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Coastal and ocean island volcanoes are renowned for having unstable flanks. This can lead to flank deformation on a variety of temporal and spatial scales ranging from slow creep to catastrophic sector collapse. A large section of these unstable flanks is often below sea level, where information on the volcano-tectonic structure and ground deformation is limited. Consequently, kinematic models that attempt to explain measured ground deformation onshore associated with flank instability are poorly constrained in the offshore area. Here, we attempt to determine the locations and the morpho-tectonic structures of the boundaries of the submerged unstable southeastern flank of Mount Etna (Italy). The integration of new marine data (bathymetry, microbathymetry, offshore seismicity, reflection seismic lines) and published marine data (bathymetry, seafloor geodesy, reflection seismic lines) allows identifying the lineament north of Catania Canyon as the southern lateral boundary with a high level of confidence. The northern and the distal (seaward) boundaries are less clear because no microbathymetric or seafloor geodetic data are available. Hypotheses for their locations are presented. Geophysical imaging suggests that the offshore Timpe Fault System is a shallow second-order structure that likely results from extensional deformation within the moving flank. Evidence for active uplift and compression upslope of the amphitheater-shaped depression from seismic data along with subsidence of the onshore Giarre Wedge block observed in ground deformation data leads us to propose that this block is a rotational slump, which moves on top of the large-scale instability. The new shoreline-crossing structural assessment may now inform and improve kinematic models.

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.

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 microeathquakes 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 hours 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.

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 investigatethe 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 avoidartifacts from the complex 3D velocity structure of the basin. Moho-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.

At the Australian-Pacific plate boundary, the northern Lau Basin is one of the fastest opening back-arc basins on earth. The current configuration of micro-plates, plate boundaries and motions within the northern Lau Basin is quite well understood, but in the southern part of the Lau Basin questions remain about the crustal structure. Here, the Central Lau Spreading Center (CLSC) and the southern tip of the Fonualei Rift and Spreading Center (FRSC) define the diffuse southern boundary of the Niuafo’ou microplate. It remains unclear where the southern plate boundary is located and what kind of boundary it is.We present 1) seismic refraction data of a 200-km long, E-W transect acquired in the transition zone from the eastern side of the CLSC to the southern tip of the FRSC and 2) seismic reflection data of four E-W profiles of varying length, acquired in both the southern part of the Niuafo’ou microplate and the transition in between the CLSC and the FRSC. The seismic data acquisition was accompanied by parametric sediment echosounder, gravimetric and magnetic measurements and was complemented by heat flow probes and dredged samples of the seafloor in the vicinity of the profile.Our travel time tomography reveals a pronounced lateral variation in seismic P-wave velocities from west to east, within the 7-8 km thick back-arc crust. Towards the east, the crust gradually thickens to 13 km of arc crust. The reflection seismic data reveals sediment pockets, varying between 300m to 1000m depth, located on both the thinner back-arc crust and thicker arc crust. In the abyssal regions, faults that cross-cut the basement, but do not reach the surface, are observed on all reflection seismic profiles and are considered inactive today. Towards the west of the profiles, faults reach the surface and are considered active. Rock sampling from this area retrieved predominantly massive aphyric basalts from the back-arc crust in the west. Olivine-rich basalts, andesites, and a broad spect

On April 1st, the Mw 8.1 Iquique earthquake ruptured the plate boundary in the North Chilean marine forearc. The earthquake did not cause enough shallow rupture in the updip area to trigger a significant tsunami in the Pacific Ocean. The segment of South American subduction-zone affected by the Iquique earthquake was previously considered a seismic gap, as it last ruptured entirely in 1877 during an M~9 earthquake, with a rupture area possibly extending from south of Arica at 19°S to the north of the Mejillones peninsula at 23°S. The smaller magnitude of the 2014 event and the reduced rupture area that only broke the central part of the seismic gap implies a seismo-tectonic segmentation of the marine forearc. To identify the structure of the marine forearc and the subduction plate interface at the shallow seismic/aseismic transition, we use two years of Ocean Bottom Seismometer (OBS) and permanent onshore seismic data recorded eight months after the Iquique mainshock. The high-resolution aftershocks of the 2014 Mw 8.1 Iquique earthquake from the local OBS network together with a seismic reflection image acquired in 2016 of the marine forearc allows us to discuss the structural control on the up-dip extent of seismic rupture in the North Chilean subduction-zone. Most aftershocks occur updip of a co-seismic shallow slip patch in the marine forearc in the vicinity of a large crustal normal fault. A second area of increased aftershocks at the plate interface coincidence with the crustal-scale normal fault that links to the coastal scarp. The updip limit under the marine forearc further correlates with a highly faulted upper-plate and the subduction of Iquique ridge related topography. Combining the results concerning the 2014 Iquique earthquake with information from the 1995 Antofagasta and 2007 Tocopilla earthquakes, we suggest a uniform structural control of seismic rupture that is uniformly active along the Northern Chile.

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

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.

The northern Lau Basin, at the Australian-Pacific plate boundary, is one of the fastest opening back-arc basins on earth. An amalgamation of active rifts and spreading centers accommodates the extension. The current configuration of micro-plates, motions and plate-boundaries within the northern Lau Basin has been studied, but remains complex. Especially in the southern part of the Lau Basin questions remain about the crustal structure. In this area, the Central Lau Spreading Center (CLSC) and the southern tip of the Fonualei Rift and Spreading Center (FRSC) define the diffuse southern boundary of the Niuafo’ou microplate. It remains unclear where the southern plate boundary is located and what kind of boundary it is. We present seismic refraction and reflection data of a 200-km long transect in the transition zone from the eastern side of the CLSC to the southern tip of the FRSC. The seismic data recording was accompanied by parametric sediment echosounder data, gravimetric and magnetic measurements and dredged samples of the seafloor in the vicinity of the profile. A travel time tomography reveals a pronounced lateral variation in seismic P-wave velocities from west to east, within the 7-8 km thick back-arc-crust. Towards the east the crust gradually thickens to 13 km of arc-crust. The reflection seismic data reveals sediment pockets that vary between 300m to 1000m depth and are located on both the thinner back-arc crust and thicker arc-crust. Rock sampling along the transect retrieved predominantly massive aphyric basalts from the back-arc-crust in the west. Ol-Px-Pl-phyric basalts, andesites, and a broad spectrum of volcaniclastic rocks are the most common rock-type collected from the arc-crust in the east. These rocks are currently analyzed to determine the age and geochemical (major, trace element and Sr-Nd-Pb-Hf isotopic) composition of the sampled structures. The lack of a thinner crust near the southern tip of the FRSC is located and a wide distribution of n

Young extension centers in the proximity of back-rolling subduction zones constitute an ideal natural laboratory to investigate the early evolutional stages of back-arc basins. We present a catalog of 711 microearthquakes recorded during a 32-days deployment of ocean bottom seismometers at the southern part of the Fonualei Rift and Spreading Center in the Lau Basin. The majority of epicenters are concentrated along the central region of the axial valley and about 450 events are associated with an earthquake swarm that is likely linked to magmatic diking in the crust. The earthquake swarm location coincides with several volcanic mounds on the seafloor, covered by fresh lava flows. The spatial distribution of microearthquakes and the maximum depth of brittle faulting is more similar to oceanic spreading centers than to continental rift zones. We interpret our results in conjunction with refraction seismic data and magnetic anomaly data from the same region. In combing all findings, we conclude that the southern Fonualei Rift and Spreading Center is currently in transition from a rifting dominated mode of extension to a seafloor spreading dominated mode of extension. This transition may be associated with fundamental changes in the underlying mechanisms of melt generation leading to a diminishing influence of hydrous flux melting and the growing influence of decompression melting at this back-arc extension zone.

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.

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 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 opening period, 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 of a long-term network consisting of 29 broad-band stations, installed from June 2017 to February 2018 in the framework of SPP2017 4D-MB, the German component of AlpArray. Two refraction seismic profiles were shot to serve two aspects: (1) Determine the orientation of the horizontal components of the long-term instruments and (2) estimate the velocity distribution of the upper lithosphere, to provide a velocity model for the passive seismic data analysis. Good quality data have been recorded, regional and teleseismic events as well as active shots could be detected by the network stations. The majority of the refraction seismic data show mantle phases at offsets up to 70 km and a very prominent wide-angle reflection originating at the crust mantle boundary. Its features share a number of characteristics (i.e. offset range, continuity) generally associated with continental settings rather than mimicking seafloor spreading lithosphere emplaced in back-arc basins. Based on traveltime tomography along the refraction lines, the crust-mantle boundary is determined at ~9.5 km depth below seafloor. The acoustic basement is difficult to map seismically. The transition to the crystalline basement is indicated at a depth of ~6.5 km below seafloor. The absolute seismic velocities can be interpreted as hyper-extended cont

Several aspects of the formation and rifting of crust in backarc environments, like the Lau Basin in the SW Pacific, are still under question. A major unresolved question is, at what stage in the structural and thermal evolution of arc rifting does magmatism begin? The Fonualei Rift and Spreading Centre in Northeastern Lau Basin separates the Tonga Volcanic Arc from the Lau Backarc Basin and shows a strong gradient in the opening rate from South to North. The rift represents an ideal study site to investigate the formation and rifting of backarc crust at different stages of its tectonic and magmatic evolution. During the multidisciplinary cruise SO267 of RV Sonne (project ARCHIMEDES I), which sailed from Dec. 11, 2018 to Jan. 26, 2019, coincident seismic refraction and reflection profiles were acquired to image the deep crustal structure covering different sections of the Fonualei Rift and Spreading Centre. A total of 50 OBS from the GEOMAR pool were available for the refraction lines. Instruments were spaced at an average distance of 6 km and recorded arrivals from up 120 km offset. Seismic phases show little to no sediment cover on the seabed. Clear mantle arrivals (PmP and Pn) were recorded by the majority of stations and will allow the assessment of crustal thickness and upper mantle velocities. Additional geophysical data included gravity, magnetics and high-resolution bathymetry as well as Parasound and heatflow data, which were acquired along all profiles and will contribute to the tectonic interpretation of this complex region that is characterized by a number of microplates, active and abandoned spreading centres and transfer zones. This contribution will present an overview of the major scientific questions driving the ARCHIMEDES I project and present first results from the acquired seismic profiles, which aim to reveal the crustal architecture and the opening history of the Lau Basin.

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.

Within the last decade, satellite-based geodetic techniques such as GPS or InSAR have increased our knowledge about tectonic plate motions and crustal deformation. However, the electromagnetic waves used by these techniques do not penetrate into water and therefore geodetic information for the offshore domain is sparse. The GeoSEA (Geodetic Earthquake Observatory on the SEAfloor) array uses acoustic ranging techniques between stations on the seafloor for relative positioning. We use data from 23 autonomous acoustic transponders installed in three deployments located on the outer rise and on the marine forearc of the northern Chile subduction zone (~21°S). The networks are located immediately south of the Iquique 2014 Mw 8.1 rupture zone. Stations were deployed in December 2015 and were mostly recording until June 2018. The geodetic networks monitor crustal deformation of the seafloor with the target to quantify interseismic deformation. Although we achieve a mm-scale precision with our acoustic ranging system, we observe no significant deformation above our resolution limits. We compare our observations with surface strain models estimated using onshore cGPS stations of the Plate Boundary Observatory Chile (IPOC). The predicted strains are of a similar order of magnitude compared to the strain resolution of the network. Furthermore, we show pressure data for all three networks. Although the resolution of the pressure sensors is in the cm-range, the data do not reveal any sudden vertical movements of the seafloor, indicating absence of vertical crustal movement.