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Geophysical sensing in the open ocean is both costly and technically challenging. Here we developed a novel distributed fiber optic sensing technique that employs microwave modulation for phase measurement in signals returned from submarine repeaters. We transformed a trans‐Atlantic telecom cable into an 81‐sensor array and measured sub‐millihertz strains. The strains correlate with ocean tide height variations in phase, suggesting a dominant factor of the cable's Poisson's effect. Large strains observed at fiber spans located in the shallow water match the strong variations of simulated seafloor temperature. This study presents the first experimental confirmation of detecting sub‐millihertz signals using trans‐oceanic distributed sensing with submarine cables at span‐wise spatial resolution (∼80 km), opening the potential for cost‐efficient tsunami early warning and long‐term ocean temperature monitoring compatible with active data‐carrying fibers.

Earthquake focal mechanisms provide critical in-situ insights about the subsurface faulting geometry and stress state. For frequent small earthquakes (magnitude< 3.5), their focal mechanisms are routinely determined using first-arrival polarities picked on the vertical component of seismometers. Nevertheless, their quality is usually limited by the azimuthal coverage of the local seismic network. The emerging distributed acoustic sensing (DAS) technology, which can convert pre-existing telecommunication cables into arrays of strain/strain-rate meters, can potentially fill the azimuthal gap and enhance constraints on the nodal plane orientation through its long sensing range and dense spatial sampling. However, determining first-arrival polarities on DAS is challenging due to its single-component sensing and low signal-to-noise ratio for direct body waves. Here, we present a data-driven method that measures P-wave polarities on a DAS array based on cross-correlations between earthquake pairs. We validate the inferred polarities using the regional network catalog on two DAS arrays, deployed in California and each comprising ~ 5000 channels. We demonstrate that a joint focal mechanism inversion combining conventional and DAS polarity picks improves the accuracy and reduces the uncertainty in the focal plane orientation. Our results highlight the significant potential of integrating DAS with conventional networks for investigating high-resolution earthquake source mechanisms. Determining earthquake faulting orientation is essential for understanding earthquake-stress interactions. Here, the authors present a technique that leverages telecom fiber optic cables to improve the estimation of this fundamental parameter.

Underwater communication cables transport large amounts of sensitive information between countries. This fact converts these cables into a critical infrastructure that must be protected. Monitoring the underwater cable environment is rare and any intervention is usually driven by cable faults. In the last few years, several reports raised issues about possible future malicious attacks on such cables. The main objective of this operational research and analysis (ORA) paper is to present an overview of different commercial and already available marine sensor technologies (acoustic, optic, magnetic and oceanographic) that could be used for autonomous monitoring of the underwater cable environment. These sensors could be mounted on different autonomous platforms, such as unmanned surface vehicles (USVs) or autonomous underwater vehicles (AUVs). This paper analyses a multi-threat sabotage scenario where surveying a transatlantic cable of 13,000 km, (reaching water depths up to 4000 m) is necessary. The potential underwater threats identified for such a scenario are: divers, anchors, fishing trawls, submarines, remotely operated vehicles (ROVs) and AUVs. The paper discusses the capabilities of the identified sensors to detect such identified threats for the scenario under study. It also presents ideas on the construction of periodic and permanent surveillance networks. Research study and results are focused on providing useful information to decision-makers in charge of designing surveillance capabilities to secure underwater communication cables.

Turbidity currents carve the deepest canyons on Earth, deposit its largest sediment accumulations, and break seabed telecommunication cables. Powerful canyon‐flushing turbidity currents break sensors placed in their path, making them notoriously challenging to measure, and thus poorly understood. This study provides the first remote measurements of canyon‐flushing flows, using ocean‐bottom seismographs located outside the flow's destructive path, revolutionizing flow monitoring. We recorded the internal dynamics of the longest sediment flows yet monitored on Earth, which traveled >1,000 km down the Congo Canyon‐Channel at 3.7–7.6 m s−1 and lasted >3 weeks. These observations allow us to test fundamental models for turbidity current behavior and reveal that flows contain dense and fast frontal‐zones up to ∼400 km in length. These frontal‐zones developed near‐uniform durations and speeds for hundreds of kilometres despite substantial seabed erosion, enabling flows to rapidly transport prodigious volumes of organic carbon, sediment, and warm water to the deep‐sea. Plain Language Summary Seafloor avalanches of sediment, called turbidity currents, transport huge volumes of sediment and organic carbon to the deep‐sea, and they break critical seabed telecommunication cables that underpin global data transfer. However, turbidity currents are very difficult to measure directly as they often damage sensors placed in their flow path, so they are poorly understood. Here we show that turbidity currents generate ground vibrations that can be measured using ocean‐bottom seismographs placed outside the flow's destructive path, revolutionizing flow monitoring. These seismographs recorded the longest sediment flows yet measured in action on Earth, which traveled >1,000 km along the submarine Congo Canyon‐Channel offshore West Africa. We use these observations to test fundamental models of turbidity current flow behavior. Our measurements show that the front of the flows contain a fast frontal‐zone with high sediment concentrations, which can be up to ∼400 km long, whilst the whole duration of the flow can last for more than 3 weeks. These frontal‐zones develop near‐uniform durations and speeds, despite extensive seabed erosion that adds sediment into the flow. New information on flow durations shows how turbidity currents rapidly deliver prodigious volumes of organic carbon, sediment, and warm water to the deep‐ocean floor. Key Points Remote seismic monitoring reveals the duration, internal structure, and evolution of powerful canyon‐flushing turbidity currents Flows contain dense and fast frontal‐zones (up to ∼400 km long) that maintain uniform durations and speeds despite huge seabed erosion Canyon‐flushing flow frontal‐zones can bring substantial fluxes of organic carbon, sediment, and warm water to the deep‐sea in <24 hr

Lead exposure reduction has been one of the most successful environmental health campaigns ever undertaken. However, for the target goal of \"zero\" micrograms of lead in blood to be met, it will be necessary to go beyond controlling lead-based paint, lead water lines, and consumer products and to seek out and reduce other overlooked exposures. This paper explores one such source--lead exposure from aerial terrestrial and submarine lead sheathed communications cables (LSCCs). LSCCs were widely used from the late 1800s to the 1960s and ultimately replaced by plastics and other synthetic coverings. The cables were suspended on utility poles and when going under water bodies, bundled together as submarine cables. In both aerial and submarine LSCCs, the metallic sheen of new cables quickly oxidizes to a dull lead oxide coating that prevents further corrosion. This coating may be friable and released as cables sway and may be subject to various weather conditions. The objective of this investigation was to conduct a preliminary assessment of the extent of soil contamination from submarine and aerial LSCCs and to assess potential risks to children's health.

Monitoring of ocean temperatures is crucial for climate studies, ocean circulation modeling and assessing potential ecosystem impacts. However, obtaining observations from the subsurface ocean is difficult and costly. We present a 3‐year time series of distributed fiber optic sensing (sensitive to temperature changes and strain) obtained at 3 to 6‐month intervals, using a network of telecommunication cables in Guadeloupe (Lesser Antilles). We demonstrate that this technique can track seasonal and annual seafloor water temperature changes to within 0.1°C, in a well‐mixed, shelf sea environment. We observe a marine heatwave, with a temperature increase of +1.5°C between 2022 and 2024 at the sea‐floor, causing coral bleaching with 30% mortality. These trends are confirmed by satellite observations of the Sea Surface Temperature in the same location. This successful demonstration, in a shallow‐water environment, opens the path forward for widespread use of submarine cables for long‐term environmental monitoring of the seafloor.

The attenuation of ocean surface waves during seasonal ice cover is an important control on the evolution of Arctic coastlines. The spatial and temporal variations in this process have been challenging to resolve with conventional sampling using sparse arrays of moorings or buoys. We demonstrate a novel method for persistent observation of wave‐ice interactions using distributed acoustic sensing (DAS) along existing seafloor fiber optic telecommunications cables. DAS measurements span a 36‐km cross‐shore cable on the Beaufort Shelf from Oliktok Point, Alaska. DAS optical sensing of fiber strain‐rate provides a proxy for seafloor pressure, which we calibrate with wave buoy measurements during the ice‐free season (August 2022). We apply this calibration during the ice formation season (November 2021) to obtain unprecedented resolution of variable wave attenuation rates in new, partial ice cover. The location and strength of wave attenuation serve as proxies for ice coverage and thickness, especially during rapidly evolving events. Plain Language Summary Coasts globally are susceptible to erosion by ocean waves. In the Arctic, sea ice near the coast can serve as protection for much of the year. It is particularly challenging to measure waves and ice in this environment, which is necessary to understand the degree of buffering and project future changes. Typical ways of observing waves (e.g., buoys and underwater moorings) have lower success in coastal ice. We show a new way to observe waves and ice in these coastal regions using cables at the seabed deployed for internet connection. With the use of an instrument called an interrogator on shore, fibers in these cables can act like a series of hundreds of wave buoys. This allows us to see that waves are reduced at a variable rate throughout the ice. There are significant opportunities to learn more about the coastal Arctic using this novel technology and method. Key Points Seafloor fiber optic cables can be used to quantify surface waves in seasonally sea ice‐covered oceans High spatial‐resolution wave observations may be used to study wave attenuation in ice at much finer resolution than previously possible The rapid evolution of the location and strength of attenuation serves as proxy for the evolution of ice itself

The sequestration of organic carbon in seafloor sediments plays a key role in regulating global climate; however, human activities can disturb previously-sequestered carbon stocks, potentially reducing the capacity of the ocean to store CO 2 . Recent studies revealed profound seafloor impacts and sedimentary carbon loss due to fishing and shipping, yet most other human activities in the ocean have been overlooked. Here, we present an assessment of organic carbon disturbance related to the globally-extensive subsea telecommunications cable network. Up to 2.82–11.26 Mt of organic carbon worldwide has been disturbed as a result of cable burial, in water depths of up to 2000 m. While orders of magnitude lower than that disturbed by bottom fishing, it is a non-trivial amount that is absent from global budgets. Future offshore developments that disturb the seafloor should consider the safeguarding of carbon stocks, across the full spectrum of Blue Economy industries. The sequestration of organic carbon in seafloor sediments plays a key role in regulating global climate. Here, the authors present an assessment of organic carbon disturbance related to the globally-extensive subsea telecommunications cable network.

Submarine mass wasting in continental margins poses a significant threat to offshore installations, submarine communication cables, and coastal communities due to tsunamis. Based on the analysis of time-lapse geophysical data, we report a very recent submarine slope failure and associated mass transport deposit (MTD) in the Krishna-Godavari (KG) basin, the Bay of Bengal that occurred between January 2009 and December 2015. The head scarp shows erosion of ~ 160 m thick sedimentary strata with an estimated volume of ~ 11 km3. The fan-shaped MTD, located at water depths of 950 to 1100 m, shows a spatial coverage of ~ 70 km2 (volume is ~ 2.2 km3) with a maximum thickness of ~ 60 m. This is one of the largest submarine slope failures reported from time-lapsed geophysical surveys. We analyzed potential triggers such as floods, earthquakes, and cyclones that could have caused the slope failure. We believe that Cyclone Helen, a category-1 storm, may have contributed to the observed slumping in unstable shelf/slope regions of the KG Basin in November 2013 as the Cyclone eye traversed over the head of the slump. However, we cannot rule out other factors such as the extreme flooding events (2010 and 2013) and a 6.0 magnitude earthquake in the Bay of Bengal in May 2014. The study shows that pre-conditioning of sediments is an important factor in the assessment of deepwater geohazards, and areas with relatively moderate external triggers are also at high risk.