Active tectonics in old oceanic lithosphere off SW Iberia based on pre-stack depth migrated mcs profiles (SWIM-2006 Survey)

Abstract

In 2006 a Multichannel Seismic (MCS) survey was performed offshore the SW Iberian Margin, in the framework of the ESF-EuroMargins SWIM marine cruise. The main objective was to characterize the geometry, deep structure and timing of deformation of the active faults located at the westernmost Gulf of Cadiz. This region present significant seismicity as a consequence of the convergence (about 4-5 mm/yr) between the African and Eurasian Plates, and has been the site of several large magnitude earthquakes, such as the 1755 Lisbon Earthquake (Mw 8.5) and the 1969 Horseshoe Earthquake (Mw 8.0). A total of sixteen MCS profiles together with complementary swath-bathymetry and sub-bottom profiler data were acquired on board the Spanish RV Hesperides, totalizing more than 2700 km of data. The seismic source was an array of 8 airguns with a volume of 1050 c.i. We used an analogical Teledyne streamer with 2.4 km of active section, formed by 96 channels (25 m separation) and towed 7 m depth. MCS data were processed using PROMAX software, and 9 of the profiles were pre-stack depth migrated (PSDM) using velocity models built with the SIRIUS software. In contrast to time-sections, PSDM images display the real depth geometry of the structures, essential for a complete stratigraphic and tectonic interpretation, as well as to calculate fault seismic parameters of specific structures. The SWIM-2006 dataset reveal E-W trending dextral strike-slip faults showing surface deformation of flower-like structures predominate in the Horseshoe Abyssal Plain, whereas NE-SW trending compressive structures prevail in the Coral Patch Ridge and Seine Hills. The evaluation of the seismic potential of these active faults based on the calculation of their seismic parameters reveals that the newly mapped faults correspond to active blind thrusts and strike-slip faults that are able to generate large magnitude earthquakes (Mw 7.2–8.4)