Understanding micro-seismicity is a critical question for earthquake hazard
assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the
seismicity along the submerged section of North Anatolian Fault within the Sea
of Marmara (comprising the “Istanbul seismic gap”) has been extensively
studied in order to infer its mechanical behaviour (creeping vs locked). So
far, the seismicity has been interpreted only in terms of being tectonic-
driven, although the Main Marmara Fault (MMF) is known to strike across
multiple hydrocarbon gas sources. Here, we show that a large number of the
aftershocks that followed the M 5.1 earthquake of July, 25th 2011 in the
western Sea of Marmara, occurred within a zone of gas overpressuring in the
1.5–5 km depth range, from where pressurized gas is expected to migrate along
the MMF, up to the surface sediment layers. Hence, gas-related processes
should also be considered for a complete interpretation of the micro-
seismicity (~M < 3) within the Istanbul offshore domain
