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[1] The late Holocene activity of a restraining bend of the northern strand of the North Anatolian Fault in Izmit Bay was investigated by a sedimentological, geochemical, and paleoecological analysis of sediment cores from Hersek coastal lagoon, NW Turkey. The sediment cores show a succession of sedimentary sequences composed of three units separated by gradual transitions. The first unit is composed of a thin layer of shell debris-rich sediment in abrupt contact with the underlying organic-rich deposits. This unit is overlain by a thick foraminifera-rich mud deposit, and the sequences are capped by an organic-rich mud unit. These sequences are interpreted as silting up, shallowing upward deposits, typical of a lagoon becoming isolated from the sea. We suggest that they represent the sedimentary signature of coseismic subsidence, which was caused by reverse slip at the Hersek bend, and tsunamis in Izmit Bay. Our radiocarbon-dated paleoseismological record indicates (1) the atypical collapse of the hanging wall during the 740 earthquake and (2) subsidence of the footwall during the 987, 1509, and 1719 earthquakes. This study contributes to the understanding of the dynamics of restraining bends, and it highlights the potential of coastal sediments for reconstructing past earthquakes and tsunamis in regions dominated by strike-slip deformations.

The Indian Ocean tsunami of 26 December 2004 shows poignantly that catastrophic tsunamis are too infrequent for their hazard to be characterized by historical records alone. Long‐term geologic records provide opportunities to assess tsunami hazards more fully. Telltale deposits left by tsunamis help assess water depth and velocity of past inundations, estimate source locations, and aid in understanding how tsunamis affect the ecology and geomorphology of coastlines. Dated deposits allow estimates of times and recurrence intervals of past tsunamis. Such information guides mitigation efforts and may reduce losses from future tsunamis.

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