Quaternary active tectonic structures in the offshore Bajo Segura basin (SE Iberian Peninsula Mediterranean Sea).

The Bajo Segura fault zone (BSFZ) is the northern terminal splay of the Eastern Betic shear zone (EBSZ), a large left-lateral strike-slip fault system of sigmoid geometry stretching more than 450 km from Alicante to Almería. The BSFZ extends from the onshore Bajo Segura basin further into the Mediterranean Sea and shows a moderate instrumental seismic activity characterized by small earthquakes. Nevertheless, the zone was affected by large historical earthquakes of which the largest was the 1829 Torrevieja earthquake (IEMS98 X). The onshore area of the BSFZ is marked by active transpressive structures (faults and folds), whereas the offshore area has been scarcely explored from the tectonic point of view. During the EVENT-SHELF cruise, a total of 10 high-resolution single-channel seismic sparker profiles were obtained along and across the offshore Bajo Segura basin. Analysis of these profiles resulted in (a) the identification of 6 Quaternary seismo-stratigraphic units bounded by five horizons corresponding to regional erosional surfaces related to global sea level lowstands; and (b) the mapping of the active sub-seafloor structures and their correlation with those described onshore. Moreover, the results suggest that the Bajo Segura blind thrust fault or the Torrevieja left-lateral strike-slip fault, with prolongation offshore, could be considered as the source of the 1829 Torrevieja earthquake. These data improve our understanding of present deformation along the BSFZ and provide new insights into the seismic hazard in the area

Acoustic and seismic imaging of the Adra Fault (NE Alboran Sea): in search of the source of the 1910 Adra earthquake

13 pages, 7 figures, 1 tableRecently acquired swath-bathymetry data and high-resolution seismic reflection profiles offshore Adra (Almería, Spain) reveal the surficial expression of a NW–SE trending 20 km-long fault, which we termed the Adra Fault. Seismic imaging across the structure depicts a sub-vertical fault reaching the seafloor surface and slightly dipping to the NE showing an along-axis structural variability. Our new data suggest normal displacement of the uppermost units with probably a lateral component. Radiocarbon dating of a gravity core located in the area indicates that seafloor sediments are of Holocene age, suggesting present-day tectonic activity. The NE Alboran Sea area is characterized by significant low-magnitude earthquakes and by historical records of moderate magnitude, such as the Mw = 6.1 1910 Adra Earthquake. The location, dimension and kinematics of the Adra Fault agree with the fault solution and magnitude of the 1910 Adra Earthquake, whose moment tensor analysis indicates normal-dextral motion. The fault seismic parameters indicate that the Adra Fault is a potential source of large magnitude (Mw ≤ 6.5) earthquakes, which represents an unreported seismic hazard for the neighbouring coastal areasThe authors acknowledge the support of the Spanish Ministry of Science and Innovation (MICINN) through National Projects IMPULS (REN2003-05996MAR), EVENT (CGL2006-12861-C02-02) and SHAKE (CGL2011-30005-C02-02); Acciones Complementarias EVENT-SHELF (CTM 2008-03346-E/MAR) and SPARKER (CTM 2008-03208-E/MAR) and the ESF TopoEurope TOPOMED project (CGL2008-03474-E/BTE). We thank the captain, crew, scientific party and technical staff of the UTM-CSIC on board the Spanish R/V “Hesperides” and R/V “García del Cid” during the IMPULS and EVENT-SHELF cruises, respectively. We thank Fabrizio Pepe (Univ. Palermo) for his assistance in using the Geotrace software.We thank guest editor Hans Nelson, Carlos Marín Lechado and an anonymous referee for constructive comments and reviews. This work has been carried out within Grup de Recerca de la Generalitat de Catalunya B-CSI (2009 SGR 146)Peer reviewe

Active tectonics and paleoseismological studies on the offshore Bajo Segura Basin faults (SE Iberian Peninsula - Mediterranean Sea)

The Deep-sea & Sub-Seafloor Frontiers Conference project (DS3F), 11-14 March 2012, Sitges, Barcelona, SpainBackground and Aims: The Neogene and Quaternary faulting activity in the SE Iberian Margin is dominated by a large left-lateral strike-slip system, the Eastern Betic Shear Zone (EBSZ) that stretches over more than 450 km (De Larouzière et al., 1988). The northern terminal splay of the EBSZ correspond to the Bajo Segura fault zone (BSFZ) that extends further into the Mediterranean Sea (Alfaro et al., 2002). The instrumental seismicity around this fault zone is mainly characterized by small to moderate earthquakes. Even though, moderate to large historical earthquakes have affected the zone, being the Torrevieja earthquake (1829; IMSK=X) the largest. Objective and methods: In September 2008 the marine geophysical cruise EVENT-SHELF was carried out (Spanish RV Garcia del Cid). The main goal was to map the sub-seafloor active structures (faults and folds) of the offshore area of BSFZ using high-resolution seismics (Spaker GeoSpark 6kJ) in order to characterize their Quaternary activity. A total of 10 regional profiles were acquired along and across the BSFZ with 30 cm of resolution at 1.5 km water depth with 400 m of penetration below seabed (Perea et al., submitted). Results: The results from the analysis of the high resolution seismic profiles have showed the presence of a serial of structures (faults and folds) in the sub-seafloor that are related to the structures described on the onshore. Moreover, these structures are active since they are deforming Quaternary units, in some locations the younger discontinuity (aprox. 20 ka) and, even, the seafloor. To characterize the seismogenic parameters of the active faults we have calculated the maximum magnitude earthquake and the recurrence interval of events using empirical relationships (Wesnousky, 1986; Wells and Coppersmith, 1999). Nevertheless, given the high resolution of the seismic profiles, it has been possible to perform a marine paleoseismological analysis in some faults allowing to identify individual seismic events and therefore obtain the time passed between specific events and the time elapsed since the last one. Conclusions: The active structures (faults and folds) described on the onshore Bajo Segura basin propagates towards the Mediterranean sea between 10 t0 60 km. Most of these structures are active since they are deforming Quaternary units and it has been possible to characterize their seismogenic behaviour. In some seismic profiles and given their high resolution, specific earthquakes and their time interval have been identified for some faultsPeer Reviewe

Quaternary active tectonic structures in the offshore Bajo Segura basin (SE Iberian Peninsula Mediterranean Sea).

The Bajo Segura fault zone (BSFZ) is the northern terminal splay of the Eastern Betic shear zone (EBSZ), a large left-lateral strike-slip fault system of sigmoid geometry stretching more than 450 km from Alicante to Almería. The BSFZ extends from the onshore Bajo Segura basin further into the Mediterranean Sea and shows a moderate instrumental seismic activity characterized by small earthquakes. Nevertheless, the zone was affected by large historical earthquakes of which the largest was the 1829 Torrevieja earthquake (IEMS98 X). The onshore area of the BSFZ is marked by active transpressive structures (faults and folds), whereas the offshore area has been scarcely explored from the tectonic point of view. During the EVENT-SHELF cruise, a total of 10 high-resolution single-channel seismic sparker profiles were obtained along and across the offshore Bajo Segura basin. Analysis of these profiles resulted in (a) the identification of 6 Quaternary seismo-stratigraphic units bounded by five horizons corresponding to regional erosional surfaces related to global sea level lowstands; and (b) the mapping of the active sub-seafloor structures and their correlation with those described onshore. Moreover, the results suggest that the Bajo Segura blind thrust fault or the Torrevieja left-lateral strike-slip fault, with prolongation offshore, could be considered as the source of the 1829 Torrevieja earthquake. These data improve our understanding of present deformation along the BSFZ and provide new insights into the seismic hazard in the area