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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.

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.

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.

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.

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.

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.

Highlights • We identify Volcano F as the source of the August 2019 pumice raft in Tonga. • Satellite and seismic data give constraints on the timing of the submarine eruption. • 2.5–12.3*106 m3 estimated eruption volume, corresponding to VEI 2–3. • First report of the morphology and geology of Volcano F. Abstract In August 2019 a large raft of pumice appeared in the territorial waters of Tonga. As in many other cases, this pumice raft was the only surface expression of a major submarine volcanic eruption. Discolored water and reconstruction of the drift path of the pumice raft using satellite imagery points towards ‘Volcano F’ in the Tofua Arc NW of the island of Vava’u as the most likely volcanic source. Here we present imagery from ESA’s Sentinel-2 satellite that captured the start of the submarine eruption on 6 August 2019 and the waning of the eruption on 8 August, followed by observations of the drifting pumice raft until 14 August. This start time is consistent with T-phase records at the seismic stations on Niue Island and Rarotonga and the signal delay time of 733 s between the two stations is consistent with an origin at or at least near Volcano F. On 8 August, a >136.7 km2 large raft of pumice appears at the sea surface. The modelled minimum raft volume is 8.2–41.0*106 m3, which is equivalent to 2.5–12.3*106 m3 dense rock. The eruption thus corresponds to a volcanic explosivity index (VEI) 2–3 eruption in the submarine environment. Prior to the volcanic eruption, a series of earthquakes close to Volcano F was recorded. The series started on 5 August with a Mb 4.7 event, followed by at least six shallow earthquakes (Mb >3.9) on 6 August. In December 2018 and January 2019, we surveyed the seafloor around Volcano F with multibeam sonar. Combining our data with pre-existing information, we present the first comprehensive bathymetric map of the volcanic edifice and its geologic setting. We show that Volcano F represents a major arc volcanic complex that is situated in an extensional setting. The basal diameter of the volcanic apron is >50 km with a large central, 8.7 x 6 km caldera with a floor at ∼700 m water depth. The top of the post-caldera constructional cone complex had a summit depth of 35 m below sea level in 2004. The volcano shows geochemical differences to the adjacent arc volcanoes on Fonualei and Late islands. The volcano’s pristine volcanic morphology and two documented eruptions (2001 and 2019) indicate a highly active volcanic system that warrants further scientific attention.

International audience; In the Ionian Sea (central Mediterranean) the slow convergence between Africa and Eurasia results in the formation of a narrow subduction zone. The nature of the crust of the subducting plate remains debated and could represent the last remnants of the Neo-Tethys ocean. The origin of the Ionian basin is also under discussion, especially concerning the rifting mechanisms as the Malta Escarpment could represent a remnant of this opening. This subduction retreats toward the southeast (motion occurring since the last 35 Ma) but is confined to the narrow Ionian basin. A major lateral slab tear fault is required to accommodate the slab roll-back. This fault is thought to propagate along the eastern Sicily margin but its precise location remains controversial. This study focuses on the deep crustal structure of the eastern Sicily margin and the Malta Escarpment. We present two two-dimensional P wave velocity models obtained from forward modeling of wide-angle seismic data acquired onboard the R/V Meteor during the DIONYSUS cruise in 2014. The results image an oceanic crust within the Ionian basin as well as the deep structure of the Malta Escarpment, which presents characteristics of a transform margin. A deep and asymmetrical sedimentary basin is imaged south of the Messina strait and seems to have opened between the Calabrian and Peloritan continental terranes. The interpretation of the velocity models suggests that the tear fault is located east of the Malta Escarpment, along the Alfeo fault system.

18 pages, 9 figures.-- Access to the field data: Bathymetric data from German research cruises (e.g., R/V SONNE SO104, SO244) can be requested through the German Bundesamt für Seeschifffahrt und Hydrographie (BSH; http://www.bsh.de). The bathymetric data collected during R/V Marcus G. Langseth cruise MGL1610 are available from the Rolling Deck to Repository (R2R) Web site (http://www.rvdata.us/). Seismic reflection data from R/V SONNE cruise SO104 are stored at the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) and can be requested through the Geo‐Seas data portal (http://www.geo‐seas.eu/). Access to processed data: The following data used in this manuscript are available from the PANGAEA data archive (https://doi.pangaea.de/10.1594/PANGAEA.893034): (I) A gridded DEM (100 m) that combines the bathymetric data from R/V SONNE cruises SO104, SO244, and R/V Marcus Langseth cruise MGL1610. (II) A GeoTIFF that contains the backscatter information from the multibeam data collected during R/V SONNE cruise SO244, New multibeam bathymetry allows an unprecedented view of the tectonic regime and its along-strike heterogeneity of the North Chilean marine forearc and the oceanic Nazca Plate between 19 and 22.75°S. Combining bathymetric and backscatter information from the multibeam data with subbottom profiler and published and previously unpublished legacy seismic reflection lines, we derive a tectonic map. The new map reveals a middle and upper slope configuration dominated by pervasive extensional faulting, with some faults outlining a >500-km-long ridge that may represent the remnants of a Jurassic or pre-Jurassic magmatic arc. Lower slope deformation is more variable and includes slope-failures, normal faulting, re-entrant embayments, and NW-SE trending anticlines and synclines. This complex pattern likely results from the combination of subducting lower-plate topography, gravitational forearc collapse, and the accumulation of permanent deformation over multiple earthquake cycles. We find little evidence for widespread fluid seepage despite a highly faulted upper-plate. An explanation could be a lack of fluid sources due to the sediment starved nature of the trench and most of the upper-plate in vicinity of the hyperarid Atacama Desert. Changes in forearc architecture partly correlate to structural variations of the oceanic Nazca Plate, which is dominated by the spreading-related abyssal hill fabric and is regionally overprinted by the Iquique Ridge. The ridge collides with the forearc around 20–21°S. South of the ridge-forearc intersection, bending-related horst-and-grabens result in vertical seafloor offsets of hundreds of meters. To the north, plate-bending is accommodated by reactivation of the paleo-spreading fabric and new horst-and-grabens do not develop, R/V SONNE Cruises SO104 (grant 03G104A) and SO244 (grant 03G0244A) were supported by the German Bundesministerium für Bildung und Forschung (BMBF). A. M. T. thanks the U.S. National Science Foundation (NSF) for support through grant OCE1459368. We also thank the crew and technical staff of the R/V Marcus Langseth, which is funded by the NSF and operated by the Lamont‐Doherty Earth Observatory. […] Jacob Geersen was partly funded by a grant (CP1404) of the Cluster of Excellence 80 “The Future Ocean,” funded within the framework of the Excellence Initiative by Deutsche Forschungsgemeinschaft (DFG) on behalf of the German federal and state governments. C. R. R. was supported by the Grup de Recerca 2014SGR940 de la Generalitat de Catalunya and Spanish Ministry of Science grant PCIN‐2015‐053. E. C.‐R. gratefully acknowledges the support of the Chilean National Science Foundation (FONDECYT) grant 1170009 and the Programa de Investigación Asociativa: ANILLOS DE INVESTIGACIÓN EN CIENCIA Y TECNOLOGÍA, CONICYT, grant ACT172002, project “The interplay between subduction processes and natural disasters in Chile.”

Three active-source seismic refraction profiles are integrated with morphological and potential field data to place the first regional constraints on the structure of the Kermadec subduction zone. These observations are used to test contrasting tectonic models for an along-strike transition in margin structure previously known as the 32°S boundary. We use residual bathymetry to constrain the geometry of this boundary and propose the name Central Kermadec Discontinuity (CKD). North of the CKD, the buried Tonga Ridge occupies the fore-arc with VP 6.5–7.3 km s^-1 and residual free-air gravity anomalies constrain its latitudinal extent (north of 30.5°S), width (110 ± 20 km), and strike (~005° south of 25°S). South of the CKD the fore-arc is structurally homogeneous downdip with VP 5.7–7.3 km s^-1. In the Havre Trough back-arc, crustal thickness south of the CKD is 8–9 km, which is up to 4 km thinner than the northern Havre Trough and at least 1 km thinner than the southern Havre Trough. We suggest that the Eocene arc did not extend along the current length of the Tonga-Kermadec trench. The Eocene arc was originally connected to the Three Kings Ridge, and the CKD was likely formed during separation and easterly translation of an Eocene arc substrate during the early Oligocene. We suggest that the first-order crustal thickness variations along the Kermadec arc were inherited from before the Neogene and reflect Mesozoic crustal structure, the Cenozoic evolution of the Tonga-Kermadec-Hikurangi margin and along-strike variations in the duration of arc volcanism.

We present a detailed 3-D P-wave velocity model obtained by first-arrival travel-time tomography with seismic refraction data in the segment boundary of the Sumatra subduction zone across Simeulue Island, and an image of the top of the subducted oceanic crust extracted from depth-migrated multi-channel seismic reflection profiles. We have picked P-wave first arrivals of the air-gun source seismic data recorded by local networks of ocean-bottom seismometers, and inverted the travel-times for a 3-D velocity model of the subduction zone. This velocity model shows an anomalous zone of intermediate velocities between those of oceanic crust and mantle that is associated with raised topography on the top of the oceanic crust. We interpret this feature as a thickened crustal zone in the subducting plate with compositional and topographic variations, providing a primary control on the upper plate structure and on the segmentation of the 2004 and 2005 earthquake ruptures.

Highlights • We report on marine 3D Magnetotelluric study on Walvis Ridge • Derived 3D electrical resistivity model shows a large scale resistive zone, which we link to crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting • Smaller scale resistive region is attributed to magma ascent during rifting • Rift basin is identified by low resistivity region The Namibian continental margin marks the starting point of the Tristan da Cunha hotspot trail, the Walvis Ridge. This section of the volcanic southwestern African margin is therefore ideal to study the interaction of hotspot volcanism and rifting, which occurred in the late Jurassic/early Cretaceous. Offshore magnetotelluric data image electromagnetically the landfall of Walvis Ridge. Two large-scale high resistivity anomalies in the 3-D resistivity model indicate old magmatic intrusions related to hot-spot volcanism and rifting. The large-scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2-D offshore seismic profiles. One of the high resistivity anomalies (above 500 Ωm) has three arms of approximately 100 km width and 300 km to 400 km length at 120 degree angles in the lower crust. One of the arms stretches underneath Walvis Ridge. The shape is suggestive of crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting. A second, smaller anomaly of 50 km width underneath the continent ocean boundary may be attributed to magma ascent during rifting. We attribute a low resistivity anomaly east of the continent ocean boundary and south of Walvis Ridge to the presence of a rift basin that formed prior to the rifting.

The pre- and current collision of the Juan Fernandez Ridge with the central Chilean margin at 31°–33°S is characterized by large-scale crustal thinning and long-term subsidence of the submarine forearc caused by subduction erosion processes. Here, we study the structure of the central Chilean margin in the ridge–trench collision zone by using wide-angle and multichannel seismic data. The transition from the upper to middle continental slope is defined by a trenchward dipping normal scarp with variable offsets of 500–2000 m height. Beneath the scarp, the 2-D velocity–depth models show a prominent lateral velocity contrast of >1 s−1 that propagates deep into the continental crust defining a major lateral seismic discontinuity. The discontinuity is interpreted as the lithological contact between the subsided/collapsed outermost forearc (composed of eroded and highly fractured volcanic rocks) and the seaward part of the uplifted Coastal Cordillera (made of less fractured metamorphic/igneous rocks). Extensional faults are abundant in the collapsed outermost forearc, however, landward of the continental slope scarp, both extensional and compressional structures are observed along the uplifted continental shelf that forms part of the Coastal Cordillera. Particularly, at the landward flank of the Valparaiso Forearc Basin (32°–33.5°S), shallow crustal seismicity has been recorded in 2008–2009 forming a dense cluster of thrust events of Mw 4–5. The estimated hypocentres spatially correlate with the location of the fault scarp, and they highlight the upper part of the seismic crustal discontinuity.

Tomography is like a photograph that was taken by a camera with blurred and defective lenses that deform the shapes and colours of objects. Reporting quantitative parameters derived from tomographic inversion is not always adequate because tomographic results are often strongly biased. To quantify the results of tomographic inversion, we propose a forward modelling and tomographic inversion (FM&TI) approach that aims to find a more realistic solution than conventional tomographic inversion. The FM&TI scheme is based on the assumption that if two tomograms derived from the inversion of observed and synthetic data are identical, the synthetic structure may appear to be closer to the real unknown structure in the ground than the inversion result. However, the manual design of the synthetic velocity distribution is usually time-consuming and ambiguous. In this study, we propose an approach that automatically searches for a probabilistic model. In this approach, a synthetic model is iteratively updated while taking into account the bias of the model in previous stages of the FM&TI performance. Here, we present an example of synthetic modelling and real data processing for an active source refraction data set corresponding to a marine profile across the subduction zone in Chile at about 32°S latitude. A key feature of the model is a low-velocity channel above the subducted oceanic crust, which was defined in the synthetic model and expected in the real case. The conventional first arrival traveltime tomography was barely able to resolve this channel. However, after several iterations of the FM&TI modelling, we succeeded in reconstructing this channel clearly. In the paper, we briefly discuss the nature of this low-velocity subduction channel, and we compare the results with other studies.

Fluids entering the subduction zone are a key factor in the subduction process. They determine the onset of melting, weakening and changes in the dynamics and thermal structure of subduction zones and trigger earthquakes when being released from the subducting plate in a series of metamorphic processes. However, the amount of water carried into the subduction zone and its distribution are not well constrained by existing data and are subject of vigorous current research in SFB574 (Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters). Electromagnetic methods like magnetotellurics have been used widely to recognize fluid release and melt production through enhanced electrical conductivities. Here we present an image of the hydration and dehydration cycle down to 120 km depth in one setting derived by an onshore-offshore transect of magnetotelluric soundings in Costa Rica. An electrically conductive zone in the incoming plate outer rise is associated with sea water penetrating down extensional faults and cracks into the upper mantle possibly causing serpentinization. Along the downward subducting plate distinct conductive anomalies identify fluids from dehydration of sediments, crust and mantle. A conductivity anomaly at a depth of approx. 12 km and at a distance of 65 km from the trench is associated with a first major dehydration reaction of minerally-bound water. This is of importance in the context of mid-slope fluid seeps which are thought to significantly contribute to the recycling of minerally-bound water. The position of the conductivity anomaly correlates with geochemical and seismic evidence stating that mid-slope fluids are originated at >=12 km depth before rising up through deep faults to the seeps. The conductivity anomaly is therefore associated with a fluid accumulation feeding the mid-slope seeps. Another fluid accumulation is revealed by a conductivity anomaly at 20-30 km depth and a distance of approximately 30 km seaward from the volcanic arc. This lower crustal fluid accumulation could likely be caused by trapping of fluids released due to de-serpentinization processes or due to other mineral dehydration processes. While we are at the moment not able to attribute one specific process causing the anomaly based on electromagnetic data alone, this feature is however of fundamental importance. A comparison with other electromagnetic studies from subduction zones around the world reveal that such a conductivity anomaly is a global feature suggesting the presence of a global fluid sink. Based on very simplified assumptions we are able derive rough estimates for the amount of water being stored in the overriding plate. Relating seismic evidence as well as petrological results collected in the multi-disciplinary study on the Costa Rican subduction zone we introduce budget estimations for the water cycle in the subduction zone.

The region offshore Eastern Java represents one of the few places where the early stage of oceanic plateau subduction is occurring. We study the little investigated Roo Rise oceanic plateau on the Indian plate, subducting beneath Eurasia. The presence of the abnormal bathymetric features entering the trench has a strong effect on the evolution of the subduction system, and causes additional challenges on the assessment of geohazard risks. We present integrated results of a refraction/wide-angle reflection tomography, gravity modelling, and multichannel reflection seismic imaging using data acquired in 2006 south of Java near 113°E. The composite structural model reveals the previously unresolved deep geometry of the oceanic plateau and the subduction zone. The oceanic plateau crust is on average 15 km thick and covers an area of about 100 000 km2. Within our profile the Roo Rise crustal thickness ranges between 18 and 12 km. The upper oceanic crust shows high degree of fracturing, suggesting heavy faulting. The forearc crust has an average thickness of 14 km, with a sharp increase to 33 km towards Java, as revealed by gravity modelling. The complex geometry of the backstop suggests two possible models for the structural formation within this segment of the margin: either accumulation of the Roo Rise crustal fragments above the backstop or alternatively uplift of the backstop caused by basal accumulation of crustal fragments. The subducting plateau is affecting the stress field within the accretionary complex and the backstop edge, which favours the initiation of large, potentially tsunamogenic earthquakes such as the 1994 Mw= 7.8 tsunamogenic event.

Earthquake history shows that the Sunda subduction zone of the Indonesian margin produces great earthquakes offshore Sumatra, whereas earthquakes of comparable magnitude are lacking offshore Java and the Lesser Sunda islands. Morphological structures in multibeam bathymetric data across the forearc relate with the extent of the seismogenic zone. Its updip limit corresponds to the slope break, most distinct off Java and Lesser Sunda islands, where we find coincident narrow, uniform, continuous outer arc ridges. Their landward termination and a shallow upper plate mantle mark the downdip limit of the seismogenic zone. In contrast the outer arc ridges off Sumatra are wider and partly elevated above sea level forming the forearc islands. The downdip limit of the seismogenic zone coincides with a deeper upper plate mantle. Sunda Strait marks a transition zone between the Sumatra and Java margins. We find the differences along the Sunda margin, especially the wider extent of the seismogenic zone off Sumatra, producing larger earthquakes, to result from the interaction of different age and subduction direction of the oceanic plate. We attribute a major role to the sediment income and continental/oceanic upper plate nature of Sumatra/Java influencing the composition and deformation style along the forearc and subduction fault.

General solutions of inverse problems often can be obtained by introducing probability distributions to sample the model space. We have developed a simple approach to define an a priori space in a tomographic study and retrieve the velocity/depth posterior distribution by a Monte Carlo method. Utilizing a fitting routine designed for very low statistics to set up and analyze the obtained tomography results, we can statistically separate the velocity/depth model space derived from inverting seismic refraction data. A profile acquired in the Lesser Antilles subduction zone reveals the effectiveness of this approach. Resolution analysis of the structural heterogeneity includes a divergence analysis that can dissect long, wide-angle profiles for deep crust and upper mantle studies. The complete information of any parameterized physical system is contained in the a posteriori distribution. Methods for analyzing and displaying key properties of the a posteriori distributions of highly nonlinear inverse problems are therefore essential in the scope of any interpretation. It is possible to map velocity variations in their extent and structure by measuring the total as well as relative divergence of the velocity structure in the a posteriori space. We have applied the divergence analysis to a part of the transect where a backstop structure has been identified, and the method resolves shallow features and returns information concerning the confidence level of results. Assuming a relationship between forearc and backstop, we can obtain a structural image in accordance with previous interpretations. ©2010 Society of Exploration Geophysicists

Amplitudes and shapes of seismic patterns derived from tomographic images often are strongly biased with respect to real structures in the earth. In particular, tomography usually provides continuous velocity distributions, whereas major velocity changes in the earth often occur on first-order interfaces. We propose an approach that constructs a realistic structure of the earth that combines forward modeling and tomographic inversion (FM&TI). Using available a priori information, we first construct a synthetic model with realistic patterns. Then we compute synthetic times and invert them using the same tomographic code and the same parameters as in the case of observed data processing. We compare the reconstruction result with the tomographicimage of observed data inversion. If a discrepancy is observed, we correct the synthetic model and repeat the FM&TI process. After several trials, we obtain similar results of synthetic and observed data inversion. In this case, the derived synthetic model adequately represents the real structure of the earth. In a working scheme of this approach, we three authors used two different synthetic models with a realistic setup. One of us created models, but the other two performed the reconstruction with no knowledge of the models. We discovered that the synthetic models derived by FM&TI were closer to the true model than the tomographic inversion result. Our reconstruction results from modeling marine data acquired in the Musicians Seamount Province in the Pacific Ocean indicate the capacity and limitations of FM&TI.

Our analysis of new bathymetric data reveals six submarine landslides at the eastern Sunda margin between central Java and Sumba Island, Indonesia. Their volumes range between 1 km³ in the Java fore-arc basin up to 20 km³ at the trench off Sumba and Sumbawa. We estimate the potential hazard of each event by modeling the corresponding tsunami and its run-up on nearby coasts. Four slides are situated remarkably close to the epicenter of the 1977 tsunamigenic Sumba M w = 8.3 earthquake. However, comparison of documented tsunami run-up heights and arrival times with our modeling results neither allows us to confirm nor can we falsify the hypothesis that the earthquake triggered these submarine landslides.

Newly pre-stack depth-migrated seismic images resolve the structural details of the western Java forearc and plate interface. The structural segmentation of the forearc into discrete mechanical domains correlates with distinct deformation styles. Approximately 2/3 of the trench sediment fill is detached and incorporated into frontal prism imbricates, while the floor sequence is underthrust beneath the décollement. Western Java, however, differs markedly from margins such as Nankai or Barbados, where a uniform, continuous décollement reflector has been imaged. In our study area, the plate interface reveals a spatially irregular, nonlinear pattern characterized by the morphological relief of subducted seamounts and thicker than average patches of underthrust sediment. The underthrust sediment is associated with a low velocity zone as determined from wide-angle data. Active underplating is not resolved, but likely contributes to the uplift of the large bivergent wedge that constitutes the forearc high. Our profile is located 100 km west of the 2006 Java tsunami earthquake. The heterogeneous décollement zone regulates the friction behavior of the shallow subduction environment where the earthquake occurred. The alternating pattern of enhanced frictional contact zones associated with oceanic basement relief and weak material patches of underthrust sediment influences seismic coupling and possibly contributed to the heterogeneous slip distribution. Our seismic images resolve a steeply dipping splay fault, which originates at the décollement and terminates at the sea floor and which potentially contributes to tsunami generation during co-seismic activity.