Deep ocean current regime as inferred from OBS noise offshore SW Iberia

From September 2007 to August 2008, an ocean bottom seismometer (OBS) experiment took place offshore of Cape S. Vincent and in the Gulf of Cádiz (Geissler et al., 2010), within the framework of the EU-funded project NEAREST (Integrated observations from NEAR shore sourcES of Tsunamis: towards an early warning system). On board the Italian research vessel URANIA, 24 LOBSTER (OBS) from the German instrument pool for amphibian seismology (DEPAS) were deployed at water depths ranging from 1990m to 5100m.

OBSs are usually deployed for seismological investigations, but these objectives are impaired by noise resulting from the ocean environment. The deep ocean, where OBSs are generally deployed, was considered until the 1980s a relatively low-energy and quiescent depositional environment where deep water masses flow as relatively slow-moving tabular bodies and deposition is episodically interrupted by down-slope gravity-driven processes. Since the 1990s, it has been demonstrated that deep-water masses can exhibit relatively high speed and play a dominant depositional role in certain areas. “Bottom current” refers to deep water capable of eroding, transporting and depositing sediments along the seafloor.

 

The permanent low-frequency geostrophic flow regime around the Atlantic Iberian margin has several water masses flowing at different depths in the same or opposite directions. Two main water masses in SW Iberia have been identified for the deep ocean. The Lower Deep Water (LDW) is composed mainly of Antarctic Bottom Water (AABW) and flows regionally below 4000m depth across the abyssal plains. The second is the North Atlantic Deep Water (NADW), which flows in various directions between 1400–4000m depth. Oceanic gateways are essential in controlling water-mass exchange between the abyssal plains and bottom current speed flow and pathways. The deep-water currents capable of eroding, transporting and depositing sediments along the seafloor exhibit relatively high speed and play a dominant depositional role in certain areas when interacting with local seafloor irregularities like seamounts, scarps, ridges, etc.

We focus on OBS-recorded noise analysis in two frequency bands, the 1-10Hz (harmonic tremors) and the long-period (10s-60s) bands, in the seismometer’s horizontal Y and vertical Z components. We see a robust seismometer response to deep ocean currents modulated by tides during the flood and ebb tides through spring and neap tides, which impact the permanent low-frequency flow from AABW and NADW.

This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020). The NEAREST project was funded by EC (GOCE, contract 037110).