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

Abstract

SUMMARY Different 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.

Insight Into Campi Flegrei Caldera Unrest Through Seismic Tremor Measurements at Pisciarelli Fumarolic Field

“…Microevents, defined as short-duration events, have been also recorded by ocean bottom seismometers (OBS) deployed over soft sediments in the western part of the Sea of Marmara (Turkey). These microevents, characterized by durations of less than 0.8 s, were studied by Tary et al (2012) and were interpreted as generated by a source very close to the sensor and associated to episodes of gas discharge from the seabed (Embriaco et al, 2014), moving inside the soft sediments. These mechanisms seem similar to those active in the hydrothermal area of Pisciarelli and suggest that the anomalous seismicity of 1 December 2018 should be interpreted as an increase in the boiling activity of the mud pool.…”

Interpretation of resonance frequencies recorded during hydraulic fracturing treatments

“…In order to determine the influence of path effects and stratigraphic filtering on the frequency content of the microseismic recordings, we perform wave‐propagation modeling considering a hypothetic source at the location of the fluid injection (Figure ). The numerical seismograms are computed using the reflectivity code of Dietrich [] [ Tary et al , ]. The source function is a zero‐phase Ricker wavelet.…”

Grain‐Displacive Gas Migration in Fine‐Grained Sediments

“…There is widespread methane release from submerged and surface sediments into the atmosphere. Persistent methane bubble curtains rise above various seafloor locations worldwide, including the West Spitsbergen continental margin (Westbrook et al, 2009), the northern U.S. Atlantic margin (Skarke et al, 2014), the sea of Marmara (Tary et al, 2012), the Gulf of Mexico (Wang et al, 2016), and the Håkon Mosby Mud Volcano (Sauter et al, 2006). Similarly, methane is released from sediments in lakes and swamps (Scandella et al, 2011).…”