Micro-séismicité et processus de fond de mer dans la partie ouest de la Mer de Marmara : nouveaux résultats fondés sur l'analyse des données de sismographes et hydrophones sous-marins

Since the devastating earthquakes of 1999, east of Istanbul, the submerged section of the North Anatolian Fault (NAF), in the Sea of Marmara (SoM) has been intensively monitored, mainly using land stations. Still, the micro-seismicity remains poorly understood. In addition, although the connection of the SoM with the hydrocarbon gas system from the Thrace Basin is now well established, along with the presence of widespread gas within the sedimentary layers, the role of gas on seismicity is still not recognized.Here, we have analyzed Ocean Bottom Seismometer (OBS) data from two deployments (April-July 2011 and September-November 2014) in the western SoM. Based on a high-resolution, 3D-velocity model, and on non-linear methods (NonLinLoc), our location results show that a large part of the micro-seismicity occurs at shallow depths ( 4.5) that frequently occur along the western segments of the MMF.In addition, OBSs also record at least two families of short duration ( 4.5 qui se produisent régulièrement le long de la MMF.Par ailleurs, 2 familles de signaux de courte durée (<1s), dits ≪ SDE ≫ (pour Short Duration Event) apparaissent sur les enregistrements : 1) les SDE se produisant à raison de quelques dizaines de SDE/jour, en réponse à des causes locales (i.e. bioturbation, activité biologique, micro-bullage de fond de mer, mouvements à l’interface eau/sédiment), etc ; 2) lesSDE se produisant par ≪paquets≫, dont certains sont enregistrés sur les 4 composantes (y compris l’hydrophone) et apparaissent de manière périodique, toutes les 1.8 s environ, en réponse à diverses causes qui restent à déterminer (parmi lesquelles : les mammifères marins ; l’activité humaine ; la sismicité ; le dégazage ; les ≪trémors≫ sismiques ; etc)

Micro-seismicity and deep seafloor processes in the Western Sea of Marmara : insights from the analysis of Ocean Bottom Seismometer and Hydrophone data

Since the devastating earthquakes of 1999, east of Istanbul, the submerged section of the North Anatolian Fault (NAF), in the Sea of Marmara (SoM) has been intensively monitored, mainly using land stations. Still, the micro-seismicity remains poorly understood. In addition, although the connection of the SoM with the hydrocarbon gas system from the Thrace Basin is now well established, along with the presence of widespread gas within the sedimentary layers, the role of gas on seismicity is still not recognized.Here, we have analyzed Ocean Bottom Seismometer (OBS) data from two deployments (April-July 2011 and September-November 2014) in the western SoM. Based on a high-resolution, 3D-velocity model, and on non-linear methods (NonLinLoc), our location results show that a large part of the micro-seismicity occurs at shallow depths ( 4.5) that frequently occur along the western segments of the MMF.In addition, OBSs also record at least two families of short duration ( 4.5 qui se produisent régulièrement le long de la MMF.Par ailleurs, 2 familles de signaux de courte durée (<1s), dits ≪ SDE ≫ (pour Short Duration Event) apparaissent sur les enregistrements : 1) les SDE se produisant à raison de quelques dizaines de SDE/jour, en réponse à des causes locales (i.e. bioturbation, activité biologique, micro-bullage de fond de mer, mouvements à l’interface eau/sédiment), etc ; 2) lesSDE se produisant par ≪paquets≫, dont certains sont enregistrés sur les 4 composantes (y compris l’hydrophone) et apparaissent de manière périodique, toutes les 1.8 s environ, en réponse à diverses causes qui restent à déterminer (parmi lesquelles : les mammifères marins ; l’activité humaine ; la sismicité ; le dégazage ; les ≪trémors≫ sismiques ; etc)

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

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 multi-peak 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

Improved detection and Coulomb stress computations for gas-related, shallow seismicity, in the Western Sea of Marmara

International audienceThe Sea of Marmara (SoM) is a marine portion of the North Anatolian Fault (NAF) and a portion of this fault that did not break during its 20th century earthquake sequence. The NAF in the SoM is characterized by both significant seismic activity and widespread fluid manifestations. These fluids have both shallow and deep origins in different parts of the SoM and are often associated with the trace of the NAF which seems to act as a conduit. On July 25 th , 2011, a M w 5 strike-slip earthquake occurred at a depth of about 11.5 km, triggering clusters of seismicity mostly located at depths shallower than 5 km, from less than a few minutes up to more than 6 days after the mainshock. To investigate the triggering of these clusters we first employ a match filter algorithm to increase the number of event located and hence better identify potential spatio-temporal patterns. This leads to a 2-fold increase in number of events relocated, coming mostly from the shallow seismic clusters. The newly detected events confirm that most of the aftershocks are shallow, with a large number of events located in the first few km below the SoM seafloor. Pore pressure diffusion from the position of the deep mainshock to the position of the shallow events is incompatible with the short time interval observed between them. We therefore investigate static and dynamic stress triggering processes. The shallow clusters fall into either positive or negative lobes with static stress variations of about ±5 kPa. Dynamic stresses may reach values of about ±40 kPa depending on the rise time and the fault orientation considered, but cannot last longer than the perturbations associated with the seismic waves from the mainshock. We then propose a mechanism of fluid pressure increase involving local fluid transfers driven by the transient opening of gas-filled fractures due to earthquake shaking, to explain the triggering of the shallow events of the clusters

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

Ocean bottom seismometers (OBS) commonly record short duration events (SDEs), that could be described by all of these characteristics: (i) duration < 1 s, (ii) one single‐wave train with no identified P‐ nor S‐wave arrivals and (iii) a dominant frequency usually between 4 Hz and 30 Hz. In many areas, SDEs have been associated with gas or fluid‐related processes near cold seeps or hydrothermal vents, although fish bumps, instrumental or current‐generated noise have been proposed as possible sources. In order to address some remaining issues, this study presents results from in situ and laboratory experiments combined with observations from 2 contrasting areas, the Sea of Marmara (Turkey) and the Chilean subduction zone. The in situ experiment was conducted at the EMSO‐Molène submarine observatory (near Brest, France) and consisted in continuously monitoring two OBSs with a camera. The images revealed that no fish regularly bumped into the instruments. Laboratory experiments aimed at reproducing SDEs’ waveforms by injecting air or water in a tank filled by sand and sea‐water and monitored with an OBS. Injecting air in the sediments produced waveforms very similar to the observed SDEs, while injecting air in the water column did not, constraining the source of SDEs in the seafloor sediments. Finally, the systematic analysis of two real data sets revealed that it is possible to discriminate gas‐related SDEs from biological or sea‐state related noise from simple source parameters, such as the temporal mode of occurrence, the back azimuth and the dominant frequency. Key Points Short duration high amplitude events routinely recorded on marine seismometers are not created by fish bumping into the geophone. In laboratory experiments, the waveforms of these events are well reproduced by fluid migration in the sediments. We propose 3 simple source parameters to discriminate gas‐related short duration events in ocean bottom seismometer data

Web INTEractive management tool for coal Regions in transition

The project WINTER, funded by the European Research Fund for Coal and Steel (RFCS) started in July 2022. The main aim of the project is to develop a web-interactive platform for the management of coal regions in transition to provide guidance and facilitate stakeholder engagement, by identifying and highlighting the available information that is required to improve the current transition plans for the pilot coal transition regions. Local authorities and coal industry stakeholders will be able to utilise the developed user-friendly web-based management tool and implement it on other regions with similar concerns. The project’s results will contribute to the repurposing of closed and/or abandoned coal sites and promote the added value of research outputs targeted at the exploitation of new products and markets regarding the coal industry.FUNDING Funding from the Research Fund for Coal and Steel under grant agreement No. 10105722810105722

An Alternative View of the Microseismicity along the Western Main Marmara Fault

A detailed study, based on ocean‐bottom seismometers (OBSs) recordings from two recording periods (3.5 months in 2011 and 2 months in 2014) and on a high‐resolution, 3D velocity model, is presented here, which provides an alternative view of the microseismicity along the submerged section of the North Anatolian fault (NAF) within the western Sea of Marmara (SoM). The nonlinear probabilistic software packages of NonLinLoc and NLDiffLoc were used for locating earthquakes. Only earthquakes that comply with the following location criteria (e.g., representing 20% of the total amount of events) were considered for analysis: (1) number of stations≥5; (2) number of phases≥6, including both P and S; (3) root mean square (rms) location error≤0.5  s; and (4) azimuthal gap≤180°. P and S travel times suggest that there are strong velocity anomalies along the Western High, with low Vp, low Vs, and ultra‐high Vp/Vs in areas where mud volcanoes and gas‐prone sediment layers are known to be present. The location results indicate that not all earthquakes occurred as strike‐slip events at crustal depths (>8  km) along the axis of the Main Marmara fault (MMF). In contrast, the following features were observed: (1) a significant number of earthquakes occurred off‐axis (e.g., 24%), with predominantly normal focal mechanisms, at depths between 2 and 6 km, along tectonically active, structural trends oriented east–west or southwest–northeast, and (2) a great number of earthquakes was also found to occur within the upper sediment layers (at depths<2  km), particularly in the areas where free gas is suspected to exist, based on high‐resolution 3D seismics (e.g., 28%). Part of this ultra‐shallow seismicity appears to occur in response to deep earthquakes of intermediate (ML∼4–5) magnitude. Resolving the depth of the shallow seismicity requires adequate experimental design ensuring source–receiver distances of the same order as hypocentral depths. To reach this objective, deep‐seafloor observatories with a sufficient number of geophone sensors near the fault trace are needed