence (), and differs from the patterns traditionally predicted in dilation zones associated with boundary faults accommodating strike-slip displacements (where = 0°). For low values of oblique divergence (

In transtensional and transpressional deformation zones, bulk 3D strains are often kinematically partitioned into regions of wrench- and extension- or shortening-dominated faulting. Most strain models assume ideal incompressible materials with a Poisson9s ratio (ν) of 0.5. It is well known from experimental and geophysical data, however, that natural rocks have values of ν

‘Inversion structures’ (e.g. folds, reverse faults) spatially associated with basin-bounding faults are very widely recognised in rift basins in both onshore and offshore settings worldwide. The great majority of such structures are attributed to local or regional crustal shortening events. There is, however, an alternative, which is investigated in this paper: inversion could reflect a horizontal shortening component of deformation formed during progressive and partitioned transtension. A case study from the Carboniferous Northumberland Basin shows that shortening structures can also form in obliquely divergent rifts if the bulk strain undergoes kinematic partitioning into distinct regions of wrench- and extension-dominated transtension. Such strain partitioning appears to be particularly favoured in basins where fault localisation is strongly influenced by pre-existing basement structures. This may occur because the pre-existing anisotropies are zones of long-lived weakness that lie in an orientation particularly favourable to the preferential accommodation of either strike-slip or dip-slip displacements. Our strain analysis applied to the Northumberland Basin, traditionally considered as a classic example of a Variscan inverted basin, reduces the deformation history to a single kinematically partitioned phase of dextral transtension during the late Carboniferous–early Permian. Our findings have profound implications for the interpretation of inversion structures in any rift basin where the direction of extension may be significantly oblique to the basin margins.

A review of about 300 published and unpublished rock friction experiments that reproduce seismic slip conditions suggests that a significant decrease in friction occurs at high slip rate. Extrapolating the experimental data to conditions that are typical of earthquake nucleation depths, the authors conclude that faults are lubricated during earthquakes, irrespective of the fault rock composition or specific weakening mechanism involved. This study reviews a large set of fault friction experiments and finds that a significant decrease in friction occurs at high slip rate. Extrapolating the experimental data to conditions typical of earthquake nucleation depths, it is concluded that faults are lubricated during earthquakes, irrespective of the fault rock composition or specific weakening mechanism involved. The determination of rock friction at seismic slip rates (about 1 m s−1) is of paramount importance in earthquake mechanics, as fault friction controls the stress drop, the mechanical work and the frictional heat generated during slip1. Given the difficulty in determining friction by seismological methods1, elucidating constraints are derived from experimental studies2,3,4,5,6,7,8,9. Here we review a large set of published and unpublished experiments (∼300) performed in rotary shear apparatus at slip rates of 0.1–2.6 m s−1. The experiments indicate a significant decrease in friction (of up to one order of magnitude), which we term fault lubrication, both for cohesive (silicate-built4,5,6, quartz-built3 and carbonate-built7,8) rocks and non-cohesive rocks (clay-rich9, anhydrite, gypsum and dolomite10 gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger11,12 a number of physicochemical processes (gelification, decarbonation and dehydration reactions, melting and so on) whose products are responsible for fault lubrication. The similarity between (1) experimental and natural fault products and (2) mechanical work measures resulting from these laboratory experiments and seismological estimates13,14 suggests that it is reasonable to extrapolate experimental data to conditions typical of earthquake nucleation depths (7–15 km). It seems that faults are lubricated during earthquakes, irrespective of the fault rock composition and of the specific weakening mechanism involved.

[1] An experimental approach has been taken to study the prefailure fluid flow properties of borehole samples of low-porosity anhydrites and the evolution of permeability (k) during deformation leading to brittle and ductile failure. The permeability measured under hydrostatic stress conditions prior to loading at room temperature is generally low (k = 10−21–10−19 m2) and displays an anisotropy and pressure sensitivity controlled by grain size and fabric orientation (foliation). Triaxial loading test results show that the brittle-ductile transition occurs for effective pressure Pe < 20 MPa and is almost independent of fabric orientation and grain size. All samples, whether deforming in a brittle (localized deformation) or ductile (distributed deformation) mode, show dilatancy after an initial phase of compaction. During loading, the k starts to increase prior to the phase of sample dilation and before the yield stress is attained. The k rise is characterized by an upward concave trend, prior to localized deformation (brittle failure), and by a downward concave trend, during distributed deformation (ductile failure). The k increase prior to brittle failure is about 1 order of magnitude higher than during ductile failure. We interpret the different shape of the k curve as due to the observed different degrees of fracture connectivity (widespread development of intragranular and intergranular fractures) reached during brittle (low) and ductile (high) deformation, respectively. Our experimental results imply that for low-porosity rocks the mode of failure, controlled by Pe, has an overwhelming effect on the evolution of permeability, compared to other factors such as grain size and fabric orientation.

[1] Recently, in the Northern Apennines, geophysical data have identified the Triassic Evaporites (TE, anhydrites and dolomites) as the source region of the major extensional earthquakes of the area (M ∼ 6). In order to characterize fault zone architecture and deformation processes within the TE, we have studied exhumed evaporite-bearing normal faults within the upper crust. The structure of large displacement (>100 m) normal faults is given by 1) a zoned fault core with a wider portion of fault-parallel foliated Ca-sulphates (ductile deformation), overprinted by an inner fault core (IFC) of localized brittle deformation, and 2) wide (dolostones) to absent (Ca-sulphates) damage zones of fault fracture patterns. Fault rock assemblage within the IFC is characterized by fault breccia, gouge, and cataclasites of different grain size. Most of the deformation within the IFC is localized along thin and fault parallel principal slip surfaces (PSS) made of dolomite-rich fine-grained cataclasite. SEM analyses show an evolution from Ca- to St- to gypsum-rich mineralization, due to episodic fluid flow events channeled along the fault zones during different stages of fault exhumation. The development of the observed fault geometry can be explained by a mechanical fault evolution model where initial faulting occurs along broad and ductile shear zones within the anhydrites and causes fracturing within the dolostones. Progressive deformation within the fault core leads to the development of fault parallel dolomite-rich cataclastic layers. Their reactivation coupled with transient fluid overpressures can produce embrittlement and localization of brittle deformation within the IFC.

Movements on low-angle normal faults (LANF) are not predicted by traditional Anderson–Byerlee frictional fault mechanics. Our investigations centre on three normal fault systems active at distinct times during the regional extension of the Northern Apennines, with each showing different degrees of crustal exhumation. These are (from E to W): the Altotiberina fault system, Umbria; the Radicofani fault system, Tuscan mainland; and the Zuccale fault system, Isle of Elba. Regional extension has been a continuous process since middle Miocene, migrating progressively from west to east, with deformation accommodated by a set of E to NE-dipping LANF and more steeply SW-dipping antithetic structures. The LANF acted as regional detachments, accommodating a majority of the extension in the Northern Apennines, with individual faults exhibiting several kilometres of displacement. Regionally, the stress field has been characterised by a vertical σ 1 and a NE–SW- (Tuscan mainland and Umbria) to E–W (Elba)-trending σ 3 . Where exposed at the surface, the main LANF detachments possess a well-developed fault core of foliated fault rocks in which fluids have played a key role in weakening due to reaction softening and the onset of stress-induced solution–precipitation mechanisms. We speculate that the E-dipping normal faults initiated with relatively low-angles of dip due to differential drag generated by mantle flow following slab retreat and roll-back beneath the Apennine chain.

Foreland (early-orogenic) and hinterland (late-orogenic) extensional domains have been widely documented in the Northern Apennines, where they are synchronous with contraction in the active part of the fold and thrust belt. The progressive eastward migration of the contractional front and the associated hinterland extensional field implies that early-orogenic extensional structures, developed in the foreland domains, may experience reactivation/inversion. We present new field data, integrated with seismic evidence, from the Gubbio normal fault, a 22 km long presently active fault, showing: a) evidence for early-orogenic extension since the Lower Miocene time; b) successive positive inversion during the Upper Miocene contraction; and c) renewed, late-orogenic, extension during the Quaternary age. Field data allowed two systems of mesoscale normal faults, respectively interpreted as early- and late-orogenic structures, to be recognised. Stress fields associated with contractional and early- late-orogenic extensional tectonic regimes, are characterised by an overall coaxiality, with directions of compression and tension consistently aligned NE-SW. The symmetry between the successive deformation stages is interpreted as a likely cause for the repeated reactivation of the Gubbio fault since the Miocene. Our analysis in the Gubbio area shows that normal faults play a key role during the evolution of a fold and thrust belt from the early-orogenic stages, when they influence the geometry and evolution of foredeep basins, to the late-orogenic stages when Quaternary activity, due to extension reactivation, controls the evolution of the intramountain basins.

The Zuccale fault is a gently east-dipping normal fault exposed on Elba. Its displacement of 7—8 km occurred from the mid-Miocene to the early Pliocene and the fault has been exhumed from 3—6 km depth. A complex hydrofracture system exposed in the footwall block consists of three vein sets: two vertical sets trending N—S and E—W and one sub-horizontal. The veins show a crack-and-seal texture and mutually crosscut each other. The regional stress field throughout the period when the Zuccale fault was active was extensional with the minimum principal stress oriented E—W, consistent only with the N—S trending set of vertical hydrofractures. We interpret all three sets of orthogonal fractures as the result of local stress changes due to exhumation, superimposed on tectonic stresses, with build-up of overpressure beneath the low-permeability phyllosilicate-rich fault core and release of overpressure when the fault slipped.