Sara Martínez-Loriente

The Carboneras Fault Zone (CFZ) is one of the most active structures of the Alborán Sea, and together with the Al-Idrissi Fault, forms part of the Trans-Alborán Shear Zone (TASZ), which accommodates ongoing Africa-Eurasia convergence. Despite its tectonic significance, the offshore geometry and segmentation of the CFZ remain poorly understood. Here, we integrate multibeam bathymetry, high-resolution multichannel seismic reflection data, and three-dimensional (3D) structural modelling to reassess its offshore architecture. Our results show that the offshore CFZ forms a deformation corridor exceeding 110 km in length, comprising five principal and seven subsidiary fault segments. Individual segments range from 6.48 to 41.1 km in length and strike between N46º; and N79º. Interpretation of nine stratigraphic horizons, from the Tortonian acoustic basement to the seafloor, enabled the reconstruction of a 3D geological model of the fault system. Along-strike variations in deformation style, uplift, and segmentation allow subdivision of the CFZ into northeastern, central, and southwestern sectors. Structural complexity is greatest in the central sector, where fault branching, pressure ridges, a sag pond, and flower structures define a deformation corridor up to 4.1 km wide, and where uplift and accommodation contrasts between structural highs and fault-controlled depocentres are strongest. This complexity coincides with the transition between the dominant NE-SW-striking segments (N46º-N59º) and the more easterly southwestern segments (~N79º). Our findings refine previous models by resolving the 3D geometry, segmentation, and along-strike variability of the offshore CFZ, providing new insights into fault localization, tectonic inheritance, tectono-sedimentary evolution, and constraints for seismic-hazard assessment.

The Yusuf Fault system (YF) is one of the largest active strike-slip structures in the Alborán Sea, where it acts as a lithospheric-scale boundary accommodating part of the NW-SE convergence between the Nubian and Eurasian plates. It trends WNW-ESE, extends for ~150 km, and is characterized by a complex fault array and a prominent pull-apart basin. Accurate representation of its 3D geometry and seismogenic behavior is essential to constrain its seismic potential and understand the tectonic evolution of the region. In this study, we integrate multibeam bathymetry, high-resolution multi-channel seismic reflection (HR-MCS) data, and scaled analogue modeling to characterize the structural architecture and kinematic evolution of the YF. Bathymetric analysis using topographic attributes enabled detailed mapping of the fault trace and associated geomorphic features, while seismic data interpretation revealed subsurface discontinuities and fault offsets affecting key seismostratigraphic units from the Upper Miocene to the Quaternary. This information allowed us to construct the first 3D structural model of the YF, showing that the system consists of multiple overlapping segments forming a complex strike-slip fault architecture. To explore the processes controlling pull-apart basin development, we conducted analogue experiments simulating strike-slip fault interaction. Results suggest that basin opening is controlled by overlapped fault geometry and lateral displacement rates, providing a physical framework for interpreting the observed morphostructural patterns. This integrated approach improves constraints on fault growth and segmentation, offering critical input for seismic hazard models in the western Mediterranean.

The NE-SW Carboneras Fault is one of the main active structures in the Alborán Sea and, together with the Al-Idrisi Fault, forms the Trans-Alborán Shear Zone, which connects the Betic and Rif Cordilleras. The accurate representation of the offshore 3D geometry and seismogenic characteristics (e.g., slip rate or maximum magnitude) of this large left-lateral strike-slip fault system is essential for assessing the seismic potential in a slow-deforming region, such as the Alborán Sea. Here, we combine the interpretation of multibeam bathymetric and high-resolution multi-channel seismic reflection (HR-MCS) data to reassess the offshore extent of the Carboneras Fault and the adjacent morphostructural elements. Topographic attributes were applied to the bathymetric data to enhance the visualization of the fault trace at the seafloor. Our findings suggest that the offshore Carboneras Fault extends for 110.3 km, exceeding previous estimates. We identified twenty fault segments along the Carboneras Fault trace, with individual lengths up to 46 km and azimuths varying from N40º to N238º. Geomorphic features typically associated with strike-slip systems such as deflected drainage systems, shutter and pressure ridges, and horsetail splays were also identified. Structural seismic attributes applied to the HR-MCS profiles highlight major subsurface discontinuities, allowing detailed mapping of the fault geometry at depth. Seismic interpretation also accounted with the identification of seven horizons offset by the Carboneras Fault, comprising the Paleozoic-Triassic basement, the Messinian unconformity and Early Pliocene to Late Quaternary seismic units. Based on this information, we constructed fault and horizon surfaces to develop the first 3D model of the Carboneras Fault. This model provides key constraints on crustal architecture and offers new insights into fault growth mechanisms, thereby reducing uncertainties and improving the assessment of the seismogenic potential of this major offshore fault system.

Investigating the dynamic evolution of active fault systems is fundamental for deciphering regional tectonics and assessing seismic risk, particularly in offshore regions where direct observation is challenging. Our ultra‐high‐resolution bathymetric study of the North‐South Faults (NSF) documents an incipient and dynamically evolving left‐lateral transtensional system, comprising 41 right‐stepping, highly segmented fault scarps and associated pockmarks. The NSF’s fault interactions and morphological scarp profiles analysis support a hybrid fault growth model, with coherent constant‐length growth predominating, and a sharp northwestern boundary reflecting an actively propagating shear zone. This kinematic interpretation is reinforced by en echelon patterns and focal mechanisms displaying strike‐slip and extensional components. The NSF appears to be the northward‐propagating tip of the larger Al‐Idrissi Fault, and its present‐day activity may be supported by fault‐aligned pockmarks. Empirical calculations show that faults can generate earthquakes up to Mw 6.5, and we propose the NSF as a plausible alternative source for the historical Mw 6.1, 1910 Adra earthquake, suggesting that a full system rupture could pose a greater seismic hazard to coastal populations than currently recognized. This research advances our understanding of early stage fault growth and provides valuable insights into the dynamics of evolving transtensional fault systems, applicable to other tectonically active offshore environments.

Geomorphological approaches are essential for advancing our understanding of fault dynamics and assessing better their seismic hazard, especially offshore where direct geological observations are inherently challenging. This study employs high-resolution bathymetric data (1 m) to conduct a detailed quantitative morphometric analysis of individual fault scarps along the North-South Faults (NSF). Our analysis provides a comprehensive characterization of this fault system, including key morphotectonic features such as tectonic depressions, horst and graben structures, half grabens, and pockmarks. Specifically, the fault scarps morphometric analysis derived from evaluating diverse bathymetric profiles across each fault scarp, reveals distinct patterns of vertical displacement, fault growth, and connectivity along the NSF. Vertical displacement ranges from centimetres to decametres, with the largest scarp and fault displacements consistently located in the southern area. This spatial distribution highlights a progressive northward propagation of the fault system, reflecting its evolving dynamics. The presence of relay ramps, stepovers, and interconnected segments indicates that the NSF is an incipient fault system developing within a left-lateral transtensional regime. Our findings support the interpretation of the NSF as the northern extension of the Al-Idrissi Fault, emphasizing its role within the broader tectonic framework of the Alboran Sea. Furthermore, the potential connection between the faults within the NSF suggests that this system could generate earthquakes up to magnitude Mw 6.1. Considering this, and based on the proposed location and the calculated focal mechanism of the 1910 Adra earthquake, we also hypothesize that the NSF may represent an alternative source for this event. This research highlights the importance of surface process analysis in unravelling fault evolution and its broader implications for regional geodynamics.

Mud volcanoes (MVs) are significant geologic and ecological features, widely distributed across the Gibraltar Strait region. While initial discoveries during UNESCO TTR expeditions in the 1990s documented mud volcanism on both sides of the Strait, subsequent studies east of Gibraltar have primarily concentrated on the Ceuta contourite drift, where rapidly deposited sediments created conditions for shale diapirism and related expulsion structures. Here, we report the discovery of a previously unknown field of mud volcanoes on the Moroccan continental slope, west of Melilla. This finding arises from the STRENGTH Leg 3 expedition aboard the R/V Sarmiento de Gamboa in April 2023. Collected bathymetric data and imagery from a towed side-scan sonar revealed 15 distinct MVs, characterized by conical morphologies, basal moats, and mud flows extruded from summit emission sites. These features, reaching up to 30 meters in height and 300 meters in diameter, are interpreted as dormant MVs due to the absence of active mud expulsion. Nevertheless, ROV observations documented extensive biological colonization, including corals, sponges, and other sessile organisms, highlighting their role as ecological hotspots, due to the more competent substrate MVs provide. Geophysical data, particularly sparker profiles, have provided detailed seismic imaging of the upper ~300 meters below the seafloor. These profiles revealed extensive fluid migration pathways feeding the MVs, with distinct gas-related wipeouts both beneath the MVs structures and laterally within surrounding sediments at a consistent depth. These subsurface anomalies combined with the presence of sessile organisms (colonizing carbonate crusts deposited by authigenic mineral precipitation) suggests ongoing fluid dynamics despite their apparent dormancy. Sediment cores from several MVs were retrieved for geochemical and geochronological analyses, which will shed light on the origin and evolution of these features. This discovery expands our understanding of mud volcanism in the region and provides a foundation for future interdisciplinary studies of fluid migration, tectonics, and cold-seeps associated ecosystems.

The Alboran Sea is one of the most seismically regions of Western Europe, accommodating an important part of the current NW-SE convergence between the African and Eurasian plates (4-5 mm/yr). Despite the Alboran Sea is considered as a region of relatively low tectonic deformation and diffuse seismicity, the major faults within have been responsible of large earthquakes (IEMS>IX) since historical times (e.g., 1522 Almería, 1790 Oran, 1804 Alboran, 1910 Adra or 2016 Al-Idrissi earthquakes). One of the main issues for the characterization of the seismic hazard in the Alboran Sea, common to many low-deformation regions, lies in accurately constraining the seismic parameters that define fault activity and their behavior (i.e., slip rates, earthquake recurrence and multi-fault rupture capability, among others). This issue is further aggravated by the fact that these faults are mostly located offshore, making their investigation more challenging. As a result, most faults in the Alboran Sea have poor slip and activity rate estimates, while their capability to interact and rupture in complex rupture patterns has not been explored yet. In this study, we compute several models of earthquake rupture rates for the Alboran Sea with the SHERIFS code and using a systematic parameter exploration tree to determine the parameters of each model. We base the exploration tree on the slip rate and multi-fault rupture scenarios, allowing us to investigate the epistemic uncertainty linked to these parameters. To check the feasibility of the computed earthquake rates of each model, we compare them with the observed seismicity rates in the region. As a result, this enables us to identify which parameter combinations best match the recorded seismicity, prioritizing those that perform better for the hazard assessment. In addition, these optimal values might be used as indicators for further studies focused on better constraining the fault parameters in some faults, as well as for preliminary fault-based seismic hazard assessments. By extension, the limitations of the modelling in terms of slip rate budget distribution and fault rupture scenarios can also be used to determine which areas should be prioritized for further research. We expect that the results of our work enhance discussion among researchers working in the area and motivate further investigations into fault dynamics.

The present-day Eurasia-Africa plate convergence offshore SW Iberia gives rise to a diffuse plate boundary marked by deep lithospheric thrust and strike-slip faults. The Horseshoe Abyssal Plain Thrust (HAT) stands out as a key structure accommodating plate convergence, and it has been the site of deep (> 30 km depth) and large magnitude (Mw > 6) earthquakes. Additionally, the HAT has been proposed to be the source of the 1755 Lisbon earthquake (estimated Mw≥8.5), one of the most destructive earthquakes and tsunami in the history of Europe. The geometry of the fault and the physical properties of rocks surrounding it have been determined through tomographic models derived from controlled-source seismic data. Although large earthquakes along the HAT primarily occur at considerable depths within the peridotitic mantle (~40 km depth), the fault intersects a region of serpentinized mantle at shallower depths (10-20 km depth). In contrast to peridotite that undergoes seismic deformation, the frictional behaviour of serpentinized peridotite depends on factors such as pressure, water content, temperature, and slip velocity. Laboratory measurements indicate that serpentinite transitions from rate-strengthening behaviour at plate tectonic rates to rate-weakening at seismic slip rates. This dual nature suggests that large deep earthquakes, nucleated in pristine peridotite, could rupture seismically through the weaker serpentinized peridotite. While this mechanism has been proposed to explain the HAT's potential to generate large tsunamigenic earthquakes, it remains untested. In this study, we use dynamic rupture numerical simulations to investigate the role of serpentinized peridotite in the rupture process and the tsunamigenic potential of the HAT. In particular, we explore various frictional scenarios to determine the slip pattern necessary to account for the previously estimated tsunamigenic uplift associated with the 1755 Lisbon earthquake.

Seismic hazard assessment requires a detailed understanding of the evolution of fault systems, rupture processes, and linkage between segments. Identifying and characterizing Quaternary surface faulting features, such as fault scarps, provide valuable morphotectonic data on cumulative displacement over time, especially in regions with moderate to low seismic activity. Although fault cumulative vertical surface offsets have been traditionally measured using topographic profiles, these profiles are unevenly spread along the faults and may not reflect all the morphological changes along them. To address this situation, expanding the analysis to encompass a larger number of profiles is a viable option; nevertheless, manually executing this task would prove significantly time-consuming. Machine Learning (ML) has shown unprecedented capacities to evaluate large datasets in reduced time and provide a wealth valuable information with their related uncertainties. With this in mind, we propose a ML algorithm called ScarpLearn based on Convolutional Neural Networks (CNN) to compute the vertical cumulative displacement and its uncertainty for normal fault scarps. Despite ScarpLearn being initially developed to characterize simple scarps in onshore areas, we have enhanced its capabilities so that it can also be used in offshore areas subject to oceanic processes. This includes, among others, more intense diffusion, or the presence of seabed features such as pockmarks. Additionally, we have improved the code's versatility by providing a method modification that allows it to better characterization of scarps in more complex areas where multiple faults offset the seafloor. To this, we have trained the algorithm using a large database of realistic synthetic bathymetric profiles, including different parameters such as fault dip, slip velocity, scarp spread, scarp diffusion coefficient, and variable resolutions to ensure adaptability to all datasets. These modifications have resulted in the improvement of the ScarpLearn algorithm’s adaptability, elevating its accuracy and reliability in capturing the complexity of marine fault systems, but also applicable to terrestrial systems. We have applied the new ScarpLearn version to the North-South faults of the northern Alboran Sea, contributing to the accurate analysis of this Plio-Quaternary transtensional system and its complex geological structures. This innovative approach has allowed us to refine the morphotectonic analysis of the area and to understand better the geodynamics of the North-South faults system. In this research, we have explored the advances of the CNN method achieved in oceanic environments, considering intensive data compilation, computational time, accuracy, uncertainties, and current limitations. Our advances demonstrate the ScarpLearn ML potential, specifically tailored to analyze marine environments and multiple fault segments both onshore and offshore. Our research results contribute to the progress of marine geosciences by improving morphotectonic analysis using ML algorithms. Keywords: Convolutional Neural Networks (CNN), Oceanic processes, Normal faults, Multiple scarps.

Earthquake hazard analyses rely on seismogenic source models. These are designed in various fashions, such as point sources or area sources, but the most effective is the three-dimensional representation of geological faults. We here refer to such models as fault sources. This study presents the European Fault-Source Model 2020 (EFSM20), which was one of the primary input datasets of the recently released European Seismic Hazard Model 2020. The EFSM20 compilation was entirely based on reusable data from existing active fault regional compilations that were first blended and harmonized and then augmented by a set of derived parameters. These additional parameters were devised to enable users to formulate earthquake rate forecasts based on a seismic-moment balancing approach. EFSM20 considers two main categories of seismogenic faults: crustal faults and subduction systems, which include the subduction interface and intraslab faults. The compiled dataset covers an area from the Mid-Atlantic Ridge to the Caucasus and from northern Africa to Iceland. It includes 1248 crustal faults spanning a total length of ∼95 100 km and four subduction systems, namely the Gibraltar, Calabrian, Hellenic, and Cyprus arcs, for a total length of ∼2120 km. The model focuses on an area encompassing a buffer of 300 km around all European countries (except for Overseas Countries and Territories) and a maximum of 300 km depth for the subducting slabs. All the parameters required to develop a seismic source model for earthquake hazard analysis were determined for crustal faults and subduction systems. A statistical distribution of relevant seismotectonic parameters, such as faulting mechanisms, slip rates, moment rates, and prospective maximum magnitudes, is presented and discussed to address unsettled points in view of future updates and improvements. The dataset, identified by the DOI https://doi.org/10.13127/efsm20 (Basili et al., 2022), is distributed as machine-readable files using open standards (Open Geospatial Consortium).