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The Algerian continental margin in the western Mediterranean formed as a back-arc basin and is today reactivated by the convergence between the African and Eurasian plates. It is one of the very rare examples of passive margins undergoing inversion expressed in a moderate seismicity of the margin and is possibly giving way for future subduction. With the objective to better know the deep structure of the margin and its associated basin, the origin of its seismicity and to study the mechanism of reactivation, five existing wide-angle seismic profiles along the margin are revisited. They were located offshore Mostaganem, Tipasa, Greater Kabylia, Jijel and Annaba. These profiles show that the basin is underlain by a 5 km thick crust of oceanic magmatic origin, possibly created at non-continuous small accretionary ridge segments. The continent-ocean transition zone is narrow, except at the easternmost profile, possibly due to an opening including a shear movement. No high velocity zone in the lower crust corresponding to mantle rocks has been imaged at the Algerian margin. The continental crust is thinned in a narrow and strongly segmented manner. It is widest (70 km) in the central segment offshore Greater Kabylia where there is a wider zone of distal thinned continental crust than on the other margin segments. The thickest crust detected during this survey corresponds to the African continental crust and the Kabylides blocks and is about 22–25 km thick. This reduced thickness in comparison with unthinned continental crust might be due to the influence of earlier subduction at the margin, in form of erosion by the subducting slab.

The brutal onset of seismicity offshore Mayotte island North of the Mozambique Channel, Indian Ocean, that occurred in May 2018 caught the population, authorities, and scientific community off guard. Around 20 potentially felt earthquakes were recorded in the first 5 days, up to magnitude Mw 5.9. The scientific community had little pre-existing knowledge of the seismic activity in the region due to poor seismic network coverage. During 2018 and 2019, the MAYOBS/REVOSIMA seismology group was progressively built between four French research institutions to improve instrumentation and data sets to monitor what we know now as an on-going exceptional sub-marine basaltic eruption. After the addition of 3 medium-band stations on Mayotte island and 1 on Grande Glorieuse island in early 2019, the data recovered from the Ocean Bottom Seismometers were regularly processed by the group to improve the location of the earthquakes detected daily by the land network. We first built a new local 1D velocity model and established specific data processing procedures. The local 1.66 low VP/VS ratio we estimated is compatible with a volcanic island context. We manually picked about 125,000 P and S phases on land and sea bottom stations to locate more than 5,000 events between February 2019 and May 2020. The earthquakes outline two separate seismic clusters offshore that we named Proximal and Distal. The Proximal cluster, located 10km offshore Mayotte eastern coastlines, is 20 to 50 km deep and has a cylindrical shape. The Distal cluster start 5 km to the east of the Proximal cluster and extends below Mayotte's new volcanic edifice, from 50 km up to 25 km depth. The two clusters appear seismically separated, however our dataset is insufficient to firmly demonstrate this.

The measurement of methane clumped isotopologues ( and ) allows exploring isotope bond ordering within methane molecules, and may reveal equilibrium temperatures. Whether such temperature reflects the formation or re-equilibration temperature of the methane is not well understood, but would have critical implications for the use of methane clumped isotopologues as geo-thermometers. Here we investigate gas bubbles from vigorous emissions at cold seeps (n = 14) in the Sea of Marmara, Turkey. These cold seeps are sourced from deeper sedimentary reservoirs. Conventional geochemical tracers such as carbon and hydrogen bulk isotopic ratios (13C/12C and D/H) or n-alkane molecular ratios, suggest these gases reflect various degrees of mixing between thermogenic and microbial sources. Some samples would generally be considered purely microbial in origin (; ‰). We report measurements of and showing that a fraction of those gases are in internal thermodynamic equilibrium, with the abundances of the two mass-18 isotopologues indicating concordant temperatures of ∼90 °C and ∼130 °C. These concordant temperatures are recorded by gases of putative microbial and thermogenic origin; the temperatures of equilibration are irrespective of the formation mechanism of the gases. We conclude that the two high-temperatures recorded by and are best explained by non-enzymatic re-equilibration at two local subsurface temperatures. First principles suggest that unequal rates of exchange are possible. Disequilibrium signatures where the two isotopologues yield discordant apparent temperatures are exhibited by other samples. In those cases the data define a trend of variable at nearly constant . These signatures are enigmatic, and we investigate and reject multiple possible explanations including mixing, diffusion or Anaerobic Oxidation of Methane. Different rates of re-equilibration between the two rare isotopologues are implied, although lacks experimental foundation at present. In general, all

Ocean bottom seismometers (OBS) commonly record short duration events (SDEs), that could be described by all of these characteristics: (i) duration < 1 s, (ii) one single‐wave train with no identified P‐ nor S‐wave arrivals and (iii) a dominant frequency usually between 4 Hz and 30 Hz. In many areas, SDEs have been associated with gas or fluid‐related processes near cold seeps or hydrothermal vents, although fish bumps, instrumental or current‐generated noise have been proposed as possible sources. In order to address some remaining issues, this study presents results from in situ and laboratory experiments combined with observations from 2 contrasting areas, the Sea of Marmara (Turkey) and the Chilean subduction zone. The in situ experiment was conducted at the EMSO‐Molène submarine observatory (near Brest, France) and consisted in continuously monitoring two OBSs with a camera. The images revealed that no fish regularly bumped into the instruments. Laboratory experiments aimed at reproducing SDEs’ waveforms by injecting air or water in a tank filled by sand and sea‐water and monitored with an OBS. Injecting air in the sediments produced waveforms very similar to the observed SDEs, while injecting air in the water column did not, constraining the source of SDEs in the seafloor sediments. Finally, the systematic analysis of two real data sets revealed that it is possible to discriminate gas‐related SDEs from biological or sea‐state related noise from simple source parameters, such as the temporal mode of occurrence, the back azimuth and the dominant frequency. Key Points Short duration high amplitude events routinely recorded on marine seismometers are not created by fish bumping into the geophone. In laboratory experiments, the waveforms of these events are well reproduced by fluid migration in the sediments. We propose 3 simple source parameters to discriminate gas‐related short duration events in ocean bottom seismometer data.

Marine transform faults and associated fracture zones (MTFFZs) cover vast stretches of the ocean floor, where they play a key role in plate tectonics, accommodating the lateral movement of tectonic plates and allowing connections between ridges and trenches. Together with the continental counterparts of MTFFZs, these structures also pose a risk to human societies as they can generate high magnitude earthquakes and trigger tsunamis. Historical examples are the Sumatra-Wharton Basin Earthquake in 2012 (M8.6) and the Atlantic Gloria Fault Earthquake in 1941 (M8.4). Earthquakes at MTFFZs furthermore open and sustain pathways for fluid flow triggering reactions with the host rocks that may permanently change the rheological properties of the oceanic lithosphere. In fact, they may act as conduits mediating vertical fluid flow and leading to elemental exchanges between Earth’s mantle and overlying sediments. Chemicals transported upward in MTFFZs include energy substrates, such as H2 and volatile hydrocarbons, which then sustain chemosynthetic, microbial ecosystems at and below the seafloor. Moreover, up- or downwelling of fluids within the complex system of fractures and seismogenic faults along MTFFZs could modify earthquake cycles and/or serve as “detectors” for changes in the stress state during interseismic phases. Despite their likely global importance, the large areas where transform faults and fracture zones occur are still underexplored, as are the coupling mechanisms between seismic activity, fluid flow, and life. This manuscript provides an interdisciplinary review and synthesis of scientific progress at or related to MTFFZs and specifies approaches and strategies to deepen the understanding of processes that trigger, maintain, and control fluid flow at MTFFZs.

The Sea of Marmara (SoM) is a marine portion of the North Anatolian Fault (NAF) and a portion of this fault that did not break during its 20th century earthquake sequence. The NAF in the SoM is characterized by both significant seismic activity and widespread fluid manifestations. These fluids have both shallow and deep origins in different parts of the SoM and are often associated with the trace of the NAF which seems to act as a conduit. On July 25th, 2011, a 5 strike-slip earthquake occurred at a depth of about 11.5 km, triggering clusters of seismicity mostly located at depths shallower than 5 km, from less than a few minutes up to more than 6 days after the mainshock. To investigate the triggering of these clusters we first employ a match filter algorithm to increase the number of event located and hence better identify potential spatio-temporal patterns. This leads to a 2-fold increase in number of events relocated, coming mostly from the shallow seismic clusters. The newly detected events confirm that most of the aftershocks are shallow, with a large number of events located in the first few km below the SoM seafloor. Pore pressure diffusion from the position of the deep mainshock to the position of the shallow events is incompatible with the short time interval observed between them. We therefore investigate static and dynamic stress triggering processes. The shallow clusters fall into either positive or negative lobes with static stress variations of about ±5 kPa. Dynamic stresses may reach values of about ±40 kPa depending on the rise time and the fault orientation considered, but cannot last longer than the perturbations associated with the seismic waves from the mainshock. We then propose a mechanism of fluid pressure increase involving local fluid transfers driven by the transient opening of gas-filled fractures due to earthquake shaking, to explain the triggering of the shallow events of the clusters.

In the Sea of Marmara, areas of gas seepage or cold seeps are tightly related to the faults system and understanding the spatial and temporal dynamics in gas-related processes is crucial for geohazard mitigation. Although acoustic surveys proved to be efficient in detecting and locating cold seeps, temporal variability or trends in the gas-related processes are still poorly understood. Two arrays of 10 ocean bottom seismometers were deployed in the western part of the Sea of Marmara in 2011 and 2014, respectively. In addition to the local seismic events, the instruments recorded a large number of short duration events and long-lasting tremors. Short duration events are impulsive signals with duration < 1 s, amplitude well above the noise level and a frequency spectrum with one or two narrow peaks. They are not correlated from one site to another, suggesting a very local source. Tremors consist of sequences of clustered impulsive signals lasting for minutes to more than an hour with a multi-peak frequency spectrum. Based on evidence of known seepage and by analogy with volcanic and hydrothermal models, we suggest that short duration events and tremors are associated with gas migration and seepage. There is a relationship between tremors associated with gas emission and the local seismicity, although not systematic. Rather than triggering gas migration out of the seabed, locally strong earthquakes act as catalysts when gas is already present or gas emission is already initiated.

Marine transform faults and associated fracture zones (MTFFZs) cover vast stretches of the ocean floor, where they play a key role in plate tectonics, accommodating the lateral movement of tectonic plates and allowing connections between ridges and trenches. Together with the continental counterparts of MTFFZs, these structures also pose a risk to human societies as they can generate high magnitude earthquakes and trigger tsunamis. Historical examples are the Sumatra-Wharton Basin Earthquake in 2012 (M8.6) and the Atlantic Gloria Fault Earthquake in 1941 (M8.4). Earthquakes at MTFFZs furthermore open and sustain pathways for fluid flow triggering reactions with the host rocks that may permanently change the rheological properties of the oceanic lithosphere. In fact, they may act as conduits mediating vertical fluid flow and leading to elemental exchanges between Earth’s mantle and overlying sediments. Chemicals transported upward in MTFFZs include energy substrates, such as H2 and volatile hydrocarbons, which then sustain chemosynthetic, microbial ecosystems at and below the seafloor. Moreover, up- or downwelling of fluids within the complex system of fractures and seismogenic faults along MTFFZs could modify earthquake cycles and/or serve as “detectors” for changes in the stress state during interseismic phases. Despite their likely global importance, the large areas where transform faults and fracture zones occur are still underexplored, as are the coupling mechanisms between seismic activity, fluid flow, and life. This manuscript provides an interdisciplinary review and synthesis of scientific progress at or related to MTFFZs and specifies approaches and strategies to deepen the understanding of processes that trigger, maintain, and control fluid flow at MTFFZs

On continental margins, upward migration of fluids from various sources and various subsurface accumulations, through the sedimentary column to the seafloor, leads to the development of cold seeps where chemical compounds are discharged into the water column. MarsiteCruise was undertaken in November 2014 to investigate the dynamics of cold seeps characterized by vigorous gas emissions in the Sea of Marmara (SoM).A previous paper published by Bourry et al. (2009) presented the gas geochemistry of three seeps sampled along three different segments in the SoM. Their findings showed that the seeps were sourced by three different reservoirs. In this paper, seventeen seeps were investigated to determine the gas sources, unravel reservoir contributions, and estimate their level of mixing. The molecular and stable isotope compositions of the gas compounds were determined to establish the empirical diagrams that usually allow to delineate source domains. The results provide insights into the complexities of source mixing within the sedimentary column of the SoM before emission of the gases into the water column. The seep gases originate from deep thermogenic or microbial hydrocarbon sources, or from a CO2-rich source. Microbial sources producing methane from primary methanogenesis have been identified in the Tekirdağand the Çinarcik basins. In addition, six different thermogenic reservoirs or six different pathways of migration are responsible for the supply of gas to the seeps on the highs and in the western basin. Five of them are undergoing biodegradation followed by secondary methanogenesis, thereby providing additional sources of microbial methane to the seeps. Overall, the gases emitted by the seventeen seeps consist of variable mixtures of different components from two or three sources.

A rotating, acoustic gas bubble detector, BOB (Bubble OBservatory) module was deployed during two surveys, conducted in 2009 and 2011 respectively, to study the temporal variations of gas emissions from the Marmara seafloor, along the North Anatolian Fault zone. The echosounder mounted on the instrument insonifies an angular sector of 7° during a given duration (of about 1 h). Then it rotates to the next, near-by angular sector and so forth. When the full angular domain is insonified, the “pan and tilt system” rotates back to its initial position, in order to start a new cycle (of about 1 day). The acoustic data reveal that gas emission is not a steady process, with observed temporal variations ranging between a few minutes and 24 h (from one cycle to the other). Echo-integration and inversion performed on the acoustic data as described in the companion paper of Leblond et al. (Mar Geophys Res, 2014), also indicate important variations in, respectively, the target strength and the volumetric flow rates of individual sources. However, the observed temporal variations may not be related to the properties of the gas source only, but reflect possible variations in sea-bottom currents, which could deviate the bubble train towards the neighboring sector. During the 2011 survey, a 4-component ocean bottom seismometer (OBS) was co-located at the seafloor, 59 m away from the BOB module. The acoustic data from our rotating, monitoring system support, but do not provide undisputable evidence to confirm, the hypothesis formulated by Tary et al. (2012), that the short-duration, non-seismic micro-events recorded by the OBS are likely produced by gas-related processes within the near seabed sediments. Hence, the use of a multibeam echosounder, or of several split beam echosounders should be preferred to rotating systems, for future experiments.

The southwestern Pacific Ocean region hosts submerged continental margins, ridges, sedimentary basins, and volcanic arcs located around Papua New Guinea, New Zealand, Australia, and Fiji. The geological history of this vast region has remained controversial, and to improve understanding of the processes that controlled its geodynamical evolution, it is essential to place each piece of available data in a regional spatiotemporal framework. To this end, a new map, entitled “Structural Provinces of the Southwest Pacific,” was released by the Geological Survey of New Caledonia in May 2011. The publication consists of two parts: (1) a 40-page booklet of geological notes, which documents the nature and age of each structure and contains an associated list of references; and (2) a 3- x 4-foot poster of a structural map revealing the nature of the basement, location, and type of the main structural features (see simplified version in Figure 1) and the age of formation using the international standards for geological color codes established by the Commission for the Geological Map of the World (CGMW) (see http://ccgm.free.fr/index.html).

Society’s needs for a network of in situ ocean observing systems cross many areas of earth and marine science. Here we review the science themes that benefit from data supplied from ocean observatories. Understanding from existing studies is fragmented to the extent that it lacks the coherent long-term monitoring needed to address questions at the scales essential to understand climate change and improve geo-hazard early warning. Data sets from the deep sea are particularly rare with long-term data available from only a few locations worldwide. These science areas have impacts on societal health and well-being and our awareness of ocean function in a shifting climate. Substantial efforts are underway to realise a network of open-ocean observatories around European Seas that will operate over multiple decades. Some systems are already collecting high-resolution data from surface, water column, seafloor, and sub-seafloor sensors linked to shore by satellite or cable connection in real or near-real time, along with samples and other data collected in a delayed mode. We expect that such observatories will contribute to answering major ocean science questions including: How can monitoring of factors such as seismic activity, pore fluid chemistry and pressure, and gas hydrate stability improve seismic, slope failure, and tsunami warning? What aspects of physical oceanography, biogeochemical cycling, and ecosystems will be most sensitive to climatic and anthropogenic change? What are natural versus anthropogenic changes? Most fundamentally, how are marine processes that occur at differing scales related? The development of ocean observatories provides a substantial opportunity for ocean science to evolve in Europe. Here we also describe some basic attributes of network design. Observatory networks provide the means to coordinate and integrate the collection of standardised data capable of bridging measurement scales across a dispersed area in European Seas adding needed ce

Over the past 3 decades, satellite altimetry has been a key tool for dynamic ocean studies and for accurately estimating sea surface heights. The geodetic reference surface—the “geoid”—can be approximated as the mean sea surface height of an ocean corrected for dynamic terms such as tides and currents. It is an equipotential surface of the gravity field; and variations of this field are quantified as free‐air anomalies, from which density heterogeneities of the oceanic basement can be inferred. Using such data in combination with other geophysical data, scientists have improved their knowledge of the nature of submarine relief and underlying structures. In solid Earth geophysics, major breakthroughs came from the development of high‐resolution marine gravity models based on closely spaced altimetry profiles collected during the U.S. Navy's Geosat satellite geodetic mission (launched in 1985) and the first version of the European Remote Sensing satellite geodetic mission (ERS 1, launched in 1991). These were combined with other repetitive profiles from the international TOPEX/ POSEIDON satellite (launched in 1992); ERS 1; TOPEX/POSEIDON's successor, Jason (launched in 2001); and the European Space Agency's (ESA) Envisat missions (ERS's successors [see, e.g., Sandwell and Smith, 1997; Andersen and Knudsen, 1998]).

The MARMESONET cruise is part of the Marmara Demonstration Mission Program supported by ESONET Network of Excellence (European Seafloor Observatory Network), within the 6th European Framework Programme. Main partners are: Ifremer, CNRS/CEREGE, Istanbul Technical University, TUBITAK, Institute of Marine Science and Technology of Dokuz Eylül Universitesi (Izmir), INGV (Rom) and ISMAR (Bologna). Marmesonet is also the follow-on of the Franco-Turk collaborative programme that resulted in numerous cruises in the Sea of Marmara since 2000. The objectives of the MARMESONET cruise were: 1) to study the relationship between fluids and seismicity along the Sea of Marmara fault system ; 2) to carryout site surveys prior to the implementation of permanent seafloor observatories in the Marmara Sea through ESONET. The cruise is divided in 2 parts: Leg I (from november 4th to november 25th, 2009), mainly dedicated to: i) the high resolution bathymetry at potential sites of interest for future permanent instrumentation using the Autonomous Unmanned Vehicle (AUV)Asterx of Ifremer/Insu ; ii) the systematic mapping of the gas emissions sites on the Marmara seafloor ; iii) the deployment of the Bubble Observatory Module (BOB) in the Çinarçik basin. Leg II (from november 28th november to december 14th, 2009), for 3D, High Resolution Seismic imagery of the fluid conduits below the observatory site planned at the Western High. The present report only concerns Leg I. A total of 19 dives were completed during Leg I: 16 with the multibeam echosounder SIMRAD EM2000 (200 kHz), among which 12 were successful and 4 failed ;3 with the CHIRP sédiment penetrator (1 test dive and 2 operational, both were unfortunately with early stop recording). Main results are: The absence of recent, visible deformation on the segment south of Istanbul. Wether or not this segment is locked or creeping remains an open question. The site south of Istanbul thus requires a massive effort to assess the deformation, pa, La campagne constitue l’une des missions de démonstration soutenues par ESONET. Elle résulte d’un partenariat entre l’Ifremer, le CNRS, l’INSU, l’Université Technique d’Istanbul, l’Institut des Sciences Marines d’Izmir, le CNR-ISMAR (Bologne) et l’INGV (Rome). La plupart des objectifs du premier leg de la campagne Marmesonet, du 4 au 25 novembre 2009, ont été atteints, grâce à trois facteurs principaux: i) la météo exceptionnellement favorable ; ii) la bienveillance des garde-côtes de la Marine Turque ; iii) le professionnalisme des équipes (équipage et sédentaires)

Reflection seismics has been intensively used for the last four decades by marine geologists and geophysicists for imaging Earth structures below the seafloor. Because their subject of interest is below the sea bottom, solid Earth scientists do not usually consider the seismic signal propagating in the water column and most often do not even record it, in order to save data storage space. Two physical oceanographers, Gonella and Michon [1988],first reported internal waves revealed by reflection seismics in the northeastern Atlantic. Only recently, though, has the scientific community realized the importance of this issue, after Holbrook et al. [2003] published reflection seismic sections of the water column off Newfoundland, Canada, showing reflectors related to the major oceanographic front between the Labrador Current and the North Atlantic Current.

A deep seismic survey conducted within the western part of New Caledonia's Exclusive Economic Zone (EEZ) (Figure 1) from 8 September–5 October 2004 revealed for the first time the thinned continental and oceanic natures of the crust beneath the eastern Tasman Sea. The survey which was conducted by an international group of scientists a board the Institut Francais de Recherche pour l'Exploitation de la MER (Ifremer) R/V L'Atalante, aimed at improving the understanding of the geological framework, crustal characteristics, and evolution of the submarine basin and ridge system located west of New Caledonia's mainland. The study area, located east of both Australia and the oceanic Tasman Sea Basin, is composed of continental fragments: the Lord Howe Rise and the Norfolk Ridge, which are separated by the New Caledonia and Fairway basins.

During the SOPACMAPS 2 crusie carried out by IFREMER (Institut Français de Recherche pour l'Exploitation de la Mer) and ORSTOM (Institut Français de Recherche Scientifique pour le dévelopement en Coopération) on theR/V L'Atalante, in the Central Solomon Arc area, multibeam bathymetric and imagery data and single-channel seismic reflection profiles were collected from an area of about 3500 km2, to evaluate regional tectonics. Structural data geophysical profiles interpretation provide evidence for left-lateral transtensional tectonics on the southern edge of the Central Solomon Trough. This transtensional deformation is represented by faulting, block tilting, and rhombohedral deformation. The regional geology and the analysis of the sedimentary cover allow us to demonstrate that this tectonic occurred in two different phases during Oligocene to Miocene and Pliocene to Pleistocene times