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Gas hydrates and gas bubbles were collected during the MARNAUT cruise (May-June 2007) in the Sea of Marmara along the North Anatolian Fault system, Turkey. Gas hydrates were sampled in the western part of the Sea of Marmara (on the Western High), and three gas-bubble samples were recovered on the Western High, the Central High (center part of the Sea of Marmara) and in the Çinarcik Basin (eastern part of the Sea of Marmara). Methane is the major component of hydrates (66.1%), but heavier gases such as C 2 , C 3 , and i-C 4 are also present in relatively high concentration. The methane contained within gas hydrate is clearly thermogenic as evidenced by a low C 1 /C 2 + C 3 ratio of 3.3, and carbon and hydrogen isotopic data (δ 13 C CH4 of − 44.1‰ PDB and δD CH4 of − 219‰ SMOW). A similar signature is found for the associated gas bubbles (C 1 /C 2 + C 3 ratio of 24.4, δ 13 C CH4 of − 44.4‰ PDB) which have the same composition as natural gas fromK. Marmara-af field. Gas bubbles from Central High show also a thermogenic origin as evidenced by a C 1 /C 2 + C 3 ratio of 137, and carbon and hydrogen isotopic data (δ 13 C CH4 of − 44.4‰ PDB and δD CH4 of − 210‰ SMOW), whereas those from the Çinarcik Basin have a primarily microbial origin (C 1 /C 2 + C 3 ratio of 16,600, δ 13 C CH4 of − 64.1‰ PDB). UV-Raman spectroscopy reveals structure II for gas hydrates, with CH 4 trapped in the small (5 12) and large (5 12 6 4) cages, and with C 2 H 6 , C 3 H 8 and i-C 4 H 10 trapped in the large cages. Hydrate composition is in good agreement with equilibrium calculations, which confirm the genetic link between the gas hydrate and gas bubbles at Western High and the K.Marmara-af offshore gas field located north of the Western High. We calculate the characteristics of the hydrate stability zone at Western High and in the Çinarcik Basin using the CSM-GEM computer program. The base of the structure II hydrate stability field is at about 100 m depth below the seafloor at the Western High site, whereas in the Çinarcik Basin, P-T conditions at the seafloor correspond to the uppermost range for structure I hydrate formation from microbial gas.

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[1] Physical and chemical properties of warm and hot spring waters as well as soil radon concentrations were measured continuously during a 3-year period in the Marmara region; following the devastating I : zmit earthquake of 17 August 1999 (Mw = 7.4). Promising and encouraging anomalies in ground radon emanation have been recorded and found to be closely related to seismic activity. The temporal and spatial variations in the soil radon data are presented. The earthquakes with magnitude >4 in the region were correlated with positive radon anomalies. Furthermore, during quiescence (absence of seismic activity) the radon data indicate random walk behavior of radon in soil and show Rayleigh-type probability density function (pdf), however, during the earthquake build-up period, the data show deviations from Rayleigh-type pdf. The radon positive anomalies indicate disturbance of the path of gas movement or gas release pattern prior to earthquakes. However, systematic and consistent anomalies in physical and/or chemical properties of the spring waters have not been detected for earthquakes occurring in the observation period (M < 5.3).

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A catalog for earthquakes that occurred in western Turkey during the period 1964-2010 is compiled for achieving homogeneity for magnitudes. Data are obtained from the International Seismological Center (ISC), where earthquake magnitudes are reported in different scales and come from a variety of sources. For establishing a common magnitude expression, namely an equivalent moment magnitude M w , new relations correlating the different magnitude scales with each other are derived from converting as many as possible of the magnitudes reported in the ISC bulletins. After magnitude conversions, the completeness magnitude M c is sought by modifying the goodness-of-fit method of Wiemer and Wyss (2000) to become more appropriate for datasets with smaller sample size and higher M c thresholds. The study region is divided into four smaller regions on the basis of spatial data homogeneity, while different periods of similar seismic network performance are recognized and tested to seek spatiotemporal variation of M c . The results derived in each case are compared with those yielded by the application of both the original goodness-offit and maximum curvature methods and are found to be quite similar, although there are still cases with a difference exceeding 0.3 magnitude units. The goodness-of-fit method is very sensitive in the selection of the desirable percentage of fitting a power law (90% or 95%), whereas the proposed modification makes it independent of this level selection, and performing better especially for datasets that include events before 1990, when higher completeness magnitudes are evident.Online Material: Earthquake catalog with equivalent moment magnitude for western Turkey.

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