The State of the Art

The region off the SW Iberian Peninsula and the Gulf of Cadiz shows an important seismic activity and complicated tectonics. The area encompasses the plate boundary between the Eurasian and African plates which, in the Atlantic, runs from the Azores Islands to the Strait of Gibraltar. Along this line the relative motion is divergent in the Azores, transcurrent in the central segment and convergent East of the Tore-Madeira Rise until the Strait of Gibraltar Strait. In the easternmost sector diffuse compressional deformation is present as testified by scattered seismicity (Fig. 2). Topographic, geoid and gravity anomalies indicate significant deformation in a wide area, either in oceanic and in continental lithosphere. Around SW Iberia the regional stresses are of compressive nature, oriented in NNW-SSE to WNW-ESE direction (Fig. 3). The tectonics of the region is complicated by the presence of a complex ocean bottom topography and the offshore continuation of the inland active faults. The presence of intermediate depth earthquakes (between 30 and 80 km of depth) suggests a complex transition from oceanic to continental domain. Classic rigid plate tectonics theory do not fit this geodynamic regime and it is suggested that new plate boundaries are being nucleated with significant intraplate deformation.

During the Eighties and the Nineties a lot of effort has been devoted by Academic Institutions to understand the geodynamic evolution of the continental margins of the South-western European block. Multichannel seismic data collected in this area (Sartori et al., 1994; Torelli et al., 1997: AR92 lines in Fig. 1) gave a new insight of the tectonic structure of the South-western Iberian margin. One of these seismic lines (AR-9210 in Fig. 1) shows a large compressive structure located offshore Cape San Vincente (Portugal), called Marquês de Pombal, whose motion probably caused the famous 1755 earthquake (Zitellini et al., 1999). Following this discovery the European Community funded the project BIGSETS (Big Sources of Earthquakes and Tsunami in SW Iberia) and an MCS survey (multichannel seismic survey) has been carried out to better investigate this structure (BIGSETS lines in Fig. 1).

The BIGSETS survey confirmed that the Marquês de Pombal is the likely source of the 1755 event (Zitellini et al., 2001). The deformed area, associated to the emplacement of Marquês de Pombal structure, is at least 100 km long, 5 km wide, the maximum up-lift is of about 1100 m and is located where the thrust fault emerges at the surface with a strike of N20E. The BIGSETS survey has shown that beside the Marquês the Pombal, other active, compressive, tectonic structures of regional significance are present: the Horseshoe fault (HSF), the Guadalquivir Bank (GB) and a large hill of tectonic origin (A) (Fig. 1). Besides the Marquês the Pombal structure the other above mentioned tectonic structures are poorly known and the VOLTAIRE campaign was designed to investigate them. This new set of data will point to a re-evaluation of the potential tsunami hazards in this very highly populated area and can constitute a firm basis both for more elaborated geodynamic models in the whole region and to assess seismic and tsunami risk associated with the largest events in Western Europe and North-western Africa.

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Figure 1: Geographical area setting. Bathymetry contours every 500 m from GEBCO97 Digital Atlas at www.nbi.ac.uk. HSF = Horseshoe Fault; GB = Guadalquivir Bank; MP = Marquês de Pombal; A = submarine hill of tectonic origin. Also shown RIFANO92 and BIGSETS98 MCS lines.