For millions of years, a slow and mostly undetectable process has been taking place off the southwest coast of Portugal, where the Atlantic deepens and the water takes on a specific dark blue hue that indicates great depth rather than just distance from shore. Here, the African and Eurasian plates are coming together and rubbing against one another along one of the world’s most intricate geological borders. On top of it is the Gulf of Cadiz. Furthermore, until recently, there weren’t enough tools available to study what was going on in the water column and below the seafloor to get the whole picture at once.
That is being changed by a dataset that researchers from the University of Porto and the Instituto Superior Técnico in Lisbon published in Scientific Data in October 2025. The team produced a comprehensive seismic oceanography dataset covering 869 kilometers of Portugal’s southern and southwestern margins using eight multichannel seismic reflection sections that were first collected between June and August 2001 for energy exploration and then reprocessed.
Hydrophones are used to record the reflected signals after an airgun towed by a research vessel fires acoustic pulses. The trick is that the same acoustic data reveals the structure of the water column itself as well as the geology beneath the seafloor, particularly the boundaries between water masses with varying salinities and temperatures that reflect sound in quantifiable ways.
What that dual perspective is demonstrating to researchers is very important. The Mediterranean Undercurrent is not a passive presence in the Gulf; it is warm, extremely salty water that flows through the Strait of Gibraltar and mixes with cooler Atlantic water to descend to depths of 800 to 1200 meters. The intricate bathymetry of features like the Portimão Canyon channels and shapes it as it closely follows the seafloor. It erodes as it travels. Sediment becomes unstable as a result. Meddies, or circular eddies of trapped Mediterranean water, are created by it and carry their unique chemical signatures as they drift out into the North Atlantic. Because traditional probes and moorings are too sparsely distributed to provide a continuous view at the sub-mesoscale, the seismic data is capturing these structures at spatial resolutions that conventional oceanographic measurement just cannot.
| Field | Details |
|---|---|
| Region | Gulf of Cadiz, Southwestern Iberian Peninsula (Southern Portugal / SW Spain) |
| Tectonic Context | Part of the Azores-Gibraltar plate boundary (Africa-Eurasia convergence zone) |
| Research Published | October 21, 2025 — Scientific Data (Springer Nature) |
| Lead Authors | Ana F. Duarte (Instituto Superior Técnico, Lisbon), Renato Mendes (University of Porto), Leonardo Azevedo (IST Lisbon) |
| Dataset Coverage | 869.39 km of multichannel seismic reflection sections |
| Data Acquisition Period | June–August 2001 (reprocessed for seismic oceanography) |
| Technique | Multichannel seismic reflection (MCS) — imaging water column and subsurface simultaneously |
| Key Oceanographic Feature | Mediterranean Undercurrent (MU) — warm, salty water from Strait of Gibraltar, descending to 800–1200m |
| Meddies | Mediterranean water eddies trapping warm, saline water — mapped using seismic data |
| Portimão Canyon | Key bathymetric feature shaping MU flow and sediment erosion |
| Hazards Identified | Pockmarks, slope failures, gas-charged sediments covering 240+ km² of upper slope |
| Historical Earthquake Reference | M~8.7 Great Lisbon Earthquake (1755); M=7.9 event of February 28, 1969 |
| Micro-seismicity Location | Predominantly upper mantle; crustal faults show low to negligible seismic activity |
| Key Seismicity Research | Silva et al. (2017), AWI/Alfred Wegener Institute collaboration |
| Crustal Feature Identified | Jurassic oceanic crust; thin-skinned crustal thrusts rooting in sub-horizontal décollements |
| Serpentinization Significance | May allow aseismic slip between large events; ruptures may nucleate in lithospheric mantle |
| Tsunami Risk | Active seismic faulting + sedimentary slope instability both identified as primary triggers |
Reading this research gives me the impression that the Gulf of Cadiz has been studied in fragments for decades without anyone being able to put the whole picture together at once. A map of the fault structures can be found below. A sample of the water dynamics was taken from above. By placing both images on the same frame at the same time, seismic oceanography creates a consequential relationship between them. In addition to shaping the ocean, strong bottom currents interacting with unstable seafloor topography may have an impact on the timing and manner of fault line failure.
This is not an abstract context for seismic hazards. The M~8.7 Great Lisbon Earthquake of 1755, one of the deadliest natural disasters in European history, occurred along the same plate boundary as the Gulf of Cadiz. Within hours, the tsunami from the earthquake reached the coasts of Portugal, Spain, Morocco, and even the British Isles. The majority of seismic activity originates in the upper mantle rather than the crustal faults that are visible in bathymetric surveys, according to research on the region’s micro-seismicity, most notably a 2017 study by Silva and colleagues using 25 ocean bottom seismometers. Until a significant rupture spreads upward from the mantle through serpentinized rock layers, the crustal thrust faults appear to be relatively quiet in between events, either slipping aseismically or remaining completely locked.
This area is genuinely challenging to evaluate because of the difference between visible and active structures. Over 240 square kilometers of the upper slope of the offshore Gulf of Cadiz slope exhibit signs of subsurface instability, including pockmarks, slope failures, and gas-charged sediments. These are more than geological oddities. Regardless of the magnitude of the seismic event, one known mechanism for producing tsunamis in this area is submarine landslides brought on by earthquakes. The precise relationship between fault stability and the erosive action of the Mediterranean Undercurrent over geological timescales is still unknown. However, researchers now have a continuous, high-resolution baseline to work from thanks to the availability of seismic oceanography data, which is more than they had previously.
