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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 Poster presentation 1942<br />

Sedimentological imprint of past earthquakes in the Algerian margin from<br />

slump and turbidite record (Maradja Project)<br />

Dr. Nathalie Babonneau<br />

IUEM - Universit de Brest UMR 6538 Domaines Ocaniques<br />

Nathalie Babonneau, Antonio Cattaneo, Jacques Dverchre, Bruno Savoye, Karim<br />

Yelles, Gabriela Dan, Anne Domzig, Pierre Giresse, Rachid Matougui, Bernard<br />

Merci De Lpinay, Henri Pauc, Virginie Gaullier, Faouzi Djadid<br />

As shown by the 2003 M 6.9 Boumerdes earthquake, northern is affected by moderate to large seismic<br />

activity damaging the Algerian coastal cities. Seismic activity results from the convergence motion<br />

between African and European plates. It occurs through large earthquakes, activating fault segments<br />

partly located offshore and causing important effects on the stability of the sediments on the Algerian<br />

continental slope. Sedimentological and geophysical data acquired offshore during the Maradja cruises<br />

(2003 and 2005) allow to identify possible Quaternary sediment instabilities generated by seismic<br />

events. The submarine slope morphology is characterised by deep canyons, indicating high efficient<br />

sediment transport by gravity processes, from the shelf break to the deep basin. Morphological and<br />

sedimentological studies of the margin suggest that several types of gravity deposits with a possible<br />

seismic origin exist. Slumps and debris flow deposits appear on multibeam-based seafloor morphology.<br />

They are mainly located at the foot of the continental slope and on steep canyon flanks, and are<br />

sometimes linked to subsurface fault motion. These sedimentary structures have a typical size of a few<br />

kms. Using detailed sonar imagery (SAR), high resolution seismics and core data, we made a first<br />

estimation of the age and distribution of slumps and debris flows. The link with seismic events is difficult<br />

to build, since deposits have limited spatial extent and rare vertical superposition. For the moment, a<br />

consistent chronostratigraphic framework cannot be established for these structures and prevents us to<br />

obtain event recurrence intervals. Large turbidity currents flowing offshore may well represent the result<br />

of sediment destabilisation induced by seismic activity, as testified by direct recording of catastrophic<br />

mass flows immediately following some of the most destructive historical earthquakes. During the 1954<br />

Orlansville, the 1980 El-Asnam and the 2003 Bourmerdes earthquakes, numerous deep-sea<br />

communication cables were broken by turbidity currents directly triggered by seismic events. Currents,<br />

probably initiated in canyon heads, propagated as far as 100 km and more from the Algerian coast into<br />

the basin. These events are recorded as turbidite deposits in the abyssal plain and in levees of turbidite<br />

channels. A detailed sedimentological and morphological study of the margin (especially in the Algiers<br />

area) is performed in order to estimate the location of the initial instabilities in the canyons, the paths of<br />

active sediment transport and the main flow characteristics of the turbidity currents. Using this<br />

knowledge of transport and sedimentation processes, we establish a coring strategy for a cruise<br />

programmed in August 2007 in order to attribute an age to the largest deep turbidite sequences and<br />

estimate the recurrence time of these events in the different structural segments of the margin. This<br />

approach will help to gain important informations on the recurrence of great earthquakes along the<br />

Algerian margin in the last 10,000 years and on their actual imprint on the seafloor.<br />

Keywords: paleoseismicity, algerian margin, slope instability

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