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Data from Ocean Bottom Seismometer (OBS) deployments nowadays are routinely integrated in federated seismological data centers, but often without following standardized procedures as recommended by the OBS community. This makes it difficult to locate and exploit these data mainly due to the restrictions imposed by the standard metadata format (stationXML) and the services.In the context of the Helmholtz Metadata Collaboration (HMC), supported by OBS experts, we selected AWI and GEOMAR datasets to be archived at the GEOFON data center with standardized procedures according to the FDSN straw man proposal of W. Crawford for OBS data and metadata using also the OBSinfo tools developed at IPGP. In addition, a special emphasis was put on enhancing FAIRness for the selected datasets, for example identifiers of individual instruments were included, which are linked to instrument databases. Keywords were also added to make the data more easily findable and interoperable. From these experiences we formulated guidelines for OBS data management, which should help researchers to archive their OBS data consistently throughout EIDA data centers.

The subduction of seamounts and basement ridges affects the structure, morphology, and physical state of a convergent margin. To evaluate their impact on the seismo-tectonic setting of the subduction zone and the tectonic development of the lower subducting and upper overriding plate, it is essential to know the precise location of subducted topographic features under the marine forearc. Offshore Northern Chile, the Iquique Ridge represents a broad zone of complex and heterogeneous structure of variable width on the oceanic Nazca Plate, which complicates attempts to project it beneath the forearc of the Chilean subduction zone. Here we use a state-of-the-art seismic reflection data processing approach to map structures related to ridge subduction under the marine forearc with unprecedented accuracy and resolution and evaluate their impact on the deformation of both the plate boundary and the upper plate. We show that significant ridge-related topography is currently subducting south of 20.5 °S and that the combined effect of horst and graben subduction with subduction of Iquique ridge-related thickened and elevated crust causes an upward bulging of the entire upper plate from the plate interface up to the seafloor as well as the presence of kilometer-scale anticlinal structures observed in multibeam bathymetric data that are approximately aligned with horsts seaward of the trench. In the area affected by the subducting ridge, a frontal prism is absent, which may relate to frontal subduction erosion caused by the excess lower plate topography. In contrast farther towards the north, where only isolated seamounts subduct, a small frontal prism and a slope/apron sediment cover down to 3000 m water depth are found.

Highlights • A fiber optic strain cable is used to monitor a fault offshore Catania, Sicily. • Brillouin laser reflectometry detects 2.5 cm of cable elongation on the seafloor. • The cable elongation may be caused by fault slip or by seabottom currents. • Submarine telecom cables are likely suitable to detect deformation on the seafloor. Abstract Oceans cover more than 70 percent of the Earth's surface making it difficult and costly to deploy modern seismological instruments here. The rapidly expanding global network of submarine telecom cables offers tremendous possibilities for seismological monitoring using laser light. Recent pioneer studies have demonstrated earthquake detection using lasers in onland and submarine fiber optic cables. However, permanent strain at the seafloor has never before been measured directly as it happens. With this aim, we deployed a dedicated 6-km-long fiber optic strain cable, offshore Catania Sicily, in 2000 m water depth, and connected it to a 29-km long electro-optical cable for science use. We report here that deformation of the cable equivalent to a total elongation of 2.5 cm was observed over a 21-month period (from Oct. 2020 to Jul. 2022). Brillouin laser reflectometry observations over the first 10 months indicate significant strain (+25 to +40 microstrain) at two locations where the cable crosses an active strike-slip fault on the seafloor, with most of the change occurring between 19 and 21 Nov. 2020. The cause of the strain could be fault slip or seabottom currents. During the following 11 months, the strain amplitude increased to +45 to +55 microstrain, affecting a longer portion of the cable up to 500 m to either side of the first fault crossing. A sandbag experiment performed on the distal portion of the cable (3.2–6.0 km) starting Sept. 2021 demonstrates how the fiber optic cable deforms in response to an applied load and how the deformation signal partially dissipates over time due to the elastic properties of the cable. These preliminary results are highly encouraging for the use of BOTDR (Brillouin Optical Time Domain Reflectometry) laser reflectometry as a technique to detect strain at the seafloor in near real time and to monitor the structural health of submarine cables.

The ocean floor is the outer solid shell for over 70% of our planet, which is continuously moved and deformed in the course of global plate tectonics. These processes lead to tectonic stresses building up in the seafloor, which over long periods of time become so large that they suddenly and usually (still) unexpectedly discharge in an earthquake. In the marine environment, the seafloor cannot be studied with the established tools of tectonic geodesy, as water is not a suitable medium for geodetic systems that depend on the relatively unperturbed transmission of electromagnetic waves. During the past three decades, advances made by using space geodetic systems, such as GPS and InSAR, have revolutionized our ability to precisely track actively deforming areas onshore in high spatial and temporal resolution. Offshore, seafloor geodesy aims at precise underwater measurements of interstation distances, absolute positions, water depth, and tilt. Seafloor displacement occurs in the horizontal (x,y) and vertical direction (z) as a function of time (t). The vertical displacement is measured by monitoring pressure variations at the seafloor. Horizontal seafloor displacement can be measured either using an acoustic/GPS combination to provide absolute positioning or by long-term acoustic telemetry between different beacons fixed on the seafloor to determine relative distances by using the travel time observations to each other, which is the technique used in the framework of the GeoSEA project (Geodetic Earthquake Observatory on the SEAfloor) with the aim to record deformation directly on the seafloor. Acoustic direct path measurements by the GeoSEA Array were conducted across the North Anatolian Fault in the Sea of Marmara, on the flank of Mt Etna in the Ionian Sea, and on the North-Chilean subduction zone in the eastern Pacific. The goal of these observations is to be able to directly measure the stress buildup and use it to refine estimates of the hazard situation. Since the expected deformation rates are low (a few cm/year at most), the stations have to remain on the seafloor for several years, where they measure autonomously in water depths up to 5,500 m with a precision of 5 mm, allowing for precise measurements of strain build-up in the seafloor. The results from the different campaigns reveal the range and degree of coupling as well as the distribution of deformation, ranging from fully locked to slow-slip movement.

The goal of the ERC (European Research Council) funded project - FOCUS is to apply laser reflectometry on submarine fiber optic cables to detect deformation at the seafloor in real time using BOTDR (Brillouin Optical Time Domain Reflectometry). This technique is commonly used monitoring large-scale engineering infrastructures (e.g. - bridges, dams, pipelines, etc.) and can measure very small strains (<< 1 mm/m) at very large distances (10 - 200 km), but until now has never been used to study tectonic faults and deformation on the seafloor.Here, we report that BOTDR measurements detected movement at the seafloor consistent with ≥1 cm dextral strike-slip on the North Alfeo fault, 25 km offshore Catania, Sicily over the past 10 months. In Oct. 2020 a dedicated 6-km long fiber-optic strain cable was connected to the INFN-LNS (Catania physics institute) cabled seafloor observatory at 2060 m depth and deployed across this submarine fault, thus providing continuous monitoring of seafloor deformation at a spatial resolution of 2 m. The laser observations indicate significant elongation (20 - 40 microstrain) at two fault crossings, with most of the movement occurring between 19 and 21 Nov. 2020. A network of 8 seafloor geodetic stations for direct path measurements was also deployed in Oct. 2020, on both sides of the fault to provide an independent measure of relative seafloor movements. These positioning data are being downloaded during ongoing oceanographic expeditions to the working area (Aug. 2021 R/V Tethys; Jan. 2022 R/V PourquoiPas) using an acoustic modem to communicate with the stations on the seafloor. An additional experiment was performed in Sept. 2021 using an ROV on the Fugro vessel Handin Tide, by weighing down unburied portions of the submarine cable with pellet bags and sandbags (~25kg each) spaced every 5m. The response was observed simultaneously by DAS (Distributed Acoustic Sensing) recordings using two DAS interrogators (a Febus and a Silixa). The strain caused by the bag deployments was observed using BOTDR and typically produced a 50 - 100 microstrain signal across the 120 meter-long segments which were weighed down. In Jan. 2022 during the FocusX2 marine expedition, 21 ocean bottom seismometers were deployed for 12-14 months, which together with 15 temporary land-stations as well as the existing network of permanent stations (both operated by INGV) will allow us to perform a regional land-sea passive seismological monitoring experiment.

The Earth’s ocean floor deforms continuously under the influence of tectonic and non-tectonic processes. In the recent decade, the installation of seafloor geodetic instruments to accurately monitor fault displacement and strain accumulation has greatly improved our understanding of seafloor deformation and our knowledge of associated offshore hazards. In particular, the application of acoustic direct-path ranging networks allows the detection of displacement and strain accumulation of faults in millimeter-level precision.On-land geodetic networks revealed that the Southeast flank of Mount Etna slides seawards at a rate of ~3 cm/yrs. The highest rates are observed near the coast and the volcano flank extends far into the Ionian Sea. The long-term deformation is superimposed by frequent slow-slip events with up to ~3 cm displacement. Our first acoustic ranging measurements between 2016 and 2018 confirmed offshore active deformation and seafloor displacement by detecting a slow-event of up to ~4 cm with a right-lateral offset. Thus, the application of direct-path ranging transponders has proven to be a promising tool to monitor horizontal and vertical displacement of such strike-slip fault zones. However, the observation of long-term deformation, as observed on onshore faults, is lacking. Therefore, we conducted a second acoustic geodetic deployment at the same site offshore Mount Etna between September 2020 and November 2021 and used a different network design. The new data set shows an indication for slow long-term seafloor deformation, which had not been resolved in the first deployment. By comparing the different configurations of the acoustic direct-path networks we were able to improve data processing to achieve millimeter-level precision. We have learned that longer-distance measurements over a sharp fault favor the detection of slow-slip events, but impede the observation of slow long-term deformation. In order to resolve the latter movement, very short baselines close to the fault trace are ideal. Therefore, a trade-off between long and short-distance measurements might be the key for compressive deformation monitoring. Our results prove that the direct-path acoustic ranging technique is well-suited to detect different styles of fault slip at faults with sharp surface traces.

Extreme climatic and geophysical events pose a threat to societies and have the capacity to cause significant damage and losses whenever they occur, both in their immediate aftermath and in the medium- to long-term. Their consequences can be amplified even further when more than one event affects the same geographical areas within a short time. Be it cascading hazards, in which one event triggers the next, or simply hazards that happen to occur simultaneously (“compounding” hazards), estimation of their cumulative consequences is challenging because the action of one event affects the exposure and vulnerability to the next one. While the efforts from the research community to develop multi-hazard perspectives have increased considerably in recent years, multiple remaining challenges require strongly-coordinated efforts across different disciplines and areas of expertise to tackle them with the most appropriate tools.With a multidisciplinary team of scientists from four different Helmholtz research centres in Germany, we have started working on the CASCO project (2022-2024), in which we will develop an integrated risk workflow for CAScading and COmpounding hazards in COastal urban areas by focusing on a series of events occurring around Mount Etna (Italy). The case-scenario starts with a strong earthquake that triggers a submarine collapse at the eastern flank of Mount Etna, an area already known to be unstable, and both the earthquake and the landslide trigger a tsunami that hits the coasts of Sicily and Calabria. Almost concomitantly, a heatwave or heavy rainfall happens to affect the same regions, further stressing the population that had been affected by the combined effects of the earthquake and tsunami.The project will be directed towards the modelling of the cascading earthquake, landslide and tsunami events, the compounding heatwave and rainfall, as well as their immediate impacts in terms of cumulative damage and casualties. Moreover, the medium- to long-term response in urban dynamics and the effect of these extreme events on the economic development of the affected populations will be explored. By focusing on a tangible scenario, CASCO will not only tackle the challenges associated with bringing together the whole risk chain (which will be valid beyond our case-study) but also produce outcomes that help increase awareness of such extreme events and the need for societies to develop suitable strategies to strengthen their resilience and improve their disaster response.

The Mangatolu Triple Junction (MTJ) and the Fonualei Rift and Spreading Center (FRSC) are two prominent bathymetric features in the northern Lau Basin in the southwest Pacific Ocean. We present the results of six W-E running Multi-Channel Seismic (MCS), magnetic and sediment echo sounding profiles acquired during the ARCHIMEDES-I expedition. These profiles cover the MTJ, the FRSC and the region just south of the FRSC to investigate the tectonic history and current tectonic activity of the Lau Basin. On all MCS profiles, we observe a heavily faulted basement on both sides of the MTJ and FRSC with faults that are covered with sediments, confirmed by the sediment echo sounding data. We consider these buried faults inactive today. We also observe faults that reach the seafloor. These faults are generally located closer to the MTJ and the FRSC and they correlate well with seismic activity recorded in the region. We thus consider these faults currently active. Seismically transparent bodies are observed on most profiles as well. We have interpreted those as volcanic intrusions, i.e. sills, or as volcanoes that pierce through the stratigraphy, especially closer to the volcanic arc. The two sets of faults, the notion that extension rates are higher at the MTJ (32 mm/yr) than at the southern tip of the FRSC (8 mm/yr) and the results from our newly acquired and interpreted magnetic data, have led to the interpretation that an earlier rift phase accommodated extension in a wide rift tectonic setting between 2.15 Ma and 0.85 Ma at the MTJ and 2.15 and 1.61 Ma at the FRSC. Today, the extension is accommodated in a narrow rift tectonic setting close to the MTJ and FRSC with a higher extension rate at the MTJ than at the southern tip of the FRSC. These findings suggest that the MTJ and FRSC are one, single intra-plate extension zone that is in the process of breaking apart the overriding Niuafo’ou-Tonga microplate along the MTJ and FRSC.

Coastal and ocean island volcanoes are renowned for having unstable flanks, which expresses as slow seawards flank sliding observable by geodetic techniques and/or catastrophic sector collapses. A large section of these unstable flanks is often below sea level, where information on the volcanotectonic structure and, in particular, ground deformation are limited. Consequently, kinematic models that attempt to explain measured onshore ground deformation associated to flank instability are poorly constrained in the offshore area. This is also the case for Mount Etna’s unstable south-eastern flank that slides seawards at rates of 2-3 cm/yr. Displacements associated to flank movement, observed onshore by geodetic and remote sensing techniques, show maximum values at the coast and kinematic models consistently predict even larger movements seawards of the coast. Our seafloor geodetic measurements between 2016 and 2018 confirmed that offshore flank slip is equal or slightly larger compared to onshore slip. The main displacement was released during one slow slip event. Here, we present new data from a second deployment of the seafloor geodetic network in the same location with the same direct-path acoustic ranging technique and a modified network design. The measurements allow reconstructing relative seafloor displacement within the network at sub-centimetre precision, from September 2020 until November 2021. The preliminary results indicate a possible eastward sliding of the flank, although the overall slip of

Accurate subsurface velocity models are crucial for geological interpretations based on seismic depth images. Seismic reflection tomography is an effective iterative method to update and refine a preliminary velocity model for depth imaging. Based on residual move-out analysis of reflectors in common image point gathers, an update of the velocity is estimated by a ray-based tomography. To stabilize the tomography, several preconditioning strategies exist. Most critical is the estimation of the depth error to account for the residual move-out of the reflector in the common image point gathers. Because the depth errors for many closely spaced image gathers must be picked, manual picking is extremely time-consuming, human biased, and not reproducible. Data-driven picking algorithms based on coherence or semblance analysis are widely used for hyperbolic or linear events. However, for complex-shaped depth events, purely data-driven picking is difficult. To overcome this, the warping method named non-rigid matching is used to estimate a depth error displacement field. Warping is used, for example, to merge photographic images or to match two seismic images from time-lapse data. By matching a common image point gather against its duplicate that has been shifted by one offset position, a locally smooth-shaped displacement field is calculated for each data sample by gather matching. Depending on the complexity of the subsurface, sample tracking through the displacement field along predefined horizons or on a simple regular grid yields discrete depth error values for the tomography. The application to a multi-channel seismic line across the Sunda subduction zone offshore Lombok island, Indonesia, illustrates the approach and documents the advantages of the method to estimate a detailed velocity structure in a complex tectonic regime. By incorporating the warping scheme into the reflection tomography, we demonstrate an increase in the velocity resolution and precision by improving the data-driven accuracy of depth error picks with arbitrary shapes. This approach offers the possibility to use the full capacities of tomography and further leads to more accurate interpretations of complex geological structures.

Cruise SO288 served two scientific projects. The main objective of the COMBO project was the recovery of three seafloor geodetic networks of the GeoSEA array which were installed on the continental margin and outer rise offshore Iquique in northern Chileduring RV SONNE cruise SO244. This work was flanked by additional seismic and bathymetric surveys to characterize the sub-seafloor structure. The South American subduction system around 21°S has last ruptured in an earthquake in 1877 and wasidentified as a seismic gap prior to the 2014 Iquique earthquake (Mw=8.1). The southern portion of the segment remains unbroken by a recent earthquake and is currently in the latest stage of the interseismic phase of the seismic cycle. The seafloor geodetic measurements of the GeoSEA array provide a way to monitor crustal deformation at high resolution comparable to the satellite-based GPS technique upon which terrestrial geodesy is largely based. The GeoSEA array consists of autonomous seafloor transponders installed on 4 m high tripods. The transponders within an array intercommunicate via acoustic signals for a period of up to three years. Recovery of the GeoSEA array using a remotely operated vehicle (ROV KIEL6000) required dedicated dives in the three network locations on the middle and lower continental slope (AREA1 and AREA3, respectively) and the outer rise of the Nazca plate (AREA2). All 23 GeoSEA transponders were successfully recovered and showed an 100% uptime during the monitoring period.The GeoSEA survey represents the first seafloor geodetic transect across a subduction zone, spanning from the oceanic outer rise to the lower and middle slope of the continental upper plate. The second project, HOMER, focused on biogeochemical and microbiological processes that affect carbon cycling of the Humboldt Current System off Northern Chile down to the deep ocean. For this purpose, water samples were collected for the detailed chemical characterization of organic matter and the activity of microorganisms. The work was complemented by onboard incubations of microbial populations from deep waters with naturally occurring organic matter.Cruise SO288 was the first expedition of RV SONNE back to the Pacific Ocean starting from a South American port during the COVID-19 pandemic. Despite strict safety and health requirements prior to boarding RV SONNE in Guayaquil, several members of the scientific and ship’s crew tested positive to COVID-19 two days after we left port. Containment measures were immediately put to action, flanked by a tight testing regime. Ten days after leaving Guayaquil, we were able to break the chains of infection and the scientific working program commenced.

15 pages, 10 figures, 3 tables, supplementary material https://doi.org/10.1016/j.margeo.2022.106749.-- Data availability: The data (3D complex mesh of the ARFS and rake values, and the resulting grid files of the tsunami simulations containing the maximum wave amplitude) are archived at PANGAEA repository (https://doi.pangaea.de/10.1594/PANGAEA.941092).-- The EMODnet bathymetry is available at https://www.emodnet-bathymetry.eu/. The stochastic slip distributions have been produced by the code ANTI-FASc (https://github.com/antonioscalaunina/ANTI-FASc) a platform partially based on the code k223d (Herrero and Murphy, 2018 available at https://github.com/s-murfy/k223d), in turn based on the slipk2 (available at https://github.com/andherit/slipk2) and the trilateration codes (available at https://github.com/andherit/trilateration), The westernmost Mediterranean hosts part of the plate boundary between the European and African tectonic plates. Based on the scattered instrumental seismicity, this boundary has been traditionally interpreted as a wide zone of diffuse deformation. However, recent seismic images and seafloor mapping studies support that most of the plate convergence may be accommodated in a few tectonic structures, rather than in a broad region. Historical earthquakes with magnitudes Mw > 6 and historical tsunamis support that the low-to-moderate instrumental seismicity might also have led to underestimation of the seismogenic and tsunamigenic potential of the area. We evaluate the largest active faults of the westernmost Mediterranean: the reverse Alboran Ridge, and the strike-slip Carboneras, Yusuf and Al-Idrissi fault systems. For the first time, we use a dense grid of modern seismic data to characterize the entire dimensions of the main fault systems, accurately describe the geometry of these structures and estimate their seismic source parameters. Tsunami scenarios have been tested based on 3D-surfaces and seismic source parameters, using both uniform and heterogeneous slip distributions. The comparison of our results with previous studies, based on limited information on the fault geometry and kinematics, indicates that accurate fault geometries and heterogeneous slip distributions are needed to properly assess the seismic and tsunamigenic potential in this area. Based on fault scaling relations, the four fault systems have a large seismogenic potential, being able to generate earthquakes with Mw > 7. The reverse Alboran Ridge Fault System has the largest tsunamigenic potential, being able to generate a tsunami wave amplitude greater than 3 m in front of the coasts of Southern Spain and Northern Africa, This work is supported by the Cluster of Excellence “The Future Ocean”, within the framework of the Excellence Initiative by the Deutsche Forschungsgemeinschaft (DFG) on behalf of the German federal and state governments. This study benefited from an EU Marie Skłodowska-Curie Individual Fellowship to LGP (H2020-MSCA-IF-2017 796013). LGP, CS, FM and RB acknowledge the resources made available by the SISMOLAB-3D at INGV. This work has been carried out in collaboration with the Grup de Recerca Consolidat de la Generalitat de Catalunya “Barcelona Center for Subsurface Imaging” (2017 SGR 1662), and acknowledges the ICM “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S)

We resolve a previously unrecognized shallow subducting seamount from a re-processed multichannel seismic depth image crossing the 1994 M7.8 Java tsunami earthquake slip area. Seamount subduction is related to the uplift of the overriding plate by lateral shortening and vertical thickening, causing pronounced back-thrusting at the landward slope of the forearc high and the formation of splay faults branching off the landward flank of the subducting seamount. The location of the seamount in relation to the 1994 earthquake hypocentre and its co-seismic slip model suggests that the seamount acted as a seismic barrier to the up-dip co-seismic rupture propagation of this moderate size earthquake. The wrapping of the co-seismic slip contours around the seamount indicates that it diverted rupture propagation, documenting the control of forearc structures on seismic rupture.

The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian–Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica–Sardinia block is associated with rifting, shaping the Ligurian Basin. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We perform ambient noise tomography, also taking into account teleseismic events, using an amphibious network of seismic stations, including 22 broadband ocean bottom seismometers (OBSs), to investigate the lithospheric structure of the Ligurian Basin. The instruments were installed in the Ligurian Basin for 8 months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we carefully pre-process the data, including corrections for instrument tilt and seafloor compliance and excluding higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the AlpArray OBS array and surrounding land stations. We also correlate short time windows that include teleseismic earthquakes, allowing us to derive surface wave group velocities for longer periods than using ambient noise only. We obtain group velocity maps by inverting Green's functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The group velocity and shear-wave velocity results compare well to existing large-scale studies that partly include the study area. In onshore France, we observe a high-velocity area beneath the Argentera Massif, roughly 10 km below sea level. We interpret this as the root of the Argentera Massif. Our results add spatial resolution to known seismic velocities in the Ligurian Basin, thereby augmenting existing seismic profiles. In agreement with existing seismic studies, our shear-wave velocity maps indicate a deepening of the Moho from 12 km at the south-western basin centre to 20–25 km at the Ligurian coast in the north-east and over 30 km at the Provençal coast. The maps also indicate that the south-western and north-eastern Ligurian Basin are structurally separate. The lack of high crustal vP/vS ratios beneath the south-western part of the Ligurian Basin preclude mantle serpentinisation there.

The aftershock distribution of the 2014 Mw 8.1 Iquique earthquake offshore northern Chile, identified from a long‐term deployment of ocean bottom seismometers installed eight months after the mainshock, in conjunction with seismic reflection imaging, provides insights into the processes regulating the up‐dip limit of coseismic rupture propagation. Aftershocks up‐dip of the mainshock hypocenter frequently occur in the upper plate and are associated with normal faults identified from seismic reflection data. We propose that aftershock seismicity near the plate boundary documents subduction erosion that removes mass from the base of the wedge and results in normal faulting in the upper plate. The combination of very little or no sediment accretion and subduction erosion over millions of years has resulted in a very weak and aseismic frontal wedge. Our observations thus link the shallow subduction zone seismicity to subduction erosion processes that control the evolution of the overriding plate. Key Points: - We investigate structure and seismicity at the up-dip end of the 2014 Iquique earthquake rupture using amphibious seismic data. - Seismicity up-dip of the 2014 Iquique earthquake occurs over a broad range likely interpreted to be related to the basal erosion processes. - Coseismic stress changes and aftershocks activate extensional faulting of the upper plate and subduction erosion.

The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block is associated with rifting, shaping the Ligurian Sea. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We invert velocity models using an amphibious network of seismic stations, including 22 broadband Ocean Bottom Seismometers (OBS) to investigate the lithospheric structure of the Ligurian sea. The instruments were installed in the Ligurian Sea for eight months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we attentively pre-process the data, including corrections for instrument tilt and seafloor compliance. We took extra care to exclude higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include teleseismic earthquakes that allow us to derive surface wave group velocities for longer periods than using ambient noise only. Group velocity maps are obtained by inverting Green’s functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The shear-wave velocity results show a deepening of the Moho from 12 km at the southwestern basin centre to 20–25 km at the Ligurian coast in the northeast and over 30 km at the Provençal coast. We find no hint on mantle serpentinisation and no evidence for an Alpine slab, at least down to depths of 25 km. However, we see a separation of the southwestern and northeastern Ligurian Basin that coincides with the promoted prolongation of the Alpine front.

Spreading centers in the proximity of back‐rolling subduction zones constitute an ideal natural laboratory to investigate the interaction of magmatism and tectonism during the early evolution of back‐arc basins. Using 32 days of ocean bottom seismometer data, we located 697 micro‐earthquakes at the southern Fonualei Rift and Spreading Center (S‐FRSC). The majority of epicenters concentrate along the central region of the axial valley, marking the active ridge axis. Only odd events were associated with the prominent faults bounding the axial valley. About 450 events are spatially clustered around 17°42′S and their waveforms show a pronounced similarity. Most of these events are associated with a 138 h lasting earthquake swarm. The tectonic structure of the ridge axis in the S‐FRSC resembles a series of left‐stepping en echelon segments, expressed at the seafloor by numerous volcanic ridges. The recorded earthquake swarm is located at the stepover of two en echelon segments suggesting that the earthquake swarm is mainly tectonically driven. The events directly beneath our seismic network indicate a maximum depth of brittle faulting down to about 14 km below the seafloor. This is within the maximum depth range of brittle faulting at ultraslow mid‐ocean ridges. Since the thickness of the brittle lithosphere is mainly controlled by temperature, our results suggest a sub‐axial thermal structure similar to that of ultraslow mid‐ocean ridges of similar opening rates., Key Points: Microseismicity is focused in the axial valley along the spreading axis and coincides with volcanic ridges and lava flows. An earthquake swarm is located at the stepover of two en echelon segments. The swarm is mainly controlled by tectonism. The brittle axial lithosphere is about 14 km thick, which is in agreement with mid‐ocean ridges of similar spreading rates., Bundesministerium für Bildung und Forschung (BMBF) http://dx.doi.org/10.13039/501100002347, GEOMAR Helmholtz‐Zentrum für Ozeanforschung Kiel http://dx.doi.org/10.13039/501100003153

The Earth’s ocean floor deforms continuously under the influence of plate tectonic processes. In recent years, the development of deep-sea instruments using acoustic direct-path ranging allows observations of ocean floor deformation with unprecedented spatial and temporal resolution. Due to rapid technological progress, acoustic ranging emerged as a central research field to monitor seafloor deformation. Here we review recent developments and the progress of direct-path ranging applications. We discuss the methodology and examine the effects of the oceanographic environment on the measurement precision. Comparing the resolution of previous deployments, we find that the baseline uncertainty increases linearly with baseline length, at least for distances up to 3 km, but with different linear relations for each deployment. Measurements of displacement at millimeter-level precision across normal, thrust or strike-slip faults are discussed to evaluate the influence of dedicated network designs appropriate for the discrete fault geometries. Furthermore, tectonically quiet areas, such as flanks of coastal or ocean island volcanoes and passive continental margins pose substantial hazards that often lack in-situ monitoring and are therefore a significant target for the application of seafloor geodetic techniques.

The Alpine orogen and the Apennine system are part of the complex tectonic setting in the Mediterranean Sea caused by the convergence between Africa and Eurasia. Between 30 Ma and 15 Ma the Calabrian subduction retreated in a southeast direction pulling Corsica and Sardinia away from the Eurasian landmass. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The rifting jumped 15 Ma ago in the Tyrrhenian Sea leaving Corsica and Sardinia in a stable position relative to Eurasia.Within the framework of the AlpArray research initiative a long-term seismological experiment was conducted in the Ligurian Sea to investigate the lithospheric structure and the seismicity in the Ligurian basin. The passive seismic network consisted of 29 broad-band ocean bottom stations from Germany and France. It was in operation between June 2017 and February 2018.Two seismicity clusters occurred in the centre of the Ligurian Basin. The 18 earthquakes are located in the lower crust and in the upper-most mantle at depths between 10 km and 16 km. Re-location was performed only using picks from the OBS in the centre of the Ligurian Sea to avoid artifacts from the complex 3D velocity structure of the basin. Mantle refractions Pn and Sn have apparent velocities of 8.2 km/s and 4.7 km/s. The low Vp-Vs-ratio of 1.72 indicates a more brittle behaviour of the mantle material.Fault plane solutions were determined for four events using also the data of land stations in southern France, Corsica, Sardinia and northern Italy. The focal mechanisms are thrust faulting. Fault planes strike in a NE-SW direction, coinciding with the alignment of the events and the direction of the basin axis.We interprete the two earthquake clusters related to the inversion of the Ligurian Basin where the basin’s centre is under compression and stresses are taken up by reactivated faults in the crust and uppermost mantle. The compressional forces could be caused by the convergence of Africa and Europe. In general, observations of earthquakes in continental mantle lithosphere are rare and they reveal on the one hand a strengthening of the crust and uppermost mantle during rifting and on the other hand they support the interpretation that rifting failed in the northern Ligurian Basin.

The Lau Basin is a young back-arc basin steadily forming at the Indo-Australian-Pacific plate boundary, where the Pacific plate is subducting underneath the Australian plate along the Tonga-Kermadec island arc. Roughly 25 Ma ago, roll-back of the Kermadec-Tonga subduction zone commenced, which lead to break up of the overriding plate and thus the formation of the western Lau Ridge and the eastern Tonga Ridge separated by the emerging Lau Basin.As an analogue to the asymmetric roll back of the Pacific plate, the divergence rates decline southwards hence dictating an asymmetric, V-shaped basin opening. Further, the decentralisation of the extensional motion over 11 distinct spreading centres and zones of active rifting has led to the formation of a composite crust formed of a microplate mosaic. A simplified three plate model of the Lau Basin comprises the Tonga plate, the Australian plate and the Niuafo'ou microplate. The northeastern boundary of the Niuafo'ou microplate is given by two overlapping spreading centres (OLSC), the southern tip of the eastern axis of the Mangatolu Triple Junction (MTJ-S) and the northern tip of the Fonualei Rift spreading centre (FRSC) on the eastern side. Slow to ultraslow divergence rates were identified along the FRSC (8-32 mm/a) and slow divergence at the MTJ (27-32 mm/a), both decreasing southwards. However, the manner of divergence has not yet been identified. Additional regional geophysical data are necessary to overcome this gap of knowledge.Research vessel RV Sonne (cruise SO267) set out to conduct seismic refraction and wide-angle reflection data along a 185 km long transect crossing the Lau Basin at ~16 °S from the Tonga arc in the east, the overlapping spreading centres, FRSC1 and MTJ-S2, and extending as far as a volcanic ridge in the west. The refraction seismic profile consisted of 30 ocean bottom seismometers. Additionally, 2D MCS reflection seismic data as well as magnetic and gravimetric data were acquired.The results of our P-wave traveltime tomography show a crust that varies between 4.5-6 km in thickness. Underneath the OLSC the upper crust is 2-2.5 km thick and the lower crust 2-2.5 km thick. The velocity gradients of the upper and lower crust differ significantly from tomographic models of magmatically dominated oceanic ridges. Compared to such magmatically dominated ridges, our final P-wave velocity model displays a decreased velocity gradient in the upper crust and an increased velocity gradient in the lower crust more comparable to tectonically dominated rifts with a sparse magmatic budget.The dominance of crustal stretching in the regional rifting process leads to a tectonical stretching, thus thinning of the crust under the OLSC and therefore increasing the lower crust’s velocity gradient. Due to the limited magmatic budget of the area, neither the magnetic anomaly nor the gravity data indicate a magmatically dominated spreading centre. We conclude that extension in the Lau Basin at the OLSC at 16 °S is dominated by extensional processes with little magmatism, which is supported by the distribution of seismic events concentrated at the northern tip of the FRSC.

The Earth’s oldest oceanic lithosphere preserved in-situ is in the eastern Mediterranean Sea. It can offer essential information on the oceanic plate evolution. Yet, its thickness and other properties have been difficult to determine by means of seismic imaging due to the high heterogeneity of the region. Here, we combine a large, new surface wave dataset with published wide-angle data in order to map the properties and lateral variability of the oceanic lithosphere, as well as the ocean-continent transition in the easternmost Mediterranean beneath the Levant Basin. We use stochastic joint inversion of broad band, phase-velocity dispersion measurements and seismic refraction P-wave velocity models to obtain 1-D, shear wave velocity models down to 300 km depth and compare the structure beneath the Ionian Sea and the Levant Basin. The thickness of the crust is about 16.4 ± 3 km and 22.3 ± 2 km beneath the chosen locations within the Ionian Sea and the Levant Basin, respectively. The Poisson’s ratio of about 0.32 and Vp/Vs of about 1.93 in the crystalline crust, yielded by the inversion, confirm the presence of oceanic crust beneath the Ionian Sea. The thickness of the Ionian oceanic lithosphere is around 180 km, whereas the continental lithosphere beneath the eastern Levant Basin is ~70 km thick, with low crustal Vp/Vs (~1.7) and Poisson’s (~0.24) ratios. According to 3-D shear wave velocity tomography using the surface wave data, the thickness of the oceanic lithosphere increases from the Triassic Ionian Sea towards the Permian-Carboniferous Libyan Sea and Herodotus Basin. Thicknesses of the Permo-Triassic oceanic lithosphere considerably larger than 100 km indicate that oceanic lithosphere can thicken by cooling substantially beyond the limits suggested by the plate cooling model. The transition from oceanic to continental lithosphere occurs at about 31°E in the crust, as indicated by magnetic and gravity measurements. The continental mantle lithosphere further to the east of this boundary is ~150 km thick beneath the westernmost Levant Basin, as indicated by shear wave velocity tomography and long wavelength gravity anomalies, and strongly thins eastward towards the area of the Levantine Coast and the Dead Sea Fault. The localization of the lithospheric deformation and crustal seismicity along the Dead Sea Fault correlates spatially with the thinning of the underlying continental lithosphere.Key words: surface wave tomography, wide angle seismic imaging, joint inversion, Vp/Vs and Poisson’s ratios, eastern Mediterranean, Oceanic Lithosphere, Continental Lithosphere, Dead Sea Fault.

The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning little is known about the style of back-arc rifting. Today, seismicity indicates the closure of this back-arc basin. In the basin, earthquake clusters occur in the lower crust and uppermost mantle and are related to re-activated, inverted, normal faults created during rifting.To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismic profile was shot across the Ligurian Basin in an E-W direction from the Gulf of Lion to Corsica. The profile comprises 35 OBS and three land stations at Corsica to give a complete image of the continental thinning including the necking zone.The majority of the refraction seismic data show mantle phases with offsets up to 70 km. The arrivals of seismic phases were picked and used to generate a 2-D P-wave velocity model. The results show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The P-wave velocities in the crust reach 6.6 km/s at the base. The uppermost mantle shows velocities >7.8 km/s. The crust-mantle boundary becomes shallower from ~18 km to ~12 km depth within 30 km from Corsica towards the basin centre. The velocity model does not reveal an axial valley as expected for oceanic spreading. Further, it is difficult to interpret the seismic data whether the continental lithosphere was thinned until the mantle was exposed to the seafloor. However, an extremely thinned continental crust indicates a long lasting rifting process that possibly did not initiate oceanic spreading before the opening of the Ligurian Basin stopped. The distribution of earthquakes and their fault plane solutions, projected along our seismic velocity model, is in-line with the counter-clockwise opening of the Ligurian Basin.

The northern Lau Basin in the southwest Pacific Ocean is one of the fastest opening back-arc basins on Earth, resulting in a mosaic of microplates, including the Niuafo’ou and Tongan microplates. The Fonualei Rift and Spreading Center (FRSC) is the eastern plate boundary that separates the Niuafo’ou from the Tongan microplate. The northern part of the FRSC is actively spreading, whereas the southern part is rifting. What is unclear, however, is how extension of the Lau Basin is accommodated north and south of the FRSC.We present the results of six Multi-Channel Seismic profiles acquired during the ARCHIMEDES-I expedition and show an analogue lithosphere-scale model example of our proposed tectonic evolution. Profiles P1 (oriented NW-SE) and P2 (oriented W-E) cover the Mangatolu Triple Junction (MTJ) and the northern part of the FRSC. P3 and P4 (both oriented W-E) cover the southern Niuafo’ou microplate. P5 and P6 (both oriented W-E) cover the area south of the FRSC.The northern profiles (P1 and P2) reveal a thick package of sediment towards the east, covering a heavily faulted basement over a wide area. Some indication for intrusive material is observed closer to the volcanic arc, but also further towards the western end of P2. Faults cross-cutting the basement but that do not reach the surface are considered inactive today. Faults reach the surface close to the MTJ and the northern tip of the FRSC and are considered active today. This leads to the interpretation that an earlier rift phase accommodated extension in a wide rift tectonic setting, whereas today, the extension is accommodated in a narrow rift or spreading tectonic setting. We will show an analogue model example that demonstrates this wide-to-narrow extensional tectonic evolution.The profiles that cover the southern extent of the FRSC (P3, P4, P5 and P6), show that active faulting occurs towards the west, close to the Central Lau Spreading Center. Hidden faults that have deformed the basement, but do not affect the surface today anymore are observed in the abyssal parts of P3, P4, P5 and P6. Active faults that reach the surface are also observed towards the east. Recent volcanism is observed, both in the form of intrusive bodies, i.e. sills, as well as volcanoes that pierce through the stratigraphy. The observations lead to the conclusion that south of the FRSC an earlier (wide) rift system affected a larger area in the current abyssal parts of the profiles, whereas extension is currently accommodated through spreading in the CLSC, west of the southern tip of the FRSC.

Understanding the physical parameters and processes that control the seismogenic behavior of subduction zones megathrust faults remains one of the outstanding challenges in Earth Sciences.Here we present important results from several large seismic experiments aimed at addressing this question. These experiments focused on the three subduction zones off Greece, the Lesser Antilles islands, and Ecuador, with different convergence rates and seismic activities. Surveys included multibeam bathymetry, multichannel reflection seismic (MCS) and wide-angle seismic (WAS) acquisitions over the forearc domain, as well as teleseismic receiver-functions and local earthquakes monitoring with temporary deployments of seismological networks.Our results demonstrate the needs of both dense and extensive geophysical investigations.In the central Lesser Antilles subduction zone, the interplate has been imaged down to the backstop at 12-15 km depth over the 350-km-long Antigua to Martinique islands segment. The outer forearc crust is strongly faulted in response to the two subducting Tiburon and Barracuda ridges (SISMANTILLES1-and-2 surveys). Two WAS profiles constrained the deeper geometry of the interplate down to the forearc Moho located at 28 km depth (TRAIL survey). The OBS networks deployed over several months (OBSANTILLES and OBSISMER surveys) revealed mantle wedge supraslab earthquakes and M4-5 possible repeaters with flat-trust mechanisms. The joint active-source/local earthquake seismic tomography let us to unveil the Vp and Vp/Vs heterogeneity along the slab surface and derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. Farther northwest, where the convergence obliquity strongly increases, we constrained the geometry of the interplate down to the forearc Moho at 25 km depth. Strain partitioning localizes on inherited major structures within the forearc domain, like the left-lateral partitioning system of the Anegada Passage and the 850-km-long Bunce fault, located along the backstop (ANTITHESIS survey).On the southwestern Hellenic subduction zone, MCS and WAS acquisitions highlight the existence of an outer forearc crust beneath the forearc Matapan Trough, but its highly complex structure prevented us to image the interplate (ULYSSE survey). Acquisition by the R/V Marcus Langseth with its 8-km-long streamer finally made it possible (SISMED survey). Dense receiver-function acquisition on a 300-km-long mobile seismic network constrained the 3D geometry of the slab top underneath central Greece. This imaging revealed that the subducting oceanic crust and backstop updip limit are segmented by 9 trench-normal subvertical faults, seismically active at intermediate depths and possibly of inherited origin (THALES WAS RIGHT survey).South of the 1906 M8.8 Ecuador-Columbia rupture area, the April 2016 Mw7.8 Pedernales subduction earthquake and its ensuing postseismic phase revealed a combination of seismic/aseismic slip behavior. Fluid-enriched parts of the megathrust fault and structural margin segmentation are hypothesized to play a major role in controlling slip behavior but direct observations are still lacking. Previous MCS acquisitions revealed very locally a fluid-rich subduction channel along with severe damage effect of the forearc margin due to seamounts subduction (SISTEUR survey). Forthcoming 3D seismic acquisition along this segment will examine the impact of the along-strike and along-dip variations of the physical properties and fluid content on the slip mode (HIPER survey).

The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast trench retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning and un-roofing of mantle material little is known about the style of back-arc rifting.To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismic profile was shot across the Ligurian Basin in an E-W direction from the Gulf of Lion to Corsica. The profile extends onshore Corsica to image the necking zone of continental thinning.The majority of the refraction seismic data show mantle phases at offsets up to 70 km. The arrivals of seismic phases were picked and inverted in a travel time tomography. The results show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The mantle shows rather high velocities >7.8 km/s. The crust-mantle boundary deepens from ~12 km to ~18 km within 25 - 30 km towards Corsica. The results do not map an axial valley as expected for oceanic spreading. However, an extremely thinned continental crust indicates a long lasting rifting process that possibly does not initiated oceanic spreading before the opening of the Ligurian Basin stopped.

Laser reflectometry (BOTDR), commonly used for structural health monitoring (bridges, dams, etc.), for the first time is being tested to study movements of an active fault on the seafloor, 25 km offshore Catania Sicily (an urban area of 1 million people). Under ideal conditions, this technique can measure small strains (10E-6), across very large distances (10 - 200 km) and locate these strains with a spatial resolution of 10 - 50 m. As the first experiment of the European funded FOCUS project (ERC Advanced Grant), in late April 2020 we aimed to connect and deploy a dedicated 6-km long strain cable to the TSS (Test Site South) seafloor observatory in 2100 m water depth operated by INFN-LNS (Italian National Physics Institute). The work plan for the marine expedition FocusX1 onboard the research vessel PourquoiPas? is described here. First, microbathymetric mapping and a video camera survey are performed by the ROV Victor6000. Then, several intermediate junction frames and short connector cables (umbilicals) are connected. A cable-end module and 6-km long fiber-optic strain cable (manufactured by Nexans Norway) is then connected to the new junction box. Next, we use a deep-water cable-laying system with an integrated plow (updated Deep Sea Net design Ifremer, Toulon) to bury the cable 20 cm in the soft sediments in order to increase coupling between the cable and the seafloor. The targeted track for the cable crosses the North Alfeo Fault at three locations. Laser reflectometry measurements began April 2020 and will be calibrated by a three-year deployment of seafloor geodetic instruments (Canopus acoustic beacons manufactured by iXblue) also started April 2020, to quantify relative displacement across the fault. During a future marine expedition, tentatively scheduled for 2021 (FocusX2) a passive seismological experiment is planned to record regional seismicity. This will involve deployment of a temporary network of OBS (Ocean Bottom Seismometers) on the seafloor and seismic stations on land, supplemented by INGV permanent land stations. The simultaneous use of laser reflectometry, seafloor geodetic stations as well as seismological land and sea stations will provide an integrated system for monitoring a wide range of types of slipping events along the North Alfeo Fault (e.g. - creep, slow-slip, rupture). A long-term goal is the development of dual-use telecom cables with industry partners.

The northeastern Lau Basin is one of the fastest opening and magmatically most active back-arc regions on Earth. Although the current pattern of plate boundaries and motions in this complex mosaic of microplates is fairly well understood, the structure and evolution of the back-arc crust are not. We present refraction seismic, multichannel seismic and gravity data from a 300 km long east-west oriented transect crossing the Niuafo’ou Microplate (back-arc), the Fonualei Rift and Spreading Centre (FRSC) and the Tofua Volcanic Arc at 17°20’S. Our P wave tomography model shows strong lateral variations in the thickness and velocity-depth distribution of the crust. The thinnest crust is present in the Fonualei Rift and Spreading Center, suggesting active seafloor spreading there. In the much thicker crust of the volcanic arc we identify a region of anomalously low velocities, indicative of partial melts. Surprisingly, the melt reservoir is located at ~17 km distance to the volcanic front, supporting the hypothesis that melts are deviated from the volcanic arc towards the FRSC in sub-crustal domains. We identify two distinct regions in the back-arc crust, representing different opening phases of the northeastern Lau Basin. During initial extension, likely dominated by rifting, crust of generally lower upper-crustal velocities formed. During an advanced opening phase, likely dominated by seafloor spreading, crust of higher upper-crustal velocities formed and is now up to 11 km thick. This thickening is the result of magmatic underplating, which is supported by elevated upper mantle temperatures in this region.

The Alpine orogen and the Apennines system are part of the complex tectonic settings in the Mediterranean Sea caused by the convergence between Africa and Eurasia. Between 30 Ma and 15 Ma, the Calabrian Subduction retreated in southeast direction pulling Corsica and Sardinia away from the Eurasian continent. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The rifting jumped 15 MA ago to the Tyrrhenian Sea leaving Corsica and Sardinia in a stable position relative to Eurasia as observed by GPS measurements.Within the framework of the AlpArray research initiative and its German component “4D Mountain building” (SPP2017 4D-MB) a long-term experiment was conducted in the Ligurian sea to investigate the lithosphere structure and the seismicity in the Ligurian basin. The passive seismic network was operated by France and Germany and consisted of 29 broad-band ocean bottom stations. It was in operation between June 2017 and February 2018. At the end of the experiment two active seismic profiles were conducted additionally.A cluster of 15 events with magnitudes lower than 2.5 occurred in the centre of the Ligurian Basin. The earthquakes are located at a depth of 20 km to 35 km, i.e. 10 - 25 km below the Moho. The cluster was not continuously active but had several active periods which lasted between 2 and 5 days.A fault plane solution could be determined of the larger events in the cluster. The mechanism is a thrust faulting. Smaller events should have a similar mechanism due to the highly coherent waveforms. A similar mechanism was observed for the Mw=4.9 earthquake on 07.07.2011 which occurred 50 km east of the cluster. Both solutions show a SW-NE striking, northwest dipping fault plane. This indicates a convergence in NW-SE direction between Corsica and Eurasia.

The Ligurian Basin is located in the Mediterranean Sea to the north-west of Corsica at the transition from the western Alpine orogen to the Apennine system and was generated by the south-eastward trench retreat of the Apennines-Calabrian subduction zone. Late Oligocene to Miocene rifting caused continental extension and subsidence, leading to the opening of the basin. Yet, it still remains enigmatic if rifting caused continental break-up and seafloor spreading. To reveal its lithospheric architecture, we acquired a state of the art seismic refraction and wide-angle reflection profile in the Ligurian Basin. The seismic line was recorded in the framework of SPP2017 4D-MB, the German component of the European AlpArray initiative, and trends in a NE-SW direction at the centre of the Ligurian Basin, roughly parallel to the French coastline. The seismic data recorded on the newly developed GEOLOG recorder, designed at GEOMAR, are dominated by sedimentary refractions and show mantle Pn arrivals at offsets of up to 70 km and a very prominent wide-angle Moho reflection. The main features share several characteristics (i.e. offset range, continuity) generally associated with continental settings rather than documenting oceanic crust emplaced by seafloor spreading. Seismic tomography results are augmented by gravity data and yield a 7.5–8 km thick sedimentary cover which is directly underlain by serpentinised mantle material at the south-western end of the profile. The acoustic basement at the north-eastern termination is interpreted to be continental crust, thickening towards the NE. Our study reveals that the oceanic domain does not extend as far north as previously assumed and that extension led to extreme continental thinning and exhumation of sub-continental mantle which eventually became serpentinised.

This policy brief provides recommendations for the science needed for the next decade and beyond to support the 2030 Agenda for Sustainable Development and its 17 Sustainable Development Goals (SDGs). The document outlines the societal outcomes of the UN Decade of Ocean Science for Sustainability (A Clean Ocean, A Sustainably Harvested Ocean, A Safe Ocean, A Healthy Ocean, A Predicted Ocean, A Transparent Ocean) and provides recommendations from the 'Navigating the Future V' Position Paper (2019) by the European Marine Board to address each societal outcome.

The Calabrian subduction zone is one of the narrowest arcs on Earth and a key area to understand the geodynamic evolution of the Mediterranean and other marginal seas. Here in the Ionian Sea, the African plate subducts beneath Eurasia. Imaging the boundary between the downgoing slab and the upper plate along the Calabrian subduction zone is important for assessing the potential of the subduction zone to generate mega‐thrust earthquakes and was the main objective of this study. Here we present and analyze the results from a 380 km long, wide‐angle seismic profile spanning the complete subduction zone, from the deep Ionian Basin and the accretionary wedge to NE Sicily, with additional constraints offered by 3‐D Gravity modeling and the analysis of earthquake hypocenters. The velocity model for the wide‐angle seismic profile images thin oceanic crust throughout the basin. The Calabrian backstop extends underneath the accretionary wedge to about 100 km SE of the coast. The seismic model was extended in depth using earthquake hypocenters. The combined results indicate that the slab dip increases abruptly from 2‐3° to 60‐70° over a distance of ≤50 km underneath the Calabrian backstop. This abrupt steepening is likely related to the roll‐back geodynamic evolution of the narrow Calabrian slab which shows great similarity to the shallow and deep geometry of the Gibraltar slab. Plain language abstract We investigate the deep crustal structure of southern Italy and the Central Mediterranean where some of the oldest oceanic crust on Earth is actively descending (subducting) into the earth's interior (Speranza et al., 2012). This process causes much of the moderate seismicity observed in this region and may be responsible for strong historical earthquakes as well (Gutscher et al., 2006). Deep seismic data recorded during a marine geophysical expedition performed in 2014, allow us to reconstruct the 3‐D geometry of this subduction zone. Our data reveal a 1‐4 km thick evaporitic (salt bearing) layer in the 13 km thick accretionary wedge. The thin underlying crust has characteristics of oceanic crust. The adjacent onshore domains (E Sicily and SW Calabria) are composed of 25‐30 km thick crust with velocities typical of continental crust. Together with earthquake travel‐time tomography (providing images of the subducting slab down to 300 km) and gravity modeling we can for the first time image the abrupt steepening of the subducting slab, the “slab hinge”, where slab dip increases from ≤5° to >60° over a downdip distance of 50 km. This slab dip is steep compared to other subduction zones, for example in Northern Honshu Japan or Sumatra, where the slab dip remains roughly 10° down to 40 km depth and therefore may have consequences on the seismicity of the region.

Highlights • Crust and mantle lithospheric structures beneath the eastern Mediterranean Sea are resolved from the joint inversion of surface wave measurements and wide-angle refraction seismics. • Vp/Vs and Poisson's ration estimates point to the presence of serpentinized oceanic crust beneath the Ionian Basin and thinned continental crust beneath the Levant Basin. • Oceanic lithosphere in the eastern Mediterranean Sea consists of three different domains: a) 180 km thick, Triassic Ionian lithosphere, b) 200 km thick, Permo-Carboniferous lithosphere beneath the Central Eastern Mediterranean and c) 180 km thick lithosphere beneath the eastern Herodotus Basin. • Thin continental lithosphere (75 km thick) beneath the Levant Basin underlain by the shallow Middle East Asthenosphere. • The spatial correlation between the shallow Middle East Asthenosphere and the Dead Sea Fault reveals focusing of lithospheric deformation in an area of thinned lithosphere. Abstract The tectonic plate under the eastern Mediterranean Sea shows a remarkable variability as it comprises Earth's oldest oceanic lithosphere as well as the transition towards continental lithosphere beneath the Levant Basin. Its thickness and other properties offer essential information on the lithospheric evolution but have been difficult to determine seismically due to the high heterogeneity of the region and its complex crustal structure. Here, we combine a large, new surface wave dataset with published wide-angle data in order to determine lithospheric properties in the eastern Mediterranean. Our stochastic inversions of broad-band, phase-velocity dispersion measurements resolve the crust-mantle structural trade-offs and yield robust, 1-D shear-wave velocity models down to 300 km depth beneath the Ionian and Levant Basins. The thickness of the crust beneath the two locations is 16.4 ± 3 km and 22.3 ± 2 km, respectively. The Poisson's ratio (σ) of 0.32 and Vp/Vs of 1.93 in the crystalline crust confirm the presence of serpentinized oceanic crust beneath the Ionian Basin. Beneath the Levant Basin, low crustal Vp/Vs (∼ 1.7) and Poisson's (∼ 0.24) ratios indicate continental crust. Beneath the Ionian Basin, the lithosphere is about 180 km thick. By contrast, thin, 75 km thick lithosphere is found beneath the Levant Basin. S-velocity tomography based on surface wave data also shows thick, spatially variable oceanic mantle lithosphere beneath the eastern Mediterranean. Thickness of the oceanic lithosphere increases eastwards from the Triassic Ionian towards the Permo-Carboniferous lithosphere in the Central Eastern Mediterranean. These results demonstrate that oceanic lithosphere can thicken by cooling substantially beyond the limits suggested by the plate cooling model. Beneath the eastern Herodotus oceanic Basin, lithospheric thickness is decreasing to about 180 km. Thin continental lithosphere and shallow asthenosphere are present beneath the Dead Sea Fault, demonstrating that the localization of the lithospheric deformation and crustal seismicity along the fault correlates spatially with the thinning of the underlying continental lithosphere.

Gutscher, Marc-André ... et al.-- 12 pages, 14 figures, 2 appendices, The detailed morphology and internal structure of the Calabrian accretionary wedge and adjacent Eastern Sicily margin are imaged in unprecedented detail by a combined dataset of multi-beam bathymetry and high-resolution seismic profiles. The bathymetric data represent the results of 6 recent marine geophysical surveys since 2010 as well as a compilation of earlier surveys presented as a 2 arc-sec (60 m) grid. Several distinct morpho-tectonic provinces are identified including: the deeply incised Malta–Hyblean Escarpment, numerous submarine canyons, broad regions of relatively flat seafloor dominated by fields of sediment waves, the gently undulating anticlinal fold-and-thrust belts of the external Calabrian accretionary wedge and the adjacent portion of the Western Mediterranean Ridge. The Calabrian arc can be divided into 4 domains (from SE to NW): 1) the undeformed Ionian abyssal plain, 2) the external evaporitic wedge, 3) the internal clastic wedge, 4) the Calabrian backstop (Variscan crystalline basement). The Calabrian accretionary wedge can also be divided laterally into two major lobes, the NE- and the SW lobes, and two minor lobes. The kinematics of the limit between the two major lobes is investigated and shown to be sinistral in the external (evaporitic) wedge. A network of radial slip lines within the southernmost external wedge unequivocally demonstrate ongoing dextral displacement of a rigid indenter (representing the corner of the clastic wedge) into the evaporitic wedge thereby confirming the geodynamic model of an active lateral slab tear fault here off eastern Sicily. The slab tear produces a series of major sub-parallel dextral strike-slip faults offshore Mt. Etna and south of the Straits of Messina consistent with the relative motions between Calabria and the Peloritan domain (NE Sicily). Abundant strike-slip faulting, and wide-spread folding and thrusting observed throughout the entire accretionary wedge, indicate regional shortening between the Ionian abyssal plain (foreland) and the Calabrian–Peloritan backstop caused by active subduction, We acknowledge INSU for funding related to the CIRCEE cruise and the European Union 7th Framework project ASTARTE, grant #603839, on Tsunami Hazard in Europe for support to MAG

Navigating the Future V gives a birds-eye big-picture view of marine science and looks into the future (2030 and beyond) to identify key topics/themes that are set to significantly advance our understanding of the marine and broader earth and climate systems; and will be of increasing importance to societal wellbeing in decades to come.

The nature of the Ionian Sea crust has been the subject of scientific debate for more than 30 years, mainly because seismic imaging of the deep crust and upper mantle of the Ionian Abyssal Plain (IAP) has not been conclusive to date. The IAP is sandwiched between the Calabrian and Hellenic subduction zones in the central Mediterranean. A NNE–SSW-oriented 131 km long seismic refraction and wide-angle reflection profile, consisting of eight ocean bottom seismometers and hydrophones, was acquired in 2014. The profile was designed to univocally confirm the proposed oceanic nature of the IAP crust as a remnant of the Tethys and to confute its interpretation as a strongly thinned part of the African continental crust. A P-wave velocity model developed from travel-time forward modelling is refined by gravimetric data and synthetic modelling of the seismic data. A roughly 6–7 km thick crust with velocities ranging from 5.1 to 7.2 km s−1, top to bottom, can be traced throughout the IAP. In the vicinity of the Medina seamounts at the southern IAP boundary, the crust thickens to about 9 km and seismic velocities decrease to 6.8 km s−1 at the crust–mantle boundary. The seismic velocity distribution and depth of the crust–mantle boundary in the IAP document its oceanic nature and support the interpretation of the IAP as a remnant of the Tethys lithosphere with the Malta Escarpment as a transform margin and a Tethys opening in the NNW–SSE direction.

International audience; Laser reflectometry (BOTDR), commonly used for structural health monitoring (bridges, dams, etc.), will for the first time be applied to study movements of an active fault on the seafloor 25 km offshore Catania Sicily. The goal of the European funded FOCUS project (ERC Advanced Grant) is to connect a 6-km long strain cable to the EMSO seafloor observatory in 2100 m water depth. Laser observations will be calibrated by seafloor geodetic instruments and seismological stations. A long-term goal is the development of dual-use telecom cables with industry partners.

During the winter 2012, from 20 January to 4 February, the German oceanographic FS METEOR cruise (M86/3) took place in the central-southern Adriatic Sea in the frame of “Adria LithosPHere InvestigAtion” (ALPHA [Kopp et al., 2013]). The primary goal of the project was high-resolution tomographic imaging of the crust and lithospheric mantle underneath the southern Adriatic Sea, the Apulia eastern margin and the external zone of the Dinaric thrust-belt by collecting offshore-onshore seismic data along three multi-fold wide-aperture profiles. The definition of reliable velocity models of the Adriatic lithosphere was considered crucial for a better understanding of the structure, fragmentation, geodynamic evolution, and seismotectonics of the Adria-Apulia microplates. The ALPHA Project was coordinated by Helmholtz Centre for Ocean Research Kiel, Germany (GEOMAR), former Leibniz Institute of Marine Sciences (German: Leibniz-Institut fur Meereswissenschaften, IFM-GEOMAR) and conducted in close cooperation with different European institutions of Germany, Albania, Croatia, Italy and Montenegro. The Istituto Nazionale di Geofisica Vulcanologia (INGV) participated by deploying land stations along two transects in the Apulia and Gargano Promontory to extend westwards the seismic profiles. The primary goal was to record shallow-to-deep seismic phases travelling along the transition between the Adriatic basin and the Apulia foreland. In this paper we present the field work related to the two Italian onshore transects, the recorded data, and the processing flow developed to highlight crustal and mantle refractions and wide-angle reflections.

The Ligurian Basin is situated at the transition from the western Alpine orogeny to the Apennine system, an area where a change in subduction polarity ('Ligurian Knot') is observed. The Back-arc basin was generated by the southeast trench retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to continental thinning and subsidence, oceanic spreading with unroofing of mantle material was proposed for the late period of opening 21-16 Ma. To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active and passive seismic data have been recorded on ocean bottom seismometers. A seismic long-term network comprising of 29 broad band stations was installed from June 2017 to February 2018 in the framework of SPP2017 4D-MB, the German component of AlpArray. Refraction seismic profiles were shot to serve two aspects: (1) Determine the orientation of the horizontal components of the long-term ocean bottom seismometers and (2) estimate the velocity distribution of the upper lithosphere, to provide a better earth velocity model for the analysis of the passive seismic data. The ambient noise technique is applied to compute surface wave phase velocity maps of Rayleigh waves for the Ligurian Basin. Good quality data have been recorded, regional and teleseismic events could be detected by stations of the network. Active shots have been recorded by all stations on the seafloor during shooting. The majority of the refraction seismic data show mantle phases at offsets up to 70 km. The crust mantle boundary in the central basin is observed at ~9 km depth below seafloor. The mantle shows rather high velocities > 7.8 km/s.

SUMMARYWe infer seismic azimuthal anisotropy from ambient-noise-derived Rayleigh waves in the wider Vienna Basin region. Cross-correlations of the ambient seismic field are computed for 1953 station pairs and periods from 5 to 25 s to measure the directional dependence of interstation Rayleigh-wave group velocities. We perform the analysis for each period on the whole data set, as well as in overlapping 2°-cells to regionalize the measurements, to study expected effects from isotropic structure, and isotropic–anisotropic trade-offs. To extract azimuthal anisotropy that relates to the anisotropic structure of the Earth, we analyse the group velocity residuals after isotropic inversion. The periods discussed in this study (5–20 s) are sensitive to crustal structure, and they allow us to gain insight into two distinct mechanisms that result in fast orientations. At shallow crustal depths, fast orientations in the Eastern Alps are S/N to SSW/NNE, roughly normal to the Alps. This effect is most likely due to the formation of cracks aligned with the present-day stress-field. At greater depths, fast orientations rotate towards NE, almost parallel to the major fault systems that accommodated the lateral extrusion of blocks in the Miocene. This is coherent with the alignment of crystal grains during crustal deformation occurring along the fault systems and the lateral extrusion of the central part of the Eastern Alps.

The submarine Istanbul-Silivri fault segment, within 15 km of Istanbul, is the only portion of the North Anatolian Fault that has not ruptured in the last 250 years. We report first results of a seafloor acoustic ranging experiment to quantify current horizontal deformation along this segment and assess whether the segment is creeping aseismically or accumulating stress to be released in a future event. Ten transponders were installed to monitor length variations along 15 baselines. A joint least squares inversion for across-fault baseline changes, accounting for sound speed drift at each transponder, precludes fault displacement rates larger than a few millimeters per year during the 6 month observation period. Forward modeling shows that the data better fit a locked state or a very moderate surface creep—less than 6 mm/yr compared to a far-field slip rate of over 20 mm/yr—suggesting that the fault segment is currently accumulating stress.

Powerful subduction zone earthquakes rupture thousands of square kilometres along continental margins but at certain locations earthquake rupture terminates. To date, detailed knowledge of the parameters that govern seismic rupture and aftershocks is still incomplete. On 2015 September 16, the Mw 8.3 Illapel earthquake ruptured a 200 km long stretch of the Central Chilean subduction zone, triggering a tsunami and causing significant damage. Here, we analyse the temporal and spatial pattern of the coseismic rupture and aftershocks in relation to the tectonic setting in the earthquake area. Aftershocks cluster around the area of maximum coseismic slip, in particular in lateral and downdip direction. During the first 24 hr after the main shock, aftershocks migrated in both lateral directions with velocities of approximately 2.5 and 5 km hr−1. At the southern rupture boundary, aftershocks cluster around individual subducted seamounts that are related to the downthrusting Juan Fernández Ridge. In the northern part of the rupture area, aftershocks separate into an upper cluster (above 25 km depth) and a lower cluster (below 35 km depth). This dual seismic–aseismic transition in downdip direction is also observed in the interseismic period suggesting that it may represent a persistent feature for the Central Chilean subduction zone.