Microevents produced by gas migration and expulsion at the seabed: A study based on sea bottom recordings from the Sea of Marmara

International audienceDifferent types of 4-component ocean bottom seismometers (OBS) were deployed for variable durations ranging from 1 week to about 4 months in 2007, over soft sediments covering the seafloor of the Tekirdag Basin (western part of the Sea of Marmara, Turkey). Non-seismic microevents were recorded by the geophones, but generally not by the hydrophones, except when the hydrophone is located less than a few tens of centimetres above the seafloor. The microevents are characterized by short durations of less than 0.8 s, by frequencies ranging between 4 and 30 Hz, and by highly variable amplitudes. In addition, no correlation between OBSs was observed, except for two OBSs, located 10 m apart. Interestingly, a swarm of ∼400 very similar microevents (based on principal component analysis) was recorded in less than one day by an OBS located in the close vicinity of an active, gas-prone fault cutting through the upper sedimentary layers. The presence of gas in superficial sediments, together with analogies with laboratory experiments, suggest that gas migration followed by the collapse of fluid-filled cavities or conduits could be the source of the observed microevents. This work shows that OBSs may provide valuable information to improve our understanding of natural degassing processes from the seafloor

Gas and seismicity within the Istanbul seismic gap

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

Gas and seismicity within the Istanbul seismic gap

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

Monitoring of gas emission in the Marmara Sea by the combined study of the Acoustic Bubble Detector and Ocean Bottom Seismometers

International audienceWe here present geophysical evidence of gas emission from the Sea of Marmara seafloor, recorded by an Acoustic Bubble Detector (BOB), deployed on the top of the Central High, giving insight on the temporal variations of the flow rates. The instrument insonified 24 sectors of 7 over several days, providing multiple passes for a given angular sector. The acquired data indicate that gas emissions are non-steady processes. Non-seismic micro events have also been identified on an ocean bottom seismometer (OBS) deployed at the same location as the BOB. These non-seismic micro-events have the same characteristics as the ones previously identified within the Tekirdag Basin, by a network of 4-component OBS: They were recorded by geophones, but generally not by the hydrophones, except when the hydrophone is located less than a few tens of cm above the seafloor. They are characterized by short durations of less than 0.6 seconds, by frequencies ranging between 5 and 30 Hz, and by highly variable amplitudes. In addition, no correlation was observed between OBSs. The presence of gas in superficial sediments, together with analogies with laboratory experiments, suggest that gas migration followed by the collapse of fluid-filled cavities or conduits could be the source of the observed micro-events. The present work combines the BOB recordings with the OBS recordings covering the whole North Marmara Trough. Our study shows that the monitoring of the gas emission by a multi parameter approach may provide valuable information to better understand the relationships between deformation and non-seismic transients related to degassing from the sub-seafloor layers near the fault zone

Impaired Plasmacytoid Dendritic Cell (PDC)-NK Cell Activity in Viremic Human Immunodeficiency Virus Infection Attributable to Impairments in both PDC and NK Cell Function▿

Human immunodeficiency virus (HIV) and hepatitis C virus (HCV) infections impair plasmacytoid dendritic cell (PDC) and natural killer (NK) cell subset numbers and functions, though little is known about PDC-NK cell interactions during these infections. We evaluated PDC-dependent NK cell killing and gamma interferon (IFN-γ) and granzyme B production, using peripheral blood mononuclear cell (PBMC)-based and purified cell assays of samples from HCV- and HIV-infected subjects. CpG-enhanced PBMC killing and IFN-γ and granzyme B activity (dependent on PDC and NK cells) were impaired in viremic HIV infection. In purified PDC-NK cell culture experiments, CpG-enhanced, PDC-dependent NK cell activity was cell contact and IFN-α dependent, and this activity was impaired in viremic HIV infection but not in HCV infection. In heterologous PDC-NK cell assays, impaired PDC-NK cell killing activity was largely attributable to an NK cell defect, while impaired PDC-NK cell IFN-γ-producing activity was attributable to both PDC and NK cell defects. Additionally, the response of NK cells to direct IFN-α stimulation was defective in viremic HIV infection, and this defect was not attributable to diminished IFN-α receptor expression, though IFN-α receptor and NKP30 expression was closely associated with killer activity in viremic HIV infection but not in healthy controls. These data indicate that during uncontrolled HIV infection, PDC-dependent NK cell function is impaired, which is in large part attributable to defective IFN-α-induced NK cell activity and not to altered IFN-α receptor, NKP30, NKP44, NKP46, or NKG2D expression