Spatial and temporal dynamics of gas-related processes in the Sea of Marmara monitored with ocean bottom seismometers

Abstract

In the Sea of Marmara, areas of gas seepage or cold seeps are tightly related to the faults system and understanding the spatial and temporal dynamics in gas-related processes is crucial for geohazard mitigation. Although acoustic surveys proved to be efficient in detecting and locating cold seeps, temporal variability or trends in the gas-related processes are still poorly understood. Two arrays of 10 ocean bottom seismometers were deployed in the western part of the Sea of Marmara in 2011 and 2014, respectively. In addition to the local seismic events, the instruments recorded a large number of short duration events and long-lasting tremors. Short duration events are impulsive signals with duration <1 s, amplitude well above the noise level and a frequency spectrum with one or two narrow peaks. They are not correlated from one site to another, suggesting a very local source. Tremors consist of sequences of clustered impulsive signals lasting for minutes to more than an hour with a multipeak frequency spectrum. Based on evidence of known seepage and by analogy with volcanic and hydrothermal models, we suggest that short duration events and tremors are associated with gas migration and seepage. There is a relationship between tremors associated with gas emission and the local seismicity, although not systematic. Rather than triggering gas migration out of the seabed, locally strong earthquakes act as catalysts when gas is already present or gas emission is already initiated.

Nonseismic Signals in the Ocean: Indicators of Deep Sea and Seafloor Processes on Ocean‐Bottom Seismometer Data

“…In particular, the occurrence of short‐duration events (SDEs) has been reported worldwide and with different types of instruments. Compiling the observations from several studies all over the world (e.g., Buskirk et al, ; Diaz et al, ; Franek et al, ; Hsu et al, ; Ostrovsky, ; Tary et al, ; Tsang‐Hin‐Sun et al, and references hereafter), SDEs can mostly be described by all of these characteristics: (i) short duration (<1 s), (ii) a high‐frequency content (usually in the 4‐ to 30‐Hz frequency range), (iii) amplitudes well above the noise level, and (iv) a single impulsive wave train, that is, no P and S arrivals (Figure ). Moreover, they are recorded on one OBS at a time, suggesting a local source in the vicinity of the instrument.…”

“…The back azimuth is estimated from the particle motion in the horizontal plane (Figure S3). Prior to the back azimuth estimation, the misorientation of the horizontal components of the OBS deployed in the SoM was determined as in Tsang‐Hin‐Sun et al (), using the technique of Niu and Li (). The horizontal components of the OBS deployed in Chile are not oriented with respect to the geographic north, so the estimated back azimuth does not represent the true direction of the source.…”

“…Widespread emissions of gas from the seafloor seem to be controlled by a combination of factors such as the tectonic regime, the sedimentary cover, and the connections with the gas source (e.g., Géli et al, ; Dupré et al, ; Kuşçu et al, ). Acoustic techniques and submersible dives demonstrated that most of these gas emissions occur near the surface expression of known active faults [Bayrakci et al, ; Dupré et al, ], supporting a correlation between microseismicity and gas emissions (Dupré et al, ; Géli et al, ; Tsang‐Hin‐Sun et al, ).…”

“…In the Niger Delta, peaks in the number of SDEs were followed by an increase in the pore pressure and thus associated with the release of gas that accumulated in excess in the subsurface sediments (Sultan et al, ). In the Sea of Marmara (SoM; Turkey), multidisciplinary studies related SDEs to acoustic gas flares [Bayrakci et al, ], peaks in the methane concentration (Embriaco et al, ), conduits collapse as gas migrate (Tary et al, ), or conduits opening in the sediments triggered by locally large earthquakes (Tsang‐Hin‐Sun et al, ). Finally, many SDEs were associated with gas migration and seepage on the Svalbard Margin, as they occur near cold seeps and together with bottom temperature anomalies and could be easily distinguished from whale calls (Franek et al, ).…”

“…In the laboratory, a second experiment consisted in reproducing and recording SDEs in a tank filled by sand and water to constrain the location of the source, either in the sediments or in the water column. Then, in order to investigate the deeper and more realistic oceanic conditions, SDEs were extracted and analyzed for two networks of OBS, in the SoM (Figure ), where SDEs were mainly associated with gas emission (Embriaco et al, ; Tary et al, ; Tsang‐Hin‐Sun et al, ) and offshore Central Chile (Figure ), where little is known about SDEs.…”