Paleotsunami deposits along the coast of Egypt correlate with historical earthquake records of eastern Mediterranean

摘要

We study the sedimentary record of past tsunamis along the coastal area west of Alexandria (NW Egypt) taking into account the occurrence of major historical earthquakes in the eastern Mediterranean. The two selected sites at Kefr Saber (similar to 32 km west of Marsa-Matrouh city) and similar to 10 km northwest of El Alamein village are coastal lagoons protected by 2-20 m-high dunes parallel to the shoreline. Field data were collected by (1) coastal geomorphology along estuaries, wedge-protected and dune-protected lagoons; and (2) identification and spatial distribution of paleotsunamis deposits using five trenches (1.5 m-depth) at Kefr Saber and twelve cores (1 to 2.5 m-depth) at El Alamein. Detailed logging of sedimentary sections was conducted using X-rays, grain size and sorting, total organic and inorganic matter, bulk mineralogy, magnetic susceptibility, and radiocarbon dating to identify past tsunamis records. Generally of low energy, the stratigraphic succession made of coastal lagoon and alluvial deposits includes intercalated high-energy deposits made of mixed fine and coarse sand with broken shells, interpreted as catastrophic layers correlated with tsunami deposits. Radiocarbon dating of 46 samples consist in mixed old (13000 BP) and young (5500 BP), dated charcoal and shells in sedimentary units correlate with the 24 June AD1870 (M-w 7.5), 8 August AD1303 (M-w similar to 8) and 21 July AD365 (M-w 8-8.5) large tsunamigenic earthquakes that caused inundation along the Alexandria and northern Egyptian shoreline. Our results point out the size and recurrence of past tsunamis and the potential for future tsunami hazards on the Egyptian coastline and the eastern Mediterranean regions.