Large boulder accumulation on the Algerian coast evidence tsunami events in the western Mediterranean

Evidence of catastrophic mega-block deposition is presented for the Algerian coast from Tipaza to Dellys. The region is prone to large earthquakes, several of which are inferred to be tsunamigenic in origin, as attests the 2003 Zemmouri earthquake (Mw 6.8). It is argued here that several former tsunamis have resulted in the detachment of large boulders from the nearshore zone and their deposition inland. The estimated size, weight (volumetric mass) and distance from the shoreline of more than 100 boulders has enabled estimates to be made of the nature of the hydrodynamic waves responsible for their transport. The boulders weigh up to 200 tons and are scattered along ∼ 150 km of rocky headlands and pocket beaches, in isolated or grouped elements, from the subtidal to supratidal zones. Boulders covered by biogenic incrustations show morphological features which suggest detached, reversed and reworked pieces. Statistical and hydrodynamic analyses indicate that large boulder transport requires either ∼ 30-m-high waves or 5 to 10-m-high waves for catastrophic storm or tsunami events, respectively. Bio-indicators allow us to date two inferred tsunamis as having struck the Algerian coastline between AD 400–600 and ∼ AD 1700.