A statistical approach to relationships between fluid emissions and faults: The Sea of Marmara case

Abstract

The Sea of Marmara is traversed by the North Anatolian Fault system and also presents abundant emission sites of methane gas into the water column. In order to assess the spatial relationship between gas emissions and active faults, the distribution of distances between gas emission sites and the nearest fault is calculated and compared with the distribution of distances between a uniform random distribution of points (Poisson process representing the null hypothesis of an absence of relationship between gas emissions and faults) and the nearest fault. Interestingly, the distance distribution for the Poisson process is nearly exponential, indicating that the fault map does not have a characteristic scale other than that representing the intensity of the fault network. The distance distribution for the observed gas emissions is significantly narrower than that arising from the Poisson process, with a Kolmogorov distance of 0.25±0.02. The crossing point between the two distributions defines the characteristic halfwidth of the swath of gas emission sites around the mapped active faults. For the whole Sea of Marmara data set a characteristic half-width of 900-1000 m is found which matches the half-width of the seafloor deformation zone observed around the main active fault. When the same analysis is applied to zones covering the Western High and the Central High it is found that the swath of gas emissions is wider on the Central High (2 km half-width), and not clearly related to the seafloor deformation zone there. This difference is put in perspective with recent work showing that creep is occurring on the western segment of the Main Marmara Fault (this also causing microseismicity) while the central Istanbul-Silivri segment may have remained locked since the 1766 magnitude 7+ earthquake. This suggests that aseismic slip (and not only earthquake occurrence) effectively maintains high permeability conduits in fault zones in sediments.

Interseismic strain build-up on the submarine North Anatolian Fault offshore Istanbul

“…With the possibility of distributed strike-slip across a few kilometre-wide zone of faults at the seafloor 26,30,42,43 , we might not have captured the complete possible slip. However, deformation is clearly focussed beneath the geodetic network since the fault trace can be unambiguously identified in the bathymetry 26,30 .…”

“…The offset can be explained with the increased strain and hence stress accumulation in the vicinity of the locked fault 37 (Fig. 5c) and gas migrations at depth 42,44 . The local observation is in-line with the sparse seismicity recorded by onshore stations during the last decade along this segment of the NAF (Fig.…”

Upward migration of gas in an active tectonic basin: An example from the sea of Marmara

“…They form systems that can be viewed as flower structures and are possibly connected to a narrower damage zone at depth (Géli et al, 2018;Henry et al, this issue). The width of the shear zone at the surface, in this latter case, will depend of the depth in which the subsidiary shear planes splay from the main fault zone and on their dip.…”

Fluids and processes at the seismically active fault zone in the Sea of Marmara

“…The presence of metastable ferromagnetic iron sulfide minerals on the Western High indicates a more reducing environment resulting from the degradation of heavy hydrocarbons and organic matter. The last article (Henry et al, 2018) links fluid emission and fault activity. It has been shown that most of the gas emissions are located within a distance shorter than 750 m from the faults, and irrespective of creeping or locked segments.…”