Sedimentary earthquake records in the İzmit Gulf, Sea of Marmara, Turkey

Abstract

Sedimentary earthquake records of the last 2400a, including that of the devastating 17 August 1999 İzmit earthquake (Mw=7.4), were studied in cores from the 210m-deep central Karamürsel Basin of the İzmit Gulf in the eastern Sea of Marmara, using laser grain-size, physical properties, stable O and C isotopes and XRF Core Scanner analyses, and dated by radionuclide and radiocarbon methods. The earthquake records are represented by turbidite–homogenite mass-flow units (THU) that commonly contain a basal coarse layer, a middle laminated silt layer and an overlying homogeneous mud layer. The coarse basal part has a sharp and sometimes scoured lower boundary, and includes multiple coarse (sand/silt) layers or laminae showing normal size grading. Multiple coarse layers and occasional bi-directional cross-bedding suggest deposition from a bed-load during water column oscillations, or seiche effect. The grain-size characteristics of the overlaying laminated silt and the homogeneous mud units indicate deposition from weak oscillating currents and homogeneous suspension, respectively. High Mn value just below the base of THUs suggests diagenetic enrichment at oxic/anoxic redox boundary before the mass-flow event. Sharp decrease in Mn with very low values within the THUs suggests transient redox conditions following the mass-flow. Variable geochemical compositions of the basal coarse layers indicate different sediment sources for different THUs. Eight sedimentary earthquake records observed in the last 2400a in the İzmit Gulf can be confidently correlated with the historical earthquakes of 1999, 1509AD (Ms=7.2), 1296AD (I=VII), 865AD (I=VIII), 740AD (I=VIII), 268AD (I=VIII), 358AD (I=IX), and 427 BC. This gives an earthquake recurrence time of ca. 300a, with the interval between consecutive events ranging from 90 to 695a.