The importance of multi-parametric analysis in long- term submarine gas emission monitoring: the SN4 ob- servatory at the North Anatolian Fault (Marmara Sea, Turkey)

Abstract

Methane-rich fluid vents have been widely observed and associated to active faults in the Sea of Marmara, along the submerged portion of the North Ana- tolian Fault (NAF). Episodic gas seepage also occurs in the Izmit Gulf, along the NAF segment that ruptured during the 1999 Izmit earthquake. This site is thus a unique area to test the hypothesis on the relation between strike-slip deforma- tion, seismic activity and gas expulsion within an active fault zone. A long-term multi-parametric experiment can be an effective way to study the irregular dy- namics of gas emission from seafloor and to understand its possible relation with seismic activity. A benthic seafloor observatory (SN-4) was deployed in the Izmit Gulf in 2009 using the R/V Urania as a demonstration mission in the framework of the EC ES- ONET (European Seas Observatory NETwork) project. Instrumental redundancy and specific cross-correlation of data from different sensors, proves to be funda- mental to distinguish actual seepage events from other signals related to ocean- ographic behaviour or even sensor biases. The observatory was equipped with a three component broad-band seismometer, a CTD with turbidity meter, two methane detectors, an oxygen sensor and a current-meter. All sensors installed on the observatory were managed by dedicated low-power electronics, which can manage a wide set of data streams with quite different sampling rates. A unique reference time, set by a central high-precision clock, is used to tag each datum. After six months of continuous monitoring, SN-4 was recovered in March 2010 in order to download the data and replace the batteries for a further six month mission period and finally recovered in October 2010. The data analysis clearly shows frequent degassing events, recorded as methane anomalies in seawater and as high-frequency short-duration signals recorded by the seismometer.. The time series of other oceanographic parameters (tempera- ture, oxygen concentration, turbidity and salinity) shows patterns that seem to be linked to both local gas seepage and to the circulation of water masses in the Gulf of Izmit. A comparative analysis of the various observables and their mutual correlation, can be a key tool to understand actual degassing events along the NAF. This analysis is first attempt in finding possible correlations be