Geohazards rooted from the northern margin of the Sea of Marmara since the late Pleistocene: a review of recent results

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Owing to its special geodynamic setting on the western extension of the North Anatolian Fault (NAF) and oceanographic setting between Mediterranean and the Black Seas, the Sea of Marmara (SoM) is prone to various geohazards, such as earthquakes, submarine landslides, tsunamis and hypoxia. The NAF is a major transform plate boundary that has produced devastating historical earthquakes. The most active northern strand of the NAF cuts across sea floor, delimiting the northern margin of the SoM. With the 1999 Izmit (M 7.4) and 1912 Şarkoy (M 7.4) earthquakes having occurred on its eastern and western ends, a large part of the SoM basin presently constitutes a seismic gap. The northern margin is composed of the Palaeozoic shales, weakly cemented Tertiary sedimentary rocks, and overlaying wedge of Quaternary sediments on the shelf edge. The mechanically weak lithology of the margin, coupled with steep slopes (20°–26°) and the activity of the NAF, makes the SoM susceptible to submarine landslides. The mass-wasting was more common during times of sea level lowstands when brackish-water lacustrine conditions prevailed than under highstand marine periods. It is most likely that gas escape and gas hydrate dissociation also contributed to the mass-wasting in the SoM during the sea level lowstands. Historical records reveal that more than 30 tsunami events occurred in the past two millennia. It is more likely that most tsunamis in the SoM have been associated with submarine landslides triggered by large earthquakes. However, the normal faulting on the southern margin of the Cinarcik Basin might have also caused tsunamis. Successive development of hypoxic conditions extending to the shelf areas occurred every time the SoM was flooded by marine waters during the interglacial periods. The SoM has been affected by the geohazards in the past and will likely face them again in the future. Long-term multi-disciplinary seafloor observatories are needed to monitor the geohazards in real time in the SoM.