Paleoseismology off Algeria

Collaborations extérieures: Gueorgui Ratzov, Françoise Courboulex (GéoAzur Nice), Antonio Cattaneo (Géosciences Marines, Ifremer), Roza Si Bachir, Karim Yelles (CRAAG Alger, Algérie), Rabah Bracene (Sonatrach Exploration, Boumerdès, Algérie), Maria-Angela Bassetti, Pierre Giresse (CEFREM, UPVD, Perpignan).

Marine paleoseismology is an emerging discipline aiming to identify the indirect signature of earthquakes from the analysis of turbiditic successions. Its first challenge is to establish a causal link between earthquakes and turbidites, that can be assessed by comparison with historical records and observed events such as submarine landslides (Cattaneo et al., 2010) and cable breaks (Cattaneo et al., 2012). The second challenge is to extend back in time the record of earthquakes beyond the limit of other methods in order to better constrain seismic cycles (e.g., Ratzov et al., 2015). Previous and ongoing studies show that the marine paleoseismological approach applies well to the Algerian margin both in the Kramis (Ratzov et al., 2015) and in the Algiers sectors (Babonneau et al., 2017) during the Holocene.

In the last years, we have performed a marine paleoseismological analysis of the turbiditic deposits in 3 distinct areas of the Algerian offshore (Fig. 1): Kramis (West, Ratzov et al., 2015); Algiers (Centre, Babonneau et al., 2017) and Jijel/Bejaia (East).
Sediment cores were collected during the cruises MARADJA 2003, MARADJA2/SAMRA 2005 (R/V Le Suroît) and PRISME 2007 (R/V L’Atalante). Core correlation is based on age (14C of planktonic foraminiferas from hemipelagites), bed-by-bed correlation, and market bed identification. Age models are obtained by Oxcal.

Figure 1: Location map of the Algerian margin with bathymetry (isobaths every 500 m) and main historical earthquakes (red circles). The 3 frames locate the 3 areas investigated using turbidite paleoseismology by our research team. Redrawn from Babonneau et al. (2017).

Our main results are the following:
– Marine paleoseismology appears to be a suitable approach in the moderate seismicity context of the Algerian margin, with proved impact of large (M > 6.5) earthquakes on turbidite generation and convincing correlation of turbidites and historical earthquakes.
– Seismicity seems to have occurred, over the Holocene, in irregular cycles (Fig. 2) that are not synchronous in 3 sectors of the Algerian margin located 400 km apart.
– We attribute the apparent differences in the cycles between the Kramis and Algiers zones to a variable distance from the earthquake sources (M 6.5-7.5) to areas of sedimentary destabilization at the top of the slope. If this variability is taken into account, then the 1st and 2nd order cycles of these two margin segments could be comparable.
– Other possible sources of complexity could be: (i) spatial and temporal variability in sedimentary processes, due to changes in sediment availability, and (ii) a possible difference in tectonic behavior from west to east.
– Our analyses allow us to discuss the synchronous or non-synchronous character of the ruptures for remote seismic sources. It confirms the strong potential of the paleo-seismological approach by turbidites to constrain seismic recurrences near coastal areas exposed to seismic hazards.

Figure 2: An example of irregular seismic cycles determined by correlating land (El Asnam) paleo-ruptures and turbidites over 8 ka in the Kramis area (Fig. 1). We evidence 13 synchroneous turbidites considered as paleo-earthquakes, forming 3 earthquake clusters (C1, C2, C3) with recurrence intervals (RI) of 0.3-0.6 ka and 2 quiescence periods (Q1, Q2) of ~1.6 ka without earthquakes on any major fault in the area. These results suggest ~3-4 ka-long earthquake supercycles (broad phases of strain loading and multiple shorter phases of strain release). Picture from Ratzov et al. (2015).

Babonneau et al., 2017. Turbidite chronostratigraphy off Algiers, central Algerian margin: A key for reconstructing Holocene paleoearthquake cycles. Marine Geology, 384, 63-80, doi:10.1016/j.margeo.2016.10.017
Cattaneo et al., 2010. Submarine landslides along the Algerian margin: A review of their occurrence and potential link with tectonic structures, Advances in Natural and Technological Hazards Research, 28, 515-526.
Cattaneo et al., 2012. Searching for the seafloor signature of the 21 May 2003 Boumerdès earthquake offshore central Algeria, Natural Hazard and Earth System Sciences 12, 2159-2172, doi:10.5194/nhess-12-2159-2012

Ratzov et al., 2015. Holocene turbidites record earthquake supercycles at a slow-rate plate boundary, Geology, 43, 331-334, doi:10.1130/G36170.1

Informations complémentaires

Campagnes océanographiques ayant permis la collecte des turbidites :
SAMRA 2005
Programme soutenant cette action: Projet européen FP7 ASTARTE (Assessment, STrategy And Risk Reduction for Tsunamis in Europe) – Nov 2013 – Avril 2017.

Contact: Nathalie Babonneau