Abstract Condor Seamount is among the most seismically active regions on the Azores Plateau. In December 2019, an earthquake crisis occurred at the tip of the seamount, which was strongly felt on nearby islands. Two months after the peak of the crisis, we deployed a local network of five ocean‐bottom seismometers around the seamount, recording seismicity over 3 months. Using a machine‐learning‐based phase picker, we constructed a high‐resolution earthquake catalog. Jointly interpreting our local data and a long‐term regional catalog reveals episodic but spatially stable earthquake activity in the uppermost mantle. We propose the presence of an inflating magma reservoir that may feed a future eruption and extend the edifice westward. Furthermore, reversed P‐wave polarities were observed at the margin of the main earthquake swarm, but were absent in three other smaller swarms, supporting a local stress rotation model for magma intrusion.

Knowledge of the strength of submarine volcaniclastic deposits is important for assessing the stability of slopes of such materials and their geohazards but is difficult to measure. An opportunity for an alternative evaluation has been presented by an earthquake swarm under a volcanic seamount in the Azores. Attenuation relationships applied to earthquake data suggest that a cone field and flanks of the seamount experienced horizontal accelerations of >0.3 g during the swarm. However, multibeam sonar data collected before and after the swarm suggest that no slope failures occurred. Furthermore, in backscatter data collected after the swarm, low intensities below slopes suggest that muddy aprons were undisturbed by landslide debris. The swarm overlies cones with slopes near typical repose angles of non-cohesive particles. During earthquake shaking, the direction of maximum acceleration deviates from that due to gravity alone. We show that cone slopes effectively experienced much steeper gradients than their repose angles during the swarm. As they survived the shaking without failing, they were effectively stronger than non-cohesive sediment. We use a pseudo-static analysis to investigate the implied sediment strength, finding a ratio of undrained shear strength to vertical stress of >0.4-0.5. This implies shear strength of >24-30 kPa at 10 m depth below seabed. We speculate that carbonate cements and/or compaction may be responsible. If shallow areas are more widely strengthened, slope failure may then be less likely during moderate (M-L - 4.0 or less) seismic shaking and hence be less hazardous than if the slopes comprised wholly non-cohesive materials.

The central islands of the Azores Archipelago in the North Atlantic straddle a diffuse zone of dextral transtension between the African and Eurasian plates, providing an ideal setting for studying the interplay between tectonics and magmatism. São Jorge is a narrow island dominated by a westward progression of past basaltic fissure eruptions, where fault zones act as volcanic rifts. After two inland eruptions with significant socioeconomic impact in 1580 and 1808, the most recent probable eruption occurred offshore in 1964, after two years of seismic activity. In March 2022, a seismic crisis began on São Jorge (magnitudes up to ML 3.8). Our analyses of InSAR and GNSS data are consistent with a dike intrusion that stalled at 2 km depth below sea level. Here, we use seismicity to probe the space-time evolution of the intrusion. The unique geography and near-coastal position of seismicity yield inherently uncertain locations. To address this, we supplemented on-land stations with 6 ocean-bottom seismometers (OBSs) around the island later in the crisis. We use NLL-SSST-coherence, a location method ideal for changing station density, to exploit later OBS data to form robust source-specific station terms that allow precise relocation of the earlier part of the seismic sequence when coverage was sparser. In a final step, we combine waveform coherence and location uncertainty stacks to enhance hypocenter location precision to

A variety of subaerial and submarine events, including mass-wasting and volcanism, can generate sediment gravity flows and fallout deposits that are preserved in deep-water stratigraphic records. This study examines whether event beds with differing depositional and transport histories exhibit distinct thickness-frequency distributions. Analyzing over 4,500 event beds from seven drilling sites near Montserrat, the Izu Arc, the Kyushu-Palau Ridge, and Gran Canaria, the analyses explore variations in event-bed characteristics across different climatic periods, volcanic stages, and geomorphological settings. Statistical methods include characterizing thickness-frequency distributions and assessing subset similarity using t-tests and smoothed distribution patterns. The data-driven results indicate discernible differences where dominant geological processes vary. For example, volcanic growth stages at the Kyushu-Palau Ridge produced thicker, coarser, and more frequent event beds compared with quiescent stages. Similarly, beds from the north slope of Gran Canaria-where submarine canyons enhanced sediment delivery-were nearly twice as thick as those from the south. In contrast, indistinguishable characteristics between the rear and frontal Izu Arc subsets after 3 Ma are attributed to the development of an extensional zone supplying material to both arc sides. Comparable distributions were also observed within intervals with minimal geological differences. The reliability of this analytical approach depends on high-quality sediment recovery, as drilling-related disturbances may obscure primary depositional signals. Beyond stratigraphic characterization, the method shows broader potential for identifying the provenance of volcanic glass shards through geochemical comparisons and for evaluating the statistical compatibility of data sets from neighboring sites, ensuring sufficient sample size for robust integrated analyses.

The breakup of the Arabian-Nubian (Ar-Nu) shield created the Red Sea rift, a young continental rift transitioning to an ocean basin. Different Ar-Nu plate reconstruction models have emerged, which complicate interpreting the shield geology, reconstructing motions of other plates and leave open the question of how much oceanic crust underlies parts of the sea. Uncertainty of plate reconstructions also affects how we interpret the dynamics of rifting in this basin from geophysical data. Helping to address this issue, gravity anomalies reveal an oceanic segmentation fabric that constrains the opening direction since similar to 10 Ma or half the total Ar-Nu movement. We derive an Euler plate reconstruction pole from that fabric and apply it to restore the configuration of Ar-Nu shield structures at similar to 10 Ma. With the Ar and Nu structures brought closer together, the reconstruction supports a new association of those structures across the Red Sea. That association in turn constrains the pre-breakup relative positions of the Ar-Nu shield fragments in a NW-SE sense (i.e., rift obliquity). It allows us to prioritise earlierpublished total opening Euler poles that also bring these structures together. In particular, one of them was derived using the known Dead Sea Transform (DST) and Suez Rift motions, which constrain total Ar-Nu separation magnitude and direction. New Moho depth estimates further allow an assessment of how much distributed extension has left coastal units displaced relative to plate interiors. Applying the total-opening pole, and allowing for those coastal displacements, the pre-breakup shield terranes of the two sides are brought together but with significant gaps remaining between the two sides. The results support the views that the northern Red Sea is underlain largely by stretched continental crust and that the central Red Sea is underlain by both continental and oceanic crust.

The Red Sea Rift is an ultra-slow spreading rift filled with Miocene salt and younger sediments. While volcanic features can be observed in exposed areas in the southern Red Sea Rift, evidence of volcanism in the sediment-blanketed regions in the central and northern Red Sea Rift has been lacking, leaving open whether the mid-ocean rift axis continues beneath them. Here, we present new multichannel seismic and high-resolution bathymetric data of these blanketed regions. Our data reveals multiple instances where oceanic crust can be traced beneath the evaporite cover, forming volcanic edifices protruding through the sediment cover. We identify abundant circular depressions in the sediment cover as volcanic craters, which formed by deep-sea explosive volcanism or caldera collapses. The common occurrence of volcanic features in the sediment-covered regions supports the continuous formation of oceanic crust along large parts of the Red Sea Rift.

The central islands of the Azores Archipelago in the North Atlantic straddle a diffuse zone of dextral transtension between the African and Eurasian plates, providing an ideal setting for studying the interplay between tectonics and magmatism. São Jorge is a narrow island dominated by a westward progression of past basaltic fissure eruptions, where fault zones act as volcanic rifts. After two inland eruptions with significant socioeconomic impact in 1580 and 1808, the most recent probable eruption occurred offshore in 1964, after two years of seismic activity. In March 2022, a seismic crisis began on São Jorge (magnitudes up to ML 3.8). Our analyses of InSAR and GNSS data are consistent with a dike intrusion that stalled at 2 km depth below sea level. Here, we use seismicity to probe the space-time evolution of the intrusion. The unique geography and near-coastal position of seismicity yield inherently uncertain locations. To address this, we supplemented on-land stations with 6 ocean-bottom seismometers (OBSs) around the island later in the crisis. We use NLL-SSST-coherence, a location method ideal for changing station density, to exploit later OBS data to form robust source-specific station terms that allow precise relocation of the earlier part of the seismic sequence when coverage was sparser. In a final step, we combine waveform coherence and location uncertainty stacks to enhance hypocenter location precision to <100m. Relocations of ~12,000 earthquakes show precursory, weak seismicity that started ~6 months before, starting offshore, south of São Jorge before migrating to shallower depth beneath the centre of the island. The main seismic crisis on 19 March 2022 started at shallower (<8 km) depth and moved north-westward and deeper before concentrating in the central zone at ~10 km depth. Intriguingly, nearly all the seismicity is located west of and deeper than the modelled dike intrusion, suggesting the intrusion was largely aseismic. Nevertheless, the agreement between the strike of the dike and the seismicity lineations suggests that the pre-existing Pico do Carvão Fault Zone guided melt ascent in the crust. However, moment tensors from polarity and waveform inversion show double-couple left-lateral strike-slip faulting along planes striking obliquely (by ~20°) to the dike and seismicity lineation, evidencing high fluid/melt pressures. The overall b-value is high (~2).Interpreting both the seismicity and near-field GNSS displacements, we discuss the intrusion’s evolution along the preexisting fault zone, particularly focussing on potential magmatic inflow and drainage beneath the main dike intrusion.We are grateful to the UK Ocean Bottom Instrument Consortium (OBIC) and SEIS-UK teams for providing the instrumentation and installation services. This work was also supported by Portuguese FCT/MCTES through the project GEMMA (https://doi.org/10.54499/PTDC/CTA-GEO/2083/2021).

Understanding the signatures and mechanisms of failed volcanic eruptions is vital for mapping magma plumbing systems and forecasting volcanic hazards. Geological structures like fractures and faults are key to guiding magma, but their mechanisms remain unclear due to limited 3-D mapping of faults in volcanic regions and sufficiently precise earthquake locations. The triple-junction setting of the Azores Archipelago, where volcanic systems and seismogenic crustal faults coexist, offers a unique window into how faults impact magmatism. Using ~12,000 earthquakes relocated to ultra-high precision with onshore and ocean-bottom seismometer data, along with geodetic observations and seismic autocorrelation imaging, we analyse a failed eruption in 2022 on São Jorge Island. Magma from the upper mantle ascended rapidly and largely aseismically over several days along a crustal fault, before stalling beneath the island edifice. Adjacent seismicity with rotated focal mechanisms suggests that the ascending magma became less buoyant due to devolatilisation, with fluids leaking laterally along the fault zone, triggering an intense, months-long seismic swarm. This study reveals the dual role of fault zones in both facilitating and arresting magma ascent, highlighting the interplay between tectonism and magmatism.

The break-up of continents involves a stage of continental rifting proceeding to sea-floor spreading creating new oceanic crust. Such transitions are potentially accompanied by changes in the rugosity of the basement as sea-floor spreading processes take over from those of continental rifting, for example, changes in style and relief of normal faults or relief of volcanic features. The along-rift structure may also change, as oceanic segmentation replaces the continental rift segmentation. Whereas previous studies have investigated basement rugosity of continent-ocean transitions of mature margins, the Red Sea presents an opportunity to do so where the transition is young, only ~10 Ma in the central Red Sea studied here. We assess basement roughness (root-mean-square variation of basement relief) along profiles across and parallel to the spreading axis. The across-axis roughness is derived from the depths of basement interpreted from across-ridge seismic reflection profiles. The best estimate of mean across-ridge roughness (230 m) overlaps with, but is generally smaller than, those observed over ultra-slow spreading ridges, consistent with a ridge affected by a hotspot (here the Afar), which typically leads to smaller fault relief. Basement roughness values along ridge-parallel profiles are computed from the free-air gravity field using densities appropriate for the oceanic crust. Roughness values are found to be within the range of values for the ultra-slow spreading Southwest Indian Ridge, which appears similarly organized into segments. These axis-parallel roughness values reach minima roughly mid-way between the coast and the axial trough, where the suggested transition from stretched continental to predominantly oceanic crust occurs. Although the detailed mechanical processes are unclear, the results suggest that along-axis basement relief due to ridge segmentation can grow gradually after a continental break-up, perhaps marking a gradual establishment of the magmatic system.

Millions of tons of material are flushed through submarine canyons during infrequent high-magnitude events, transporting coastal sediment to the deep ocean. However, observations related to individual canyon flushing events are challenging due to the destructive nature and infrequency of flushing events. The impacts of one of the largest gravity flows in the past decade were documented in Kaik & omacr;ura Canyon, Aotearoa-New Zealand, where >1 km(3) of sediment was mobilized by the 2016 CE Kaik & omacr;ura earthquake (M-w 7.8). We present new high-resolution (<1 m) multibeam data collected with an autonomous underwater vehicle (AUV) along the Kaik & omacr;ura Canyon axis, together with side-scan sonar and seafloor video imagery. These data sets reveal a wide range of erosional and depositional features that were not previously identified. Eroded bedrock and deep erosional structures are found in the upper canyon, including linear grooves, and rockfall debris (>5-m-diameter boulders). This erosional area transitions downcanyon to coarse-grained depositional bedforms, including cyclic steps and gravel waves (average wavelengths of 250 m and wave heights of similar to 20 m), covering the mid- and lower canyon. Our observations provide high-resolution field-based evidence of (1) flow transformation, from a debris flow to a high-density turbidity current; and (2) variations of flow dynamics within turbidity currents both across- and downcanyon, during an infrequent, high-energy canyon flushing event. This research offers new insights into the processes that create and shape nearshore bedrock-incising submarine canyons.

Associating lithospheric suture zones of the Arabian-Nubian Shield on either side of the central Red Sea is important for constraining Arabia-Nubia plate motion and potentially testing plate rigidity during continental break-up. However, that association is difficult because of the non-unique character of the shield suture zones, some shield areas are obscured by later volcanics and sediments, and because some intervening shield crust that was extended during continental rifting now lies submerged beneath thick Miocene evaporites in the Red Sea and is thus obscured. Here, we show that an association is clearly favoured if shield structures are interpreted along with an oceanic segmentation revealed by the satellite-derived gravity field. The Proterozoic subduction suture zones associated include the Bi'r Umq on the Arabian shield with a previously unrecognised suture zone in the Nubian Shield (herein called Shagara), Fatma with Nakaseib and Ad Damm with Ashat suture zone. Bouguer gravity anomaly lows, produced by oceanic spreading discontinuities of the new mid-ocean ridge formed in the Red Sea, connect two sets of suture zones: Bi'r Umq-Shagara and Fatma-Nakaseib. As far as we are aware, these are the first short-offset oceanic discontinuities to be clearly linked to adjacent shield structures.

Red Sea Deep Water is presently slow-moving, but was this true of the earlier Plio-Pleistocene (PP)? In seismic reflection records, the PP deposits are distorted by halokinetic deformation of their underlying Miocene evaporites. However, if reflections are flattened to a prominent reflector representing the top of the Miocene, they reveal mounded deposits within the earlier PP along both sides of the sea. Off Egypt, a plastered drift occurs along a salt wall. In the central Red Sea, they are mounded drifts. Seismic reflections from these deposits change shape gradually upwards to the modern seabed, which is commonly flatter, suggesting a gradual change in depositional conditions. To explain their origins, we appeal to other evidence. DSDP cores from the Late Pleistocene contain the rigid aragonite cements formed by restricted conditions, but not the lower and middle PP. Furthermore, mid-PP sedimentary 518O values are similar to global ocean 518O for that time, not enhanced as expected from excess evaporation. These data suggest that there was a greater exchange of Red Sea waters with the Indian Ocean during the mid-PP. That exchange may have allowed waters densified by evaporation in shallow regions of the northern Red Sea to flow south vigorously (the mounds would then be contourites). Alternatively, as the Pliocene seabed was shallower, wind-driven eddies may have affected more of the water column. Overall, the results indicate for the first time that deep circulation was stronger in the earlier PP compared with the present day. That circulation needs to be considered when evaluating organism dispersions across the Red Sea, regional climate, and influence of Red Sea Outflow Water on Indian Ocean Intermediate Water. Plain language summary: During the Pliocene (about 3-5 million years ago), temperatures on Earth's surface were similar to those predicted in some models of Earth's future climate. This has led to researchers becoming interested in studying geological evidence of conditions during the Pliocene as a clue to Earth's future. In detail, however, the comparison may not be exact. In our article, we describe mounds of sediment that were originally formed on the bed of the Red Sea in the Pliocene. They appear similar to snow drifts, and like snow drifts, may have formed under steady currents. Sea water becomes dense with evaporation (making it more saline). At the present day, dense water created in the Gulf of Suez cascades down into the deep Red Sea as a slow current. Perhaps such movements were more vigorous in the Pliocene, helped by the water escaping through a deeper barrier in the south into the Indian Ocean? Alternatively, these mounds were formed by giant eddies, such as those on the surface of the modern Red Sea, which are moved by strong winds. In either case, the deep waters of the Pliocene Pliocene Red Sea were more vigorously moving compared to the quiescent deep waters of the modern-day.

The northern Red Sea coastal ecosystem is one of the most diverse coastal ecosystems in the world. Fortunately, it has shown extraordinary resilience against climate change and is predicted to survive global warming during the coming decades. However, with warming waters, increased sediment and pollutants, and other human impacts, the ecosystem and consequently thriving reef tourism which forms a pillar of the ongoing economic diversification policies of the northern Red Sea region are under threat. A variety of evidence indicates significant damage has already been done to terrestrial and ocean ecosystems on both sides of the northern Red Sea. Expenditures on ecosystem protection and research lag behind Egypt’s billions in USD revenue from tourism. Unfortunately, the economic drive to generate profit has resulted in sprawling touristic, industrial, and mixed development without careful planning or assessment of the fragility and sustainability of the natural ecosystem. As a result, the future of coastal urban growth is murky. Given its natural, social, and touristic value, the northern Red Sea system requires a special ecological security system with detailed analysis, inclusive development, and proactive governance across coastal cities and their adjacent inland secondary cities. This study identifies the geological research gaps, human-ecological interactions, inclusive urban development challenges, and related literature pertaining to the northern Red Sea. We propose immediate, targeted, multidisciplinary research trajectories and provide policy recommendations to ensure that the region's existing and future developmental pursuits are undertaken in an environmentally sustainable and inclusive approach.

The structures within continental shields potentially influence the manner in which the shields are rifted and ultimately the development of oceanic seafloor spreading cells. Lying within a young oceanic basin, the Red Sea margins have thermally subsided less than those of mature ocean basins and thus could reveal these influences better. Here, we investigate velocity structures from three seismic refraction lines off Sudan. Seismic velocities are converted to densities and used to predict free-air gravity anomalies to compare with satellite-derived anomalies. The density structures are also tested in an isostatic sense by computing their column weights at discrete locations. The predicted anomalies are found to be ~40 mGal smaller than those observed at two locations close to inferred ocean-continent boundaries on their oceanic sides. This suggests that the crust does not thin as rapidly seaward as originally inferred. In contrast, at two further locations over the continental crust, observed anomalies are up to 60 mGal larger than those predicted. The refraction experiment therefore did not detect bodies of high velocity and/or over-estimated crustal thickness in these places. They lie offshore from suture zones within the Nubian shield; high densities could be due to mantle or other high-density bodies incorporated within the crust when the original collision events created the suture zones. If correct, the Red Sea contains clues to how segmented gravity structures, which are common at continental margins, could be due to old shield structures.

Oceanic core complexes (OCCs) are upwards-convex areas of outcropping lower crustal or upper mantle rocks raised to the seabed by normal faults, commonly associated with weak lithologies such as serpentinites. They are common adjacent to transform valleys of slow-spreading ridges. In this paper, we analyse the September 2020, Mw 6.6 strike-slip earthquake and its two-week long aftershock sequence within the Vernadsky transform valley, using recordings from regional seismic stations. The aftershocks occurred in two phases. During the first four days, ∼ 131 events occurred east and northeast of the mainshock, overlapping an adjacent OCC complex along the northern flank of the transform valley. During the following nine days, 20 aftershocks occurred, including a Mw 5.8 event close to the OCC. To refine the locations of aftershocks, the spatial spread of the events (clustering) was refined by double-difference relocating 114 events, and five with teleseismic relative relocation. Modeling of static Coulomb stresses was carried out, based on a mainshock rupture length compatible with the 26–30 km horizontal extent of aftershocks. This revealed that the aftershocks occurred mainly in areas where static Coulomb stresses decreased, not increased. Other researchers have suggested that changes in fault strength can arise from dynamic stresses during major earthquakes, leading to seismicity in areas of decreased Coulomb stress. We explore this idea in the context of OCCs.

The Red Sea is an important example of a continental rift transitioning slowly to an oceanic basin. However, structures that can inform us of how that transition occurred have been poorly reported because deep seismic reflection data capable of imaging basement under the rift sediments are generally lacking publicly. Three lines of multichannel seismic reflection data have recently been published revealing structures on the Nubian side of the central part of the basin. In this study, we reassess these data in the light of recent studies of the central Red Sea. Over continental crust, the data reveal reflection sequences likely due to strata at or near the base of the evaporites, in two cases with varied dips suggesting the presence of syn-rift growth stratigraphy. Almost all of those reflections dip downwards towards the rift axis, not away as would be expected from tilted fault blocks of bookshelf faulting types. That observation, and low relief of basement, confirm inferences made earlier based on gravity anomalies that this part of the Red Sea lacks large-relief fault escarpments and is most likely a syn-rift sag basin. In the transition to oceanic crust, an abnormally broad magnetic anomaly of estimated Chron 5 age is found not to be associated with structures such as sills, so it likely arises from deeper sources. One of the seismic lines traverses a ridge in Bouguer gravity anomalies that runs across the axis. This feature has previously been interpreted as a volcanic ridge similar to those observed at other ultra-slow spreading ridges. The seismic data reveal diffuse basement reflections and confirm that the record immediately above basement lacks reflections typical of sedimentary strata. Both observations are consistent with the presence of oceanic crust. Modelling of gravity anomalies suggests the ridge is likely underlain by igneous intrusive rocks displacing mantle rocks, as expected for a volcanic ridge. The seismic data, combined with recently updated multibeam and high-resolution sparker seismic results, further suggest how the evaporite movements have been modulated by basement topography. These results add to our knowledge of the evaporite movements and continent-ocean transition structures in the central Red Sea.