Understanding the relations between seismicity and fluid compressibility in submarine environments: learnings from two case studies, e.g. the Main Marmara Fault and the East Pacific transform faults
jeudi 15 novembre 2018, de 11h00 à 12h00, Géoazur - Salle de conférences du bâtiment 1
Louis Géli - IFREMER
In submarine environments, progress in acoustic imagery of the water column reveals that gas emission from seafloor sediments is of common occurrence. Hence progress may be expected from detailed studies on
the relation between gas emissions, seismicity and crustal creeping in submarine settings, like -for instance the submerged section of the North-Anatolian Fault, within the western part of the Sea of Marmara, where intermediate magnitude earthquakes (e.g. with Ml between ~4 and 5) repeatedly occur every ~1 to 2 years. Since the devastating earthquakes of 1999, the seismicity within the so-called “Istanbul seismic gap” has been extensively studied, to determine the mechanical behaviour (creeping vs locked) of the different segments of the Main Marmara Fault (MMF). So far, the micro-seismicity has only been interpreted in terms of being tectonic-driven, even though the MMF is known to strike across sediment layers rich in hydrocarbon gas. High-resoluti! on, 3D ea rthquake depth determinations demonstrate the existence of shallow (<5 km), low magnitude seismicity, within gas-prone sediment layers along the western segments of the MMF, consistently with current views that propose deep crustal creeping where most of the gas emissions from the seafloor are found, against locking along the eastern segments of the MMF. Hence, although the role of gas is not yet clearly understood, a relation appears between crustal creeping, sediment deformation, shallow seismicity and gas emissions. In a totally different type of environment, mid-ocean ridge transform faults from the East Pacific are characterized by a global deficit in seismic moment and by the repeated occurrence of intermediate to large magnitude earthquakes every ~1 to 2 years (e. g. 15 events with Mw between 5.4 and 6.0 on Discovery FZ from 1990 to 2014). In addition, detailed seismological studies have shown the existence of strong fault patches separated by vertical swarms of micr! o-seismic ity (from the base of the crust to the seafloor) that do not appear to rupture in the largest earthquakes. Because hydrothermal fluids within young, hot crust at oceanic transform faults may become significantly more compressible with decreasing pressure, resulting in potential impacts on fault behaviour, we propose that the likely presence of highly compressible fluids at shallow depth (e.g. above the critical point) may explain the presence of these vertical swarms of micro-seismicity. We suggest that these studies could be of general use for improving our understanding of earthquake generation processes in submarine environments, wherever gas or highly compressible fluids are expected to be present at shallow depths.