Tsunami wave observations far from the coast remain challenging due to the logistics and cost of deploying and operating offshore instrumentation on a long-term basis with sufficient spatial coverage and density. Distributed Acoustic Sensing (DAS) on submarine fiber optic cables now enables real-time seafloor strain observations over distances exceeding 100 km at a relatively low cost. Here, we evaluate the potential contribution of DAS to tsunami warning by assessing theoretically the sensitivity required by a DAS instrument to record tsunami waves. Our analysis includes signals due to two effects induced by the hydrostatic pressure perturbations arising from tsunami waves: the Poisson’s effect of the submarine cable and the compliance effect of the seafloor. It also includes the effect of seafloor shear stresses and temperature transients induced by the horizontal fluid flow associated with tsunami waves. The analysis is supported by fully coupled 3-D physics-based simulations of earthquake rupture, seismo-acoustic waves and tsunami wave propagation. The strains from seismo-acoustic waves and static deformation near the earthquake source are orders of magnitude larger than the tsunami strain signal. We illustrate a data processing procedure to discern the tsunami signal. With enhanced low-frequency sensitivity on DAS interrogators (strain sensitivity ≈ 2×10−10 at mHz frequencies), we find that, on seafloor cables located above or near the earthquake source area, tsunamis are expected to be observable with a sufficient signal-to-noise ratio within a few minutes of the earthquake onset. These encouraging results pave the way towards faster tsunami warning enabled by seafloor DAS.

As tsunamis propagate across open oceans, they remain largely unseen due to the lack of adequate sensors, hence limiting the scope of existing tsunami warnings. A potential alternative method relies on the Global Navigation Satellites Systems to monitor the ionosphere for Traveling Ionospheric Disturbances created by tsunami-induced internal gravity waves (IGWs). The approach has been applied to tsunamis generated by earthquakes but rarely by undersea volcanic eruptions injecting energy into both the ocean and the atmosphere. The large 2022 Hunga Tonga-Hunga Ha’apai volcanic eruption tsunami is thus a challenge for tsunami ionospheric imprint detection. Here, we show that in near-field regions (<1500km), despite the complex wavefield, we can isolate the tsunami imprint. We also highlight that the eruption-generated Lamb wave’s ionospheric imprints show an arrival time and an amplitude spatial pattern consistent with internal gravity wave origin.

Distributed Acoustic Sensing (DAS) is a photonics technology converting seafloor telecommunications and optical fiber cables into dense arrays of strain sensors, allowing to monitor various oceanic physical processes. Yet, several applications are hindered by the limited knowledge of the transfer function between geophysical variables and DAS measurements. This study investigates the quantitative relationship between surface gravity DAS-recorded wave-generated strain signals along the seafloor and the pressure at a colocated sensor. A remarkable linear correlation is found over various sea conditions allowing to reliably determine significant wave heights from DAS data. Utilizing linear wave potential theory, we derive an analytical transfer function linking cable deformation and wave kinematic parameters. This transfer function provides a first quantification of the effects related to waves and fiber responses. Our results validate DAS’s potential for real-time reconstruction of the surface gravity wave spectrum over extended coastal areas. It also enables the estimation of waves hydraulic parameters at depth without the need of offshore deployments.

The novel technique of distributed acoustic sensing (DAS) holds great potential for underwater seismology by transforming standard telecommunication cables, such as those currently traversing most of the world’s oceans, into dense arrays of seismo-acoustic sensors. To harness these measurements for seismic monitoring, the ability to record transient ground deformations using telecommunication fibers is investigated here by analyzing ambient noise, earthquake signals, and their associated phase velocities, on DAS records from three dark fibers in the Mediterranean Sea. The recording quality varies dramatically along the fibers and is strongly correlated with the bathymetry and the apparent phase velocities of the recorded waves. Apparent velocities are determined for several well-recorded earthquakes and used to convert DAS S-wave strain spectra to ground motion spectra. Excellent agreement is found between the spectra of nearby underwater and on-land seismometers and DAS converted spectra, when the latter are corrected for site effects. Apparent velocities greatly affect the ability to detect seismic deformations: for the same ground motions, slower waves induce higher strains and thus are more favorably detected than fast waves. The effect of apparent velocity on the ability to detect seismic phases, quantified by expected signal-to-noise ratios, is investigated by comparing signal amplitudes predicted by an earthquake ground motion model to recorded noise levels. DAS detection capabilities on underwater fibers are found to be similar to those of nearby broadband sensors, and superior to those of on-land fiber segments. The results demonstrate the great potential of underwater DAS for seismic monitoring and earthquake early warning.