Resúmenes de la 1ª Reunión Ibérica sobre Fallas Activas y Paleosismología, Sigüenza, España (2010) CHARACTERIZING ACTIVE FAULTS AND ASSOCIATED MASS TRANSPORT DEPOSITS IN THE SOUTH IBERIAN MARGIN (ALBORAN SEA AND GULF OF CADIZ): ON-FAULT AND OFF-FAULT PALEOSEISMIC EVIDENCE Caracterización de fallas activas y deslizamientos asociados en el Margen Sur de Iberia (Mar de Alboran y Golfo de Cadiz): Evidencias paleosísmicas E. Gràcia (1), R. Bartolomé (1), C. Lo Iacono (1), X. Moreno (1,2), S. Martínez-Loriente (1), H. Perea (3), E. Masana (2), R. Pallàs (2), S. Diez (1), J. Dañobeitia (1), P. Terrinha (4) and N. Zitellini (5) (1) Unitat de Tecnologia Marina - CSIC, CMIMA, Pg. Marítim de la Barceloneta, 37-49, 08003 Barcelona, Spain egracia@cmima.csic.es (2) RISKNAT, Dpt. Geodinàmica i Geofísica, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain (3) Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1746-016 Lisboa, Portugal (4) LNEG- Laboratorio Nacional de Geologia e Energia, Unidade de Geologia Marinha, Aptd 7586, 2720-866 Amadora, Portugal (5) Istituto di Scienze Marine, (ISMAR) - CNR, Bologna, Italy Abstract: During the last years we have carried out successive high-resolution marine geological and geophysical surveys to investigate the seismic potential of the slow-moving seismogenic faults from the South Iberian Margin. Based on multiscale acoustic mapping, sub-seafloor seismic imaging and dating methods we have characterized submarine fault systems with unprecedented resolution. We present primary paleoseismic evidence obtained by direct investigations of selected faults from the Mediterranean Sea (i.e. Carboneras Fault, Bajo Segura Fault, Adra Fault) and the external part of the Gulf of Cadiz (i.e. Marques de Pombal Fault, Horseshoe Fault, Coral Patch Ridge Fault and SWIM lineaments). The obtained fault seismic parameters suggest that these faults are active and capable of generating large magnitude (Mw > 6) seismic events, representing an earthquake and tsunami hazard for the surrounding coastal areas. Secondary paleoseismic evidence in the SW Iberian Margin (based on landslides and turbidite deposits) yields a regional recurrence rate of large magnitude earthquakes of 1800 – 2000 years. Key words: Active faulting, submarine paleoseismology, recurrence interval, earthquakes and tsunamis. INTRODUCTION Crustal deformation in the south Iberian margin, which includes the offshore Atlantic and Mediterranean regions, is driven mainly by the NW– SE convergence (4-5 mm/year) between the African and Eurasian plates (e.g. Argus et al., 1989). Convergence is accommodated over a wide active deformation zone suggesting distributed deformation among a number of tectonic structures. Earthquake mechanisms reveal that reverse to strike-slip faulting style dominates along the eastern Alboran Sea and external Gulf of Cadiz (e.g. Buforn et al., 2004; Stich et al., 2003, 2005), as corroborated by marine geological and geophysical data (e.g. Comas et al., 1999; Gràcia et al., 2003a,b, 2006; Terrinha et al., 2003, 2009; Zitellini et al., 2004, 2009). Regional seismicity is characterized by shallow to deep earthquakes of low to moderate magnitude (Mw < 5.5) (Buforn et al., 2004; Stich et al., 2003, 2005). However, large and destructive earthquakes (Mw ≥ 8.0 and MSK Intensity X-XI) such as the 1755 Lisbon Earthquake, have also occurred in the region (e.g. Martínez Solares and López Arroyo, 2004), and may represent a significant earthquake and tsunami hazard along the Iberian Peninsula and North African coasts. In south Iberia, assessment of seismic hazard is largely based on the relatively short period of instrumental and historical earthquake catalogues. This may not be sufficient to assess seismic hazard models in the Iberian Peninsula, especially when considering high-magnitude earthquakes with long recurrence intervals (> 10 3 years) (e.g. Masana et al., 2004; Gràcia et al., 2010). To investigate the seismic potential of the slow- moving seismogenic faults of the south Iberian Margin we have carried out several marine geophysical surveys in the Alboran Sea and Gulf of Cadiz in the frame of National (IMPULS and EVENT) and European projects (ESF EuroMargins SWIM and EU FP-VI NEAREST). We present our main outcomes focusing into two aspects: a) the characterization of active and seismogenic submarine faults of the southern Iberian Margin aiming to find their paleoseismic parameters, and b) the identification and age of landslides and turbidite deposits as secondary paleoseismic evidence to obtain the recurrence rate of large magnitude (Mw > 6) earthquakes occurred in the area. DATA AND METHODS State of the art methodologies integrate the most advanced tools in marine geosciences covering different scales of resolution. Acoustic mapping techniques, such as swath-bathymetry and sidescan sonar data, allow identifying the geomorphic evidence of active faults, such as seafloor ruptures, folds, fault scarps and fault traces. Sub-seafloor seismic imaging methods, ranging from high- resolution sub-bottom profiler (uppermost tens of meters of penetration) to multichannel seismic data (several km of penetration) allow to detect the stratigraphic evidence of past seismic activity, such as displaced seismic horizons, folded and faulted reflectors, zones of shearing and discontinuities. Pre- stack depth migration of MCS profiles yield a 163 brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by Repositório do LNEG