Geometry Of Normal Faults Research Articles

Groundwater–rock interactions and mixing in fault–controlled karstic aquifers: A structural, hydrogeochemical and multi-isotopic review of the Pontina Plain (Central Italy)

Karstic aquifers represent crucial water resources and are categorized as either stratigraphically or fault–controlled. This study investigates groundwater–rock interactions and mixing processes within one of the largest fault–controlled karstic aquifers in Central Italy, adjacent to the Pontina plain, which is a highly populated area where agricultural activities and climate change challenge the groundwater assessment of a complex aquifer. We conducted structural, hydrogeochemical, and multi-isotopic screening of ten selected springs with different degrees of mineralization (ranging from Ca–HCO3 to Na–Cl hydrofacies), incorporating new analyses and modeling of δ34S(SO4), δ18O(SO4), 87Sr/86Sr, and δ11B. Additionally, the reinterpretation of a seismic section provides a more detailed framework extending to depths of approximately 5–7 km that allows the identification of the geometry of normal faults, which act as pathways for upwelling fluids. Our findings reveal that hydrogeochemical compositions result from multiple interactions between karstic water and deeper fluids that have interacted with different rocks. Concentration (Na/Li) and isotope (SO4–H2O) geothermometers, coupled with geochemical modeling and trace element analysis, enabled the estimation of a water temperature equilibrium of approximately 95.5 °C, with Triassic evaporites generally corresponding to a depth of approximately 3 km and a temperature of 40 °C with magmatic rocks at approximately 1 km depth, which is likely associated with ongoing tectonics and the Quaternary tectonically controlled Volsci Volcanic Field. To obtain the latter estimate, we used a new geothermometer activity based on the equilibrium between analcime and pollucite. Furthermore, this multidisciplinary approach enhances the understanding of groundwater behavior in fault–controlled karstic aquifers, where mantle-derived CO2 dissolved in groundwater is the driving force behind water–rock interactions. Given the potential for further variations in mixing, which may worsen water quality and increase aquifer vulnerability, periodic monitoring of these processes is essential in a human-impacted environment amidst ongoing climate change.

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

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

Geometry and 3D seismic characterisation of post-rift normal faults in the Pearl River Mouth Basin, northern South China Sea

Geometry and 3D seismic characterisation of post-rift normal faults in the Pearl River Mouth Basin, northern South China Sea

Kinematics and geometry of normal faults at the margins of pull-apart basins: An example from the Dead Sea Basin

The parameters controlling the geometrical and kinematical evolution of a normal fault system along the margins of a subsiding pull-apart basin are studied using the case of the Dead Sea Basin. The Western margins of the basin consist of sub-parallel normal faults and basinward tilted blocks that evolved in a layered sedimentary sequence. The faults exhibit narrow domains of fault damage, characterized by mechanical degradation of the surrounding rock materials. The slip surfaces display oblique-normal abrasive striations and waviness. Measurements of normal-to-slip fault surface roughness at more than eight orders of length-scale magnitudes, combined with directly obtained fault dips and slip orientations, suggest that at length-scales higher than ~102 m the dominant geometry of the faults change from generally wavy with multiscale undulations to a characteristic zigzag pattern with two distinct sets. We utilize the numerical, discrete elements, DDA method to explain the structural evolution of the margins. Our results suggest that the sedimentary cover deforms as a coherent system of uniformly sized blocks, exhibiting flexure in the western margins and gravitational collapse eastward into the subsiding basin, predominantly in tilted blocks mode. These conditions explain many of the observed structures in the field, including the extensional belt and the stepped basinward block-tilting of the sedimentary cover. Comparing the wide deformation belt on the western margins of the basin to the much narrower deformation zone on the eastern margins suggests that the existence of a thick sedimentary cover and its initial structure control the width of the margins and the general mode of structural deformation.

Topological Characterization of a Fault Network Along the Northern North Sea Rift Margin

AbstractThe factors that control the spatial variation of the topological characteristics of normal fault networks at the rift‐scale are poorly understood. Here, we use 3D seismic reflection data from the northern North Sea to investigate the spatial variation of the geometry, topology, and strain heterogeneity of the Late Jurassic normal fault network along the rift margin. Our results show that fault orientation varies spatially along the rift margin. Normal faults within fault blocks that are adjacent to the North Viking Graben exhibits dominant N‐S and NE‐SW strikes that are sub‐parallel to the graben axis and associated step‐over, whereas in fault blocks farther from the graben, there is a dominant NW‐SE strike. Furthermore, we identify two broad topological domains within the fault network: (a) dominated by isolated nodes, partially connected branches, and low fault connectivity, and (b) dominated by abutting nodes, fully connected branches, and moderate to high fault connectivity. These topological domains correlate with previous sub‐division of the rift margin in the northern North Sea into platform and sub‐platform structural domains, respectively. There is also a positive correlation between the spatial variability of the fault orientations and intensity, with the fault network connectivity, highlighting the relationship between normal fault geometry and topology. We conclude that the across and along‐strike variation in strain, presence of pre‐existing structures, and accommodation zone‐related deformation are key factors influencing the spatial variation of fault network properties at the rift scale.

The Role of Rheology and Fault Geometry on Fault Reactivation: A Case-Study from the Zsámbék-Mány Basin, Central Hungary

In this study, we investigated the structural evolution of the Vértessomló (VT) Thrust and the Környe-Zsámbék (KZ) Fault, which are located in the Transdanubian Range in the center of the Miocene Pannonian back-arc basin. Our study is based on surface and well data. The Transdanubian Range was located on the Adriatic passive margin during the Late Triassic, where a thick succession of platform carbonates was deposited. Intercalations of intraplatform basin deposits occur in the eastern part of the study area. South-directed thrusting and the formation of the VT Thrust took place during the Cretaceous, related to the Austroalpine orogeny. Asymmetric half-grabens were formed during the Eocene in the hanging wall of the segmented dextral normal KZ Fault. The geometry and kinematics of the KZ Fault were influenced by the pre-existing VT Thrust located in the Mesozoic basement of the Paleogene sub-basins. These Eocene half-grabens suffered mild inversion due to the dextral reverse reactivation of the VT Thrust and the KZ Fault during the Oligocene–Early Miocene. The geometry of Miocene normal faults indicates that the VT-KZ Fault system was an active transfer fault during the Miocene extension of the Pannonian Basin, as well. We found a positive correlation between the rheology of the Triassic basement and the mode of Paleogene fault reactivation. Our results show that reactivation of the pre-existing thrust took place along that segment, where the Triassic basement is made up of homogeneous platform carbonates. In contrast, a diffuse fault zone developed, where the Triassic basement is represented by the weak layers of intraplatform basins.

Relations between inherited basement fabric and fault nucleation in a continental setting: The Rio do Peixe Basin, NE Brazil

The control of normal fault growth by preexisting basement structures is still a worldwide matter of debate. The objectives of the present study are (i) to investigate where rift faults nucleate in continental-scale ductile shear zones and how these zones influence fault architecture; (ii) how subvertical basement fabric controls listric fault geometry and evolution; (iii) how long reactivation of faults lasts; and (iv) which are the implications of fault reactivation for the oil industry. We contribute to these issues by defining the 3-D relationship between preexisting ductile strike-slip shear zones, the geometry and evolution of subsequent rift-related normal faults using airborne magnetic, 2-D and 3-D seismic reflection, and structural data from the Rio do Peixe Basin, a Cretaceous intracontinental basin in northeastern Brazil. Our results indicate that Precambrian mylonitic foliation and faults share a common strike at the surface, but not a common dip, mostly because the Precambrian shear zones are planar and the reactivated faults are listric (curved). The combined magnetic anomaly and seismic reflection help understand the 3-D geometrical relation between inherited structures and Cretaceous brittle reactivation, which is not obvious from the field data. As inferred from subsurface information, the mylonitic foliation (sub-vertical dip) and shear zone boundary (ca. 60° dip, parallel to brittle faulting) are not parallel. The basin-bounding faults reactivated along ductile shear zones that separate major terranes with mechanical contrast. We conclude that the map view spatial coincidence of Precambrian ductile shear zones and main brittle faults can be deceiving. Mylonitic foliation and faults are parallel along strike but not along dip, indicating that the geometry of rift faults is not controlled by the mylonitic foliation, but by the mechanical contrast between tectonometamorphic units separated by ductile shear zones.

Mechanical Stratigraphy Controls Normal Fault Growth and Dimensions, Outer Kwanza Basin, Offshore Angola

AbstractMechanical stratigraphy controls the growth patterns and dimensions of relatively small normal faults, yet how it influences the development of much larger structures remains unclear. Here, we use 3D seismic reflection data from the Outer Kwanza Basin, offshore Angola to constrain the geometry and kinematics of several normal faults formed in a deep‐water clastic succession. The faults are up to 6.3‐km long and 1.9‐km tall and have up to 44 m of throw. Aspect ratios and lower‐tip throw gradients are greater for faults that terminate downward at a c. 100 m thick, mass‐transport complex (MTC; up to 5.2 and 0.12) than for those that offset it (up to 2.7 and 0.01). Faults that offset the MTC invariably have &gt;30 m of throw. Based on their geometric properties and throw patterns, we interpret that the faults nucleated above the MTC and propagated down toward it. Upon encountering this unit, which we infer behaved in a more ductile manner than encasing strata, tip propagation was halted until tip stresses were sufficiently high (corresponding to minimum throw of c. 30 m) to breach it. Faults with smaller throw were unable to breach the MTC. We argue that using only geometric criteria to determine fault growth patterns can mask the significant control mechanical stratigraphy has on fault kinematics. Mechanical stratigraphy is therefore a key control on the growth of large, seismic‐scale normal faults, in a similar way to that observed for far smaller structures.

Large-to Local-Scale Control of Pre-Existing Structures on Continental Rifting: Examples From the Main Ethiopian Rift, East Africa

In the Main Ethiopian Rift (East Africa) a complex tectonic history preceded Tertiary rifting creating pre-existing discontinuities that influenced extension-related deformation. Therefore, this area offers the opportunity to analyze the control exerted by pre-existing structures on continental rifting at different scales. In this paper we present an overview of such an influence. We show that at a large scale (up to ∼800–1,000 km) rift localization has been controlled by a lithospheric-scale inherited heterogeneity corresponding to a Precambrian suture zone, separating two different lithospheric domains beneath the plateaus surrounding the rift. The inherited rheological differences between these two lithospheric domains, as well as the presence of pre-existing lithospheric-scale transversal structures, largely controlled the along-axis segmentation and symmetry/asymmetry of different, ∼80–100 km-long rift segments. Inherited transversal structures also controlled the development of off-axis volcano tectonic activity in the plateaus surrounding the rift. At a more local scale (&amp;lt;80 km), inherited fabrics controlled the geometry of normal faults and the distribution and characteristics of rift-related volcanism. These observations document a strong control exerted by pre-existing structures on continental rifting at all different scales.

Controls on gravity-driven normal fault geometry and growth in stacked deltaic settings: a case study from the Ceduna Sub-basin

Kinematics of gravity-driven normal faults exerts a critical control on petroleum systems in deltaic settings but to date has not been extensively examined. The Ceduna Sub-basin (CSB) is a passive margin basin containing the White Pointer (Albian-Cenomanian) and Hammerhead (Campanian-Maastrichtian) delta systems that detach on shale layers of Albian-Cenomanian and Turonian-Coniacian ages, respectively. Here we present evidence for spatially variable fault growth styles based on interpretation of the Ceduna 3D seismic survey and fault kinematic analyses using displacement–distance, displacement–depth and expansion index methods. We identified faults that continuously grew either between the Cenomanian–Santonian or Santonian and the Maastrichtian located throughout the study area and faults that exhibit growth between the Cenomanian–Maastrichtian that are geographically separated into three areas according to their evolution histories: (i) Northern CSB faults exhibit constant growth between the Cenomanian and Maastrichtian. (ii) Central CSB faults show two dip-linkage intervals between (a) Cenomanian and Coniacian–Late Santonian, (b) Coniacian–Late Santonian and Late Santonian–Maastrichtian segments, respectively. (iii) Central and southern CSB faults exhibit dip-linkage intervals between Cenomanian–early Santonian and Late Santonian–Maastrichtian segments. Our study demonstrates a relationship between the location of the Cenomanian–Maastrichtian faults and their evolution history suggesting constant growth evolution at north and dip linkage at the central and south areas.

Geomechanics in Depleted Faulted Reservoirs

This paper examines the impact of the effective stresses that develop during depletion of a faulted reservoir. The study is based on finite element modeling using 2D plane strain deformation analysis with pore pressure and elastoplastic deformation of the reservoir and sealing shale layers governed by the Drucker–Prager plasticity model. The mechanical properties and response of the rock formations were derived from triaxial test data for the sandstone reservoirs and correlation functions for the shale layers. A normal fault model and a reverse fault model were built using seismic data and interpretation of field data. The estimated tectonic in-situ stress field was transformed to the plane of the modeled geometry. Sensitivity studies were performed for uncertainties on the values of the initial horizontal stress and for the friction of the fault surfaces. It was found that the stress path during depletion is mainly controlled by the initial lateral stress ratio (LSR). The developed effective stresses with depletion are influenced by the fault geometry of the compartmentalized blocks. Plastic deformation develops for low LSR whereas for high values the system tends to remain in the elastic region. When plastic deformation takes place, it affects mainly the region near the fault. The reservoir deformation is dominated by vertical displacement which is higher near the fault region and nearly uniform in the remote area. The volumetric strain is dominated by compaction. More volatile conditions in relation to change of the friction coefficient and LSR were found for the normal fault geometry.

Pre-inversion normal fault geometry controls inversion style and magnitude, Farsund Basin, offshore southern Norway

Abstract. Compressional strains may manifest along pre-existing structures within the lithosphere, far from the plate boundaries along which the causal stress is greatest. The style and magnitude of the related contraction is expressed in different ways, depending on the geometric and mechanical properties of the pre-existing structure. A three-dimensional approach is thus required to understand how compression may be partitioned and expressed along structures in space and time. We here examine how post-rift compressional strains are expressed along the northern margin of the Farsund Basin during Late Cretaceous inversion and Palaeogene–Neogene pulses of uplift. At the largest scale, stress localises along the lithosphere-scale Sorgenfrei-Tornquist Zone, where it is expressed in the upper crust as hangingwall folding, reverse reactivation of the basin-bounding normal fault, and bulk regional uplift. The geometry of the northern margin of the basin varies along strike, with a normal fault system passing eastward into an unfaulted ramp. Late Cretaceous compressive stresses, originating from the convergence between Africa, Iberia, and Europe, selectively reactivated geometrically simple, planar sections of the fault, producing hangingwall anticlines and causing long-wavelength folding of the basin fill. The amplitude of these anticlines decreases upwards due to tightening of pre-existing fault propagation folds at greater depths. In contrast, later Palaeogene–Neogene uplift is accommodated by long-wavelength folding and regional uplift of the entire basin. Subcrop mapping below a major, uplift-related unconformity and borehole-based compaction analysis show that uplift increases to the north and east, with the Sorgenfrei-Tornquist Zone representing a hinge line rather than a focal point to uplift, as was the case during earlier Late Cretaceous compression. We show how compressional stresses may be accommodated by different mechanisms within structurally complex settings. Furthermore, the prior history of a structure may also influence the mechanism and structural style of shortening that it experiences.

Interpretation of a ca. 1600–1580 Ma metamorphic core complex in the northern Gawler Craton, Australia

The Mount Woods Domain in the Gawler Craton, South Australia records a complex tectonic evolution spanning the Palaeoproterozoic and Mesoproterozoic. The regional structural architecture is interpreted to represent a partially preserved metamorphic core complex that developed during the ~1600–1580 Ma Hiltaba Event, making this one of the oldest known core complexes on Earth. The lower plate is preserved in the central Mount Woods Domain, which comprises the Mount Woods Metamorphics. These rocks yield a detrital zircon maximum depositional age of ~1860 Ma and were polydeformed and metamorphosed to upper amphibolite to granulite facies during the ~1740–1690 Ma Kimban Orogeny. The upper plate comprises a younger succession (the Skylark Metasediments) deposited at ~1750 Ma. Within the upper plate, sedimentary and volcanic successions of the Gawler Range Volcanics were deposited into half graben that evolved during brittle normal faulting. The Skylark Shear Zone represents the basal detachment fault separating the upper and lower plate of the core complex. The geometry of normal faults in the upper plate is consistent with NE-SW extension.Both the upper and lower plates are intruded by ~1795–1575 Ma Hiltaba Suite granitic and mafic plutons. The core complex was extensively modified during the ~1570–1540 Ma Kararan Orogeny. Exhumation of the western and eastern Mount Woods Domain is indicated by new 40Ar/39Ar biotite cooling ages that show that rock packages in the central Mount Woods Domain cooled past ~300 °C ± 50 °C at ~1560 Ma, which was ~20 million years before equivalent cooling in the western and eastern Mount Woods Domain. Exhumation was associated with activity along major syn-Kararan Orogeny faults.

Using polygonal layer-bound faults as tools to delimit clastic reservoirs in the Levant Basin offshore Lebanon

The Levant Basin offshore Lebanon contains an array of layer-bound normal faults in the Oligocene–Miocene units. The faults are believed to have nucleated in fine-grained sedimentary rocks similar to polygonal fault systems worldwide, and as a result, they may be influenced by lithological heterogeneities in the host-rock unit. Three-dimensional seismic data and amplitude extraction from offshore Lebanon were used to map deep-water channels and fan lobes, demonstrating that the distribution, geometry, and growth of the layer-bound normal faults are affected by these sedimentary bodies, which are impacting fault propagation. Three fault types are identified, differentiated by their growth models and overall geometry. Type I faults in the deep basin indicate that a competent unit is present in the lower Miocene and is affecting their growth. Type II and III faults in the Latakia ridge and the margin, respectively, are smaller because of restriction by upper Miocene and lower Oligocene basin-floor fans. Based on their seismic expression and on analogy with other basins containing similar faults, it is very likely that the overlying and underlying units causing restriction consist of coarse-grained clastic intervals. Therefore, reservoirs offshore Lebanon are probably lower Miocene in the deep basin, whereas in the Latakia ridge and the Levant margin, reservoirs are in upper Miocene and in lower Oligocene units. This study provides an additional evidence that layer-bound, or polygonal faults, can have practical industrial application during exploration of hydrocarbons, even in the absence of well data.

Uncertainty in strain-rate from field measurements of the geometry, rates and kinematics of active normal faults: Implications for seismic hazard assessment

Multiple measurements of the geometry, kinematics and rates of slip across the Auletta fault (Campania, Italy) are presented, and we use these to determine: (1) the spatial resolution of field measurements needed to accurately calculate a representative strain-rate; (2) what aspects of the geometry and kinematics would introduce uncertainty with regard to the strain-rate if not measured in the field. We find that the magnitude of the post last-glacial maximum throw across the fault varies along strike. If such variations are unnoticed, different values for a representative strain-rate, hence different results in seismic hazard calculations, would be produced. To demonstrate this, we progressively degrade our dataset, calculating the implied strain-rate at each step. Excluding measurements can alter strain-rate results beyond 1σ uncertainty, thus we urge caution when using only one measurement of slip-rate for calculating hazard. We investigate the effect of approximating the throw profile along the fault with boxcar and triangular distributions and show that this can underestimate or overestimate the strain-rate, with results in the range of 72–237% of our most detailed strain-rate calculation. We discuss how improved understanding of the potential implied errors in strain-rate calculations from field structural data should be implemented in seismic hazard calculations.

Pre-existing intra-basement shear zones influence growth and geometry of non-colinear normal faults, western Utsira High–Heimdal Terrace, North Sea

Pre-existing intra-basement shear zones can induce mechanical and rheological heterogeneities that may influence rifting and the overall geometry of rift-related normal faults. However, the extent to which physical and kinematic interaction between pre-existing shear zones and younger rift faults control the growth of normal faults is less-well understood. Using 3D reflection seismic data from the northern North Sea and quantitative fault analysis, we constrain the 3D relationship between pre-existing basement shear zones, and the geometry, evolution, and synrift depositional architecture of subsequent rift-related normal faults. We identify NE-SW- and N-S-striking rift faults that define a coeval Middle Jurassic – Early Cretaceous, non-colinear fault network. NE-SW-striking faults are parallel to underlying intra-basement shear zone. The faults either tip-out above or physically merge with the underlying shear zone. For faults that merges with the basement shear zone, a change from tabular to wedge-shaped geometry of the hangingwall synrift strata records a transition from non-rotational to rotational extension faulting, which we attribute to the time of rift fault's linkage with the shear zone, following downward propagation of its lower tip. N-S-striking faults are oblique to, and offset (rather than link with) intra-basement shear zones. These observations highlight the selective influence pre-existing intra-basement shear zones may (or may not) have on evolving rift-related normal faults.

Active normal faulting along the Seerteng Shan, North China: Geometry and kinematics

Although rotation of fault blocks during seismic and long-term periods has long been recognized in extensional tectonic regimes, the geometry and kinematic behaviors of normal faults at outcrop scale have scarcely been investigated in sufficient detail. In this study, combining with the analysis of high-resolution topography acquired by small unmanned aerial vehicle (sUAV) using the structure-from-motion (SfM) technique, we present the results of our field observations on the structural characteristics of the Seertengshan Piedmont Fault (SPF), a major active normal fault bounding the northern margin of the Hetao Graben, North China. Although previous studies have reported the recent tectonic activity of this fault including timing of most recent earthquake, average slip rate and repeat time of paleo-earthquakes, few of them have focused on geometric patterns of the outcropping fault zones, which provide excellent outcrop-scale examples to investigating geometry and kinematics of active normal faults. Besides the observed prominent fault scarps indicative of recent tectonic activity along the SPF, outcrop observations indicate that in some cases both planar and curved normal faults are developed, forming complex fault zones. Sequential development of normal faults may have rotated the early-formed fault blocks, resulting in shallowly dipping faults and highly tilted strata. Collectively, structural similarities probably exit among normal faults at multiple spatial scales. We suggest that the results of this study are also helpful for a better understanding of structural development and evolution in other actively extending areas.

Tectonics Inversions, Fault Segmentation, and Triggering Mechanisms in the Central Apennines Normal Fault System: Insights From High‐Resolution Velocity Models

AbstractThe potential role of subsequent tectonic phases in reworking inherited geological structures is a key issue to unravel the seismotectonics of an area. This has a direct connection with fault segmentation, earthquakes maximum magnitude, and strong implications for seismic hazard assessment. The central Apennines (Italy) represent an exemplary case, since it developed because of the overprint of different deformational phases, producing potential conditions for episodic tectonic inversions and a very complex structural architecture. In this paper, we show how inherited compressional structures, still dominating the Apennines belt architecture, interfere with the active extension, having a direct connection with active seismotectonics. We present seismicity and new velocity tomograms of an 80‐km‐long section of the normal fault system activated during the 2009 and 2016–2018 seismic sequences. The joint interpretation highlights how the extensional seismic sequences partially reactivated inherited compressive structures, which have not an undisputable relationship with the surficial geological setting. Complexity deriving from the irregular geometry of normal faults and inverted thrust ramps is responsible for the observed intense fragmentation of the extensional system. Fluid overpressure seems to be a viable mechanism behind the partial remobilization of unbroken segments of the fault system.

Faults Geometry and the Role of Fluids in the 2016–2017 Central Italy Seismic Sequence

AbstractThe 2016–2017 Central Italy seismic sequence ruptured overlapping normal faults of the Apennines mountain chain, in nine earthquakes with magnitude Mw &gt; 5 within a few months. Here we investigate the structure of the fault system using an extensive aftershock data set, from joint permanent and temporary seismic networks, and 3‐D Vp and Vp/Vs velocity models. We show that mainshocks nucleated on gently west dipping planes that we interpret as inverted steep ramps inherited from the late Pliocene compression. The two large shocks, the 24 August, Mw = 6.0 Amatrice and the 30 October, Mw = 6.5 Norcia occurred on distinct faults reactivated by high pore pressure at the footwall, as indicated by positive Vp/Vs anomalies. The lateral extent of the overpressurized volume includes the fault patch of the Norcia earthquake. The irregular geometry of normal faults together with the reactivated ramps leads to the kinematic complexity observed during the coseismic ruptures and the spatial distribution of aftershocks.

Normal fault growth in layered basaltic rocks: The role of strain rate in fault evolution

Conceptual models for the evolution of dilatant faults in volcanic rift settings involve a step-wise growth pattern, involving upward propagation of subsurface faults, and surface monocline formation, which are breached by subvertical, open faults. Immature, discontinuous normal faults are considered representative of the early stages of mature, linked faults that accommodate extensional strains. We consider the evolution of surface-breaking normal faults using a comparison of the distribution and geometry of normal faults from two volcanic rift zones: the Koaʻe fault system, Hawaiʻi, and the Krafla fissure swarm, NE Iceland. Field mapping highlights similarities to current predicted geometries, but also prominent differences that are not reconciled by current models. Variable deformation styles record magma supply changes within the rift zones, which drive local strain rate gradients. Building on existing studies, we present a conceptual model of fault growth that accounts for spatial and temporal changes in strain rate within the deforming regions. We propose that faults in separate rift systems may not advance through the same stages of evolution and that faults within individual rift systems can show differing growth patterns. Variations in surface strains may be indicative of subsurface magmatic system changes, with important implications for our understanding of volcano-tectonic coupling.