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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy<br />

(S) - <strong>IASPEI</strong> - International Association of Seismology and Physics of the Earth's<br />

Interior<br />

JSS006 Oral Presentation 1928<br />

Active faulting and holocene paleoseismic record offshore Portugal,<br />

Southwest Iberian Margin<br />

Dr. Eulalia Gracia<br />

Unitat de Tecnologia Marina CSIC - Barcelona <strong>IASPEI</strong><br />

Alexis Vizcaino, Rafael Bartolom, Alessandra Asioli, Carlota Escutia, Jordi Garcia<br />

Orellana, Pedro Terrinha, Antonio Villaseor, Susana Diez, Sara Martnez, Juanjo<br />

Daobeitia<br />

Crustal deformation in the southwestern margin of the Iberian Peninsulais controlled by the NW-SE<br />

convergence of the African and Eurasian Plates (4.5-5.6 mm/yr) at the eastern end of the Azores-<br />

Gibraltar zone. This convergence is accommodated through a wide deformation zone characterized by<br />

moderate magnitude seismicity although great earthquakes (Mw>8), such as the 1755 Lisbon<br />

Earthquake and Tsunami and the 1969 Horseshoe Earthquake, have also occurred in the region. A<br />

multidisciplinary geological and geophysical dataset acquired during the last 10 years in the course of<br />

successive marine surveys revealed a number of active NE-SW trending west-verging thrusts (e.g.<br />

Marques de Pombal, Sao Vicente, and Horseshoe faults) and WNW-ESE strike-slip faults located<br />

offshore southern . The recognition of deformed Quaternary units together with swarms of shallow<br />

seismicity associated with surface ruptures suggest that faults located beneath these ruptures are<br />

active. For instance, a detailed swath-bathymetry data together with new high-resolution multichannel<br />

seismic profiles crossing the external part of the Gulf of Cadiz evidence a 150 km long WNW-ESE dextral<br />

strike-slip fault reaching up to the seafloor. This newly mapped structure may be a plausible candidate<br />

for the 1969 Earthquake (Mw 8.0) which epicentre is located few km away in the Horseshoe Abyssal<br />

Plain agreeing with the calculated fault plane solution. Associated to the active faults, mass transport<br />

deposits and submarine landslides are commonplace. Textural, mineralogical and geochemical analyses<br />

of marine sediments collected in the area in eight different sites stretching from the Tagus to the<br />

Horseshoe Abyssal Plains revealed the presence of numerous turbidite intervals. Although a number of<br />

mechanisms may be suggested to account for turbidite triggering, in the case of the SW Iberian Margin,<br />

earthquakes are the most likely explanation for synchronous, widely-spaced distribution of turbidites<br />

during the Holocene, when sea level was relatively stable. A total of 9 widespread turbidite events have<br />

been recognized for the Holocene. Precise dating of the two recentmost events (E1 and E2) based on<br />

210Pb and 137Cs geochronology provides ages of 1971 3 AD and 1908 8 AD, respectively. These ages<br />

correspond to high-magnitude historical and instrumental earthquakes that occurred in the region: the<br />

1969 Horseshoe Earthquake (Mw 8.0) and the 1909 Benavente Earthquake (Mw 6.0). The following 14C<br />

calibrated ages of turbidite events (E3 to E5) correlate with important historical earthquakes and<br />

paleotsunami deposits from the Gulf of Cadiz area, such as AD 1755, BC 218, and 5200 BP. Considering<br />

also the deepest events (E6 to E9) ranging from 7000 BP to 10400 BP we obtain a recurrence interval of<br />

about 1500 yr for the Holocene. Such a good correlation between turbidites and instrumental and<br />

historical seismic events suggests that the turbidite record can be used as a paleoseismic indicator in a<br />

low-convergence rate margin, constituting a valuable tool for assessment of earthquake and tsunami<br />

hazard along the coasts of the Iberian Peninsula and North Africa.<br />

Keywords: marine paleoseismology, seismogenic faults, turbidite

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