Long-term sedimentary earthquake records along the northern branch of the North Anatolian Fault in the Sea of Marmara (NW T\"urkiye)

Abstract

Geological earthquake records are important for probabilistic seismic risk assessment. Such records can be obtained from studies of turbidites triggered by seismic activity in marine and lake basins. The Sea of Marmara (SoM), located on the North Anatolian Fault (NAF), serves as an important laboratory for subaqueous palaeoseismological research. This is because it has 2500 yrs. of historical earthquake records that can be correlated with radiometrically dated sedimentary earthquake records. Additionally, the relatively high sedimentation rate ($\sim$3 mm/yr) in the deep subbasins allows the recognition of individual events. Following the destructive 1912 Mw 7.4 \c{S}ark\"oy-M\"urefte and 1999 Mw 7.4 Izmit and Mw 7.2 D\"uzce earthquakes, the main submerged part of the NAF in the SoM represents a seismic gap, where the long-term earthquake history is of crucial importance for earthquake risk assessment. We have studied nine cores recovered along the most active northern strand of the NAF (i.e. the Main Marmara Fault: MMF), using high resolution digital X-ray radiography, $\mu$-XRF core scanning, MSCL physical properties and grain-size analyses. The chronology was determined using accelerated mass spectrometry (AMS) radiocarbon and radionuclide methods. In the cores, coseismic turbidites commonly consist of a basal part with multiple sand-silt laminae above a sharp and often erosional base and an overlying graded mud part (homogenite). The basal parts exhibit high gamma density and MS, and are often enriched in one or more elements, such as Si, K, Fe, Ti, Zr, Ca and Sr, indicative of coarse siliciclastic and carbonate shell fragments. Radionuclide- and radiocarbon-dated coseismic turbidites in different SoM subbasins extending back to more than 5000 yrs. indicate an average earthquake recurrence time between 220 and 300 yrs. along the different segments of the MMF. These results are compatible with the GPS velocities and geological slip rates. However, the intervals between two consecutive events vary widely between 50 and 695 yrs. for the different northern NAF segments. Integration of our results with previous studies indicates that the M > 7 events recorded are the 1999, 1509, 1296 and 740 earthquakes on the __zmit Gulf segment; the 1894, 1509, 1343, 1090 and 740 earthquakes on the Prince Islands segment in the \c{C}{\i}narc{\i}k Basin; the 1894, 1766, 1509, 989, 869 or 862, 740 and 447 earthquakes on the Central High segment and in the Central Basin; and the 1912, 1766, 1344, 989 and 447 earthquakes on the western Marmara segment. The 1912 \c{S}ark\"oy-M\"urefte earthquake record is absent in the Central Basin, which suggests that the earthquake rupture did not extend beyond the western end of the Western High. The distribution of the 2500 yr-long sedimentary earthquake records matched with the historical earthquakes along the MMF indicates the common occurrence of multiple-segment ruptures, and suggests that presently, the most earthquake-prone part of the MMF is the Central High segment, located SW of Istanbul.