Giovanni Camanni

Placing constraints on the geometry and growth of faults has significant implications for the management of resources in the subsurface; faults are widespread structures and can either compartmentalise subsurface reservoirs or provide favourable fluid migration pathways.There are two widely accepted models of fault growth: the isolated fault growth model and the constant length fault growth model. These models, largely derived from field, seismic, and analogue modelling data, both describe three stages in fault growth: (1) propagation of fault segments, (2) slip accumulation on fault segments, and (3) segment interaction and linkage. However, they differ in how the initial fault segments interact, whether they are kinematically dependent, and how rapidly their full length is established. The debate is currently still open on which model best describes natural faults, and what geological controls favor one model over the other.To further address the topic of fault growth, we investigate the different stages of growth through two sets of experiments. First, loading experiments are performed on intact samples of a rock analogue material to track both the propagation of the fracture and the displacement accumulation to test which of the two fault growth models most accurately describes the initial stages of fault propagation. Second, loading experiments are performed on samples with pre-cuts to replicate realistic fault segment geometries, to track fault tip migration and displacement partitioning during the linkage stage of fault growth, to test geometrical controls on the process of fault linkage.The samples are made of a rock analogue material capable of accommodating displacement gradients through ductile processes, similar to those observed in natural rocks over geological timescales. This material is cohesive and allows the creation of pre-cuts to replicate fault segment geometries. Loading experiments are conducted in a biaxial apparatus at low strain rates, coupled with an interferometric technique using an optical bench to obtain speckle patterns. These speckles are employed to track in high resolution the increase in the length of the fault using Diffusive Wave Spectroscopy (DWS). In addition, we concurrently use the same speckle patterns to track displacement along the fault using Digital Image Correlation.

The South China Sea (SCS) formed through Cenozoic lithospheric stretching of the southeastern edge of the Eurasian Plate and is one of the largest marginal seas in the Western Pacific region. In recent years, its northern continental margin has emerged as a global hotspot for geoscientific research, particularly for advancing our understanding of rift evolution and continental margin architecture. An exceptional wealth of new geological and geophysical data from the hydrocarbon exploration industry and recent scientific ocean drilling expeditions has particularly driven this surge. Building on this opportunity, we assembled the Thematic Collection ‘ Emerging knowledge on the tectonics of the South China Sea ' , which integrates new results with existing datasets to provide an updated and comprehensive view of the northern SCS continental margin. In this preface, we provide an overview of the collection, which comprises nine original research articles. Collectively, the contributions reveal a complex structural evolution of the continental margin from proximal to distal domains, characterized by punctuated stages of rifting and pronounced along-strike structural variability. Additionally, they document structural and sedimentological controls on potential hydrocarbon resource distribution across the region. These new insights not only advance our knowledge of the northern SCS continental margin but also contribute to deciphering the inherent complexities across plate-edge (marginal sea) rifts worldwide.

The reactivation of deep-seated, throughgoing strike-slip faults produces highly segmented fault zones at shallower levels, yet how displacement is partitioned among segments as the fault grows remains poorly constrained. We address this by examining a ~160-m-long section of the East Anatolian fault surface rupture of the Mw 7.8 2023 Kahramanmaraş strike-slip earthquake (Turkey). The rupture offsets meter-spaced ridges and furrows in a ploughed field, providing a unique opportunity for detailed displacement measurements, which were obtained from an orthophoto and a digital elevation model (DEM) built for this study. The surface rupture consists of ten primary segments separated by nine restraining stepovers and is associated with a 3-D strain field dominated by ~3.5 m of left-lateral offset. Displacement patterns allow us to derive a new model for strike-slip fault growth in which deformation is asymmetric and controlled by the 3-D structure of the fault—specifically, by the position of the underlying fault relative to shallow fault segments. Initially, displacement is accommodated by contraction within restraining stepovers. With increasing displacement, fault segments located closer to the trace of the underlying fault consistently accumulate greater displacement. This asymmetry is facilitated by enhanced synthetic rotation within stepovers. With further displacement, the higher-displacement segment bounding a stepover becomes dominant, while the other one is bypassed and layer rotation ceases. These results not only improve fault growth models but could also inform seismic hazard models and impact industrial applications dealing with subsurface faulted reservoirs.

Reconstructing the spatiotemporal evolution of fault systems in rift basins is essential for characterizing reservoirs used in geothermal exploration and CO2 or hydrogen storage projects. This study aims to elucidate the growth and reactivation of inverted normal faults in the West Netherlands Basin and their implications for subsurface renewable energy projects. With a complex tectonic history, the area experienced multiple rifting phases and basin inversion. Fault displacement-distance diagrams were produced by using an updated semi-automated workflow for nine major basin-scale faults, providing new insights into the lateral and vertical growth of faults in inverted rift basins.This study demonstrates that the faults in the West Netherlands Basin developed their lateral lengths during the early stages of Triassic rifting. Subsequent Jurassic extensional phases caused reactivation, leading to a consequent increase of vertical displacement and creating accommodation space for the deposition of the study area’s main reservoir rock. Variations in the reactivation behaviour along the different fault segments were promoted by stress field rotations, which significantly influenced the distribution and extent of sediment deposition. This resulted in a complex reservoir architecture that is characterized by spatial heterogeneities in porosity and permeability.We identified contractional features, such as pop-up structures and fault-propagation folds, formed by positive fault reactivation during Late Cretaceous basin inversion. The strength of inversion was influenced by the geometry and orientation of pre-existing faults and the thickness of the underlying sedimentary cover. Inversion-related structures further complicate the basin’s architecture, by compartmentalizing the reservoir rock and influencing sediment distribution patterns.Our findings show an example of how fault dynamics can affect the geothermal reservoir quality and storage capacity of subsurface exploration targets. This study provides valuable insights for optimizing exploration strategies and storage site selection by integrating fault growth and reactivation analysis. This helps to further reduce geothermal exploration risks and enhances storage efficiency in rift basin settings.

Shallow crustal fault zones, particularly those within carbonate successions, are deformation zones that influence subsurface fluid migration, localization, and interaction. These zones, characterized by fault-controlled fracturing and brecciation, can act as important conduits for fluid flow in the Earth's crust. The interaction between fluids and the fractured carbonates rock matrix can induce significant changes in the mineralogical and mechanical properties of fault zones. Therefore, studying fluid-rock interactions in such environments is crucial not only for understanding the natural processes governing subsurface fluid dynamics, deformation, and metamorphism but also for addressing significant challenges in energy and mineral exploration as well as in underground engineering.This study presents, for the first time in the southern Apennines (southern Italy), evidence of fault-driven hydrothermal dolomitization during the late Triassic rifting event in the western Adria plate. We investigated the fault-controlled saddle dolomite formation in Norian dolomites exposed in the western sector of the Matese Massif. The study focused on dolomite breccias associated with N-S and NNW-SSE striking normal faults. These structures include layers of mature cataclasites made of clasts with angular boundaries within a highly porous matrix, crossed by veins, mosaic and chaotic breccias. The breccias are composed of angular clasts of host rock dolomite, formed by early marine replacive dolomitization of shallow-water carbonates, surrounded by coarse saddle dolomite cement. The saddle dolomite cement is characterized by two distinct phases. The first phase (SD1) is yellow, inclusion-rich, and forms a rim around the clasts, while the second phase (SD2) is euhedral, exhibiting well-defined zoning with a transition from cloudy to limpid crystals. The saddle dolomite cement texture, coupled with decreasing δ18O and 87Sr/86Sr values, suggests that it precipitated at temperatures of 100-120°C from fluids that likely interacted with magmatic sources.U-Pb dating of the dolomite cement provides late Triassic crystallization ages of approximately 206 ± 13 Ma and 217.0 ± 6.6 Ma. Additionally, the ferroan dolomite cement contains quartz and hydrothermal minerals, including fluorite and apatite, in minor quantities. These findings suggest that the brecciation and hydrothermal saddle dolomite precipitation were linked to normal fault activity during the breakup of Pangea, contributing to the separation of the SW sector of Eurasia from the western margin of the Adria Plate.

The Irpinia region is one of the most seismically active areas of Italy owing to continuing, late-orogenic extension in the axial zone of the Apennine mountain belt. However, the 3D architecture and the nature of the faults that drive this extension are still uncertain, posing challenges to seismic hazard assessment. Here, we address these uncertainties by integrating a new catalogue of high-resolution micro-seismicity (ML < 3.5), complemented by earthquake focal mechanisms, with existing 3D seismic velocity models and geological data. We found that micro-seismicity is primarily taking place along a segmented, approximately 60 km long, deep-seated, Mesozoic normal fault that was inverted during the shortening stages of the Apennine orogeny and then extensionally reactivated during the Quaternary. These findings suggest that multiple events of reactivation of long-lived faults can weaken their strength, making them prone to co-seismic remobilization under newly imposed strain fields in active mountain belts.

Studying fault zone properties is crucial in addressing key challenges in subsurface exploration, resource management, and seismic risk evaluation. As global interest in geothermal energy, hydrogen, and carbon storage intensifies, the mechanical and structural characterization of faults, as well as their impact on fluid migration, reservoir integrity, and fault sealing analysis, is becoming increasingly important. The presented study focuses on the structural and mechanical properties of superposed fault zones in dolostones, an issue that has been poorly investigated. In particular, we addressed how the architecture of an earlier, large-scale normal fault (F1) influences the geometry and deformation mechanisms of younger, superposed strike-slip faults (F2). The F1 fault consists of four sub-parallel fault rock units, each several tens of meters thick: (i) a cataclastic core (Cu), bounded in the hanging wall by (ii) cemented micro-mosaic breccia (MB), and in the footwall by (iii) high-strained (HS) and (iv) low-strained (LS) fault rocks. To achieve this aim, we performed some geotechnical and morphometric analyses. Uniaxial compressive strength (UCS) tests revealed mean values of 83 MPa and 120 MPa for MB and LS, respectively, whereas Cu and HS exhibit lower UCS values of 58 MPa and 62 MPa, respectively. MB and Cu exhibit heterogeneous particle size distributions (PSD) and porosities of 4.02% and 4.96%, respectively, while HS and LS show more homogeneous PSDs with porosities of 2.87% and 2.19%, respectively.The F2 faults developed a spectrum of structural facies such as cataclastic shear bands (CSBs) in the LS and HS, and as compaction bands (CBs) in the MB and Cu. In the LS, cataclasis is highly localized within widely spaced, thick, tabular, and cemented CSBs. In the HS, deformation occurs through anastomosing CSBs, accommodating diffuse cataclasis and dissolution-precipitation mechanisms. In the Cu, anastomosing CBs develop through pore collapse and dissolution-precipitation processes. In the MB, compaction is driven by pore collapse and grain crushing, forming well-localized, widely spaced CBs.Overall, the microstructural properties (PSD and porosity) and mechanical strength of the F1 fault rocks are key factors influencing the deformation mechanisms (cataclasis vs. compaction) and the geometry (localized vs. anastomosing) of the F2 faults. These findings contribute to the understanding of fault permeability and fluid flow dynamics in multi-faulted dolomite reservoirs.

The Irpinia Near Fault Observatory is a dense instrumented infrastructure monitoring the normal fault system of the Irpinia region, in the Southern Apennines (Italy). The area is one of the highest seismic hazard regions in Italy; nevertheless, the background seismicity rate is low, with about 4000 earthquakes detected in the last 15 years of network operation, with a magnitude of completeness of 1.1. Thus, understanding the fault system mechanical state requires high quality data and advanced tools for seismicity location and characterization, along with new monitoring systems, able to catch the earthquake signals at or below the noise level. In this study, we investigated the geometrical and mechanical properties that can be inferred from the (micro)seismic sequences occurred during the network operation. The seismic catalogs enhanced through the use of machine learning and similarity-based techniques, and double-difference event relocation show that the events define kilometric-scale structures, sub-parallel to the main faults that generated the M 6.9, 1980 Irpinia earthquake. We estimated the stress release and the rupture area of the events within the sequences, showing that the static stress transfer is the main mechanism of triggering of the events within the sequences. The seismicity delineates slip-driven alignments that can be associated with fault roughness. In the case of the major sequence, the event distribution might also indicate the occurrence of an aseismic deformation episode, too small to be detected by surface GNSS instruments. Finally, we also analyzed the seismicity recorded along the 1-year DETECT experiment, during which 200 velocimetric stations were deployed as a constellation of seismic arrays, within Irpinia Near Fault Observatory region. We discovered that deep seismicity mainly occurs in sequences, with most events having a magnitude smaller than the unity. When jointly analyzed with the 3D P-wave tomographic model, these sequences illuminate a SW-dipping, previously unknown, segmented fault with a total length of more than 30 km. This structure could be causative for a future M6+ event, depending on the rupture ability to overcome the geometrical stepover on the fault.

We present a method for automatically identifying segmented fault surfaces through the clustering of earthquake hypocenters without prior information. Our approach integrates density-based clustering algorithms (DBSCAN and OPTICS) with principal component analysis (PCA). Using the spatial distribution of earthquake hypocenters, DBSCAN detects primary clusters, which represent areas with the highest density of connected seismic events. Within each primary cluster, OPTICS identifies nested higher-order clusters, providing information on their quantity and size. PCA analysis is then applied to the primary and higher-order clusters to assess eigenvalues, enabling the differentiation of seismicity associated with planar features and distributed seismicity that remains uncategorized. The identified planes are subsequently characterized in terms of their location and orientation in space, as well as their length and height. By applying PCA analysis before and after OPTICS, a planar feature derived from a primary cluster can be interpreted as a fault surface, while planes derived from high-order clusters can be interpreted as fault segments within the fault surface. The consistency between the orientation of illuminated fault surfaces and fault segments, and that of the nodal planes of earthquake focal mechanisms calculated along the same faults, supports this interpretation. We show applications of the method to earthquake hypocenter distributions from various seismically active areas (Italy, Taiwan, California) associated with faults exhibiting diverse kinematics.