Description: 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.

Description: The Mozambique Basin is one of the oldest extensional sedimentary basins developed along the eastern African margin. The basin hosts a continuous record of sediments since the Jurassic separation of Antarctica from Africa. The objectives of this study were to extend the regional stratigraphic framework north of the Zambezi Delta into the deep abyssal plains and review the early evolution of the Mozambique Basin using nine multi-channel seismic reflection profiles. We identify six major stratigraphic units that were deposited in Jurassic, Early Cretaceous, Late Cretaceous, Paleogene, Neogene and Quaternary times. Mesozoic sedimentation rates of 5-10 cm/kyr and 1-3 cm/kyr during the Paleogene are calculated in the deeper basin. The presence of shales in neighbouring wells on the shelf implies an euxinic environment in the rapidly subsiding basin until Early Cretaceous times. The Mesozoic sediments have a high seismic velocity that exceeds 4.5 km/s, except in a distinct Early Cretaceous low-velocity (3.7 km/s) zone that may indicate the presence of undercompacted, overpressured shales. In spite of the fact that the Zambezi catchment was much smaller in pre-Miocene times, the high Late Cretaceous sedimentation rates can be attributed to rapid denudation of the African continent after a major tectonic uplift episode at approximately 90 Ma. Increased sediment influx into the basin from the Zambezi in Late Cretaceous times resulted in the formation of an elongated submarine fan lobe into the Mozambique Channel north of Beira High. Strong north-south bottom currents commenced within the channel in Late Cretaceous times, forcing the aggradation of sediments on the southern flank of the lobe. In addition, we observe several current-controlled sediment deposits in the deeper basin that are influenced by north-south bottom currents. Low Paleogene sedimentation rates are attributed to a sediment-starved basin during a relative quiet tectonic phase onshore.