Determination of earthquake mass flow units in Lake Sapanca sediment and correlation with historical earthquakes

Lake Sapanca has a maximum depth of 55 m and a surface area of 46.8 km2, measuring 16 km in East-West and 5 km in North-South directions. The lake is important to the Marmara region because of its freshwater character and has been used as a source of drinking water by this most populated region of Turkey. The catchment area of Lake Sapanca is surrounded by mountains in the south and small hills in the north. Water is taken from the Lake for domestic and industrial needs. Several streams and ground water entering from the bottom feed the lake. There is only one stream draining the lake. The margins of Lake Sapanca are defined by a series of mountain ranges. The Samanlı Mountains to the south of the study area were uplifted as a pressure ridge structure and isolated from the surrounding morphology between the northern and southern strands of the North Anatolian Fault Zone (NAFZ). Lake Sapanca is located on a tectonic hole, which is situated between Izmit Bay and Adapazari Meadow and runs parallel to Iznik Lake. The mountain ranges are composed of Paleozoic to Early Tertiary metamorphic and sedimentary rocks which form the basement of the region. On the Eşme front of the Kocaeli Peneplain to the north of the lake, Paleozoic-Early Tertiary rocks crop out as uplifted basement against the Plio-Quaternary units and consist of sandstone, clayey limestone, marl and interbedded sandstone, shale and siltstone. The Sapanca front of the Samanlı Mountains, situated on the southern margin of the lake, consists of Paleozoic-Mesozoic rocks that include metamorphic (schist, marble, gneiss and quartzite) and meta-ophiolitic rocks (peridotite, gabbro and amphibolite). Lake Sapanca developed with its East-West trending elongated morphology on the Izmit-Sapanca Corridor and the floor of the depression is filled with Plio-Quaternary alluvial fan and alluvium deposits. Lake Sapanca is a pull-apart basin located along the North Anatolian Fault Zone in NW Anatolia, Turkey. Large earthquakes are common to different fault segments (Gölcük Segment, Sapanca Segment, Sakarya Segment, Karadede Segment, Düzce Segment, Mudurnu Segment and Abant Segment) of the North Anatolian Fault Zone in the eastern Marmara region around Lake Sapanca.The lake is loceted on quaternary sequence (alluvium). Therefore, the Lake is easily effected by earthquake's shaking and we can catch their records in lake sediments. The main objective of this study is to determine and date the records of past earthquakes in the lake's sedimentary sequence, which is important for earthquake risk assessment for the area. We carried out a systematic study of the sedimentological, physical and geochemical properties of three water-sediment interface cores (Sapanca1, Sapanca2 and Sapanca3). The cores, which were collected along a depth transect ranging from 43 to 51.7 m water depth on a high resolution seismic line, are up to 75.7 cm long and cover the last 250 years.. Within the aim of the study, the cores were described lithologicaly and analyzed for grain size using Fritsch laser diffraction at a sampling resolution of 1 cm, for total organic (TOC) and inorganic carbon (TIC) using Shimadzu Total Organic Carbon Analyzer at 2 cm resolution, for physical properties using Multi Sensor Core Logger (MSCL) and for elemental composition and digital radiography using Itrax-XRF Core Scanner. The resolution of the XRF and Multi Sensor Core Logger analyses were 0.25 mm and 0.5 mm. The chronology was determined using accelerator mass spectrometry (AMS) radiocarbon method in core Sapanca2 and radionuclide (210Pb, 137Cs) methods in core Sapanca3. 210Pb activity measurements were made by alpha counting of 210Po. 137Cs was measured by gamma-ray spectrometry using a germanium detector. The Constant Flux–Constant Sedimentation (CF–CS) model for the unsupported 210Pb supply was used to calculate the sedimentation rate. AMS radiocarbon analysis of a piece of plant collected at 37.5 cm in core Sapanca2, provided an age between the years 1900-1950. 210Pb results give an average sedimentation rate of 1.8 mm/yr and an age of 250-260 years for the base of core Sapanca3. The Chernobyl nuclear accident signal was found at at a core depth of 18 cm. This systematic multi-parameter study was performed on the dated lake sediments (Sapanca1, Sapanca2, Sapanca3), which respectively cover the uppermost 42.5 cm, 59.2 cm and 75.7 cm record of the lacustrine archive The earthquake records are represented by mass-flow deposits accumulated in the form of turbidite-homogenite (TH) units, The upper part of mass flow units are homogeneously clayey silt (homogenite), which together with the coarse basal part (turbidite), has the features of a turbidite-homogenite (TH) unit. The lake sedimentary sequence consist of brown clayey silt interrupted by mass-flow units, which are characterized by grey or dark grey coarse to fine sand and silty mud with sharp basal and transional upper boundaries. The units commonly show normal size grading with their basal parts showing high density, high magnetic susceptibility, low inorganik and organic carbon contents, enrichment in one or more elements such as Si, Ca, Ti, K, Rb, Zr, Fe which are indicative of coarse detrial input, and depletion in total carbonate (TIC). Mass flow units are formed with various effects which are earhtquake, volcanic activity, flood and storm. There isn't any volcanic formation around the Lake Sapanca. Lake Sapanca is not affected by the violent storms. Because Lake Sapanca is a sheltered field because of Samanlı Mountains and Kocaeli Penepline. Previous studies indicate that a drier climate is dominant in the region last 250 years according to present. Therefore, volcanic activity, flood and storm aren't origin of mass flow units in Lake Sapanca cores. However, tectonic activity is common for this area because of North Anatolian Fault Zone. Therefore, earthauqke is origin of mass flow units. Earthquake-induced mass flow units are called sismo turbidites. Based on radionuclide and radiocarbon datings, the mass flow units are correlated with 1999 Izmit and Düzce earthquakes (Mw=7.4 and 7.2, respectively), 1967 Mudurnu earthquake (Mw= 6,8), and 1957 Abant (Mw= 7.1) earthquake, 1943 Hendek Earthquake (Mw=6.6), 1894 East Marmara Earthquake (Mw=6.7), 1878 Esme Earthquake (Mw=6.7), and 1754 Izmit Earthquake (Mw=6.8). The relations between earthquake parameters (magnitude, distance from epicentre, intensity and sediment thickness of deposit since previous earthquake) and mass-flow parameters (thickness, sedimentary structure, grain size, composition) are discussed. Earthquake parameters influence to thickness of sismo-turbidite. Sismo-turbidites of core Sapanca3 correlated with earthquake parameters. Our data set is in sufficient for precise interpretation. Because we determined only seven earthquake records. However, preliminary results showed that increasing intensity of earthquake and decreasing distance from epicentre cause increase in thickness of the sismo-turbidite units.