Multiscale Secondary Microseism Propagation Analysis Using Full Waveform Modelling and DAS Observations offshore Catania, Sicily

Ocean generated low-frequency seismic noise signals called microseisms are linked to the ocean environment, the subsurface and the atmosphere. The energy associated with microseisms is closely related to ocean wave amplitude, globally and throughout the seasons, and shows great potential in the analysis of climate variability.

To comprehend the generation and propagation mechanisms of secondary microseisms offshore Sicily, a detailed analysis of Distributed Acoustic Sensing (DAS) data is being conducted on the MEOC fibre optic cable of the INFN-LNS submarine infrastructure offshore Catania. DAS technology exploits the backscattering properties of fiber optic cables which enables data acquisition over large distances of cables acting as a densely distributed array for recording strain rate at the seafloor. The DAS data presented in this study was collected over five days (October 10–15, 2020) and focuses on the first 20 km section of the MEOC cable.

The appearance of pronounced energy observed in both the DAS data and nearby land seismometer within the secondary microseism band (~3s), persisting for a prolonged duration and coincident with a “storm” event, confirms the impact of regional weather conditions on microseism generation. This specific time window is analysed in detail to explore the propagation effects of the secondary microseism wavefield arriving on the cable. Hindcast data from the WAVEWATCH III ocean wave model identifies a secondary microseism source location south of Sicily, generated in response to high winds from northwest to southeast. The 20 km long array with a channel spacing of 2 m  enables, through effective FK (frequency-wavenumber) domain analysis, a detailed examination of the spatial variability of the wavefield arriving at the cable. The FK analysis plots show almost equal energy on both positive and negative wavenumbers with a slight dominance in seaward propagation directions. A simple forward seismo-acoustic simulation performed to see the effects of bathymetry corroborates  the results obtained from the DAS observations. The simulations demonstrate that while the wavefield reaches the cable at a normal angle, a dominant seaward propagation is observed towards the cable's end due to the wavefront getting redirected from the continental shelf. Multiple simulations with different source locations are discussed in order to further understand the influence of the source position as well as regional bathymetry on the wavefield recorded with the DAS. These findings highlight the complex interplay between the role bathymetric features in microseism propagation and dominant microseism source location. Spatially dense DAS data recorded over long distances can play a key role in  unravelling these dynamics.