ESONET: Iberian Margin

The Gulf of Cadiz / Iberian margin is a region of complexity with the junction of the Eurasian and African plates resulting in doming of the sea floor, mud volcanoes and other complex features. The interaction of the Mediterranean outflow with Atlantic waters is significant.

Southwest Portugal, the Gulf of Cadiz and Morocco are prone to earthquake and tsunami as testified by the great 1755 Lisbon earthquake and tsunami. This event was the most catastrophic earthquake that ever occurred in historical times, Western Europe. With an estimated magnitude 8.5-9.0, this event generated anomalous sea waves that struck the coast of Portugal, Spain and Morocco and were observed all over the North Atlantic, as far as Great Britain, Finland and the Caribbean Sea. It caused severe destruction in Lisbon, Tanger and Casablanca. Most of the seismic activity is due to Europe - Africa plate convergence and it occurs at sea, along the continental margin of SW Iberia and in the Gulf of Cadiz. Due to lack of permanent seismic stations at sea the seismic activity is not properly monitored. This fact prevents either the early detection of the eventual tsunamis waves, either the precise location of low magnitude earthquakes, which are key data to understand the present stress behaviour of the margin.

Extensive mud volcanism, pockmarks, mud diapirism and carbonate chimneys related to hydrocarbon rich fluid venting are been recently observed throughout the Gulf of Cadiz. The Gulf of Cadiz is also the site to investigate of the Mediterranean Outflow Water (MOW) because it affects the deep-water circulation on global scale.

South West Iberian Margin, details of earthquakes in this region.

The main objective for the Iberian region is to realise a seismic monitoring network in the Gulf of Cadiz, thereby providing an in-depth knowledge of the seismic activity of the area and a capability in early detection of tsunami. This proposal follows the path of several projects, funded by the European Commission during the last years, for the earthquake/tsunami risk assessments of the area as BIGSETS (Big Sources of Earthquake and Tsunami in SW Iberia) and GITEC. From an oceanographic point of view the Gulf of Cadiz is of great importance for the study of the Mediterranean Outflow Water (MOW), which affects global deep-water circulation. An additional objective in this region is to monitor the temporal variation of the warm (13°) and saline (>37 g/l) MOW. The MOW flows out from the strait of Gibraltar and spreads in the Gulf of Cadiz at depth of 800-1200 m with two main branches. One branch diverge northward, toward the Bay of Biscay, the other crosses the North Atlantic reaching the Labrador and Norwegian – Greenland seas after 20-30 years. The station will allow, through continuous measurements over the years, the correct assessment of the salinification and warming of MOW and the study of its inter and intra-annual variability in relation to atmospheric forcing. Measurements of MOW in the Gulf of Cadiz may anticipate climate change at the scale of tens of years.

The proposed Iberian Margin network of observatories.

The location for the main deep-sea long-term observatory is at about 100 Km SW Cape San Vicente at water depth of 3000-4000m. This observatory will be equipped with broad band seismometers with control on seismometer tilt and orientation, pressure transducer, magnetometer, gravimeter and will be integrated with an oceanographic mooring with acoustic current meters, transmissiometers and T/P sensors and it will be integrated with the network of seismic station present onshore, in Portugal and Spain. The selected site owns the following characteristics: it is easily and rapidly reachable in any season; it presents key features for the scientific/monitoring targets; it is suitable for investigations on various scientific/monitoring targets; it offers safe operating conditions for the deployed instruments. Along the cable pathway it is planned to install additional sensor for the measurements of the MOW which main branches turns just South of Cape San Vicente at water depth 800-1000 m. Future expansion of the main station are planned for the continuous monitoring of the fluid venting occurring at seafloor and the biological community associated to it. Precise location of the "fluid escape" site will be defined after completion of the on going high resolution bathymetric mapping of the area. The planned deep sea observatory will be starting point for both real time warning network and long term seismic observation, to recover important measurements of the tsunami generation critical parameters, to grant long term oceanographic data, to monitor geochemical and physical parameter of the fluid escape on the seafloor and the biological community associated to it.

3D Map of the proposed Iberian Margin network of observatories.