Creep and seep: the North Anatolian Fault in the Sea of Marmara

Since the 1999 Izmit-Kocaeli earthquake, the offshore segments of the North Anatolian Fault system in the Sea of Marmara have been considered at an imminent risk for a large earthquake. The expected earthquake has not happened yet and studies based on land geodesy could not detect interseismic loading on the fault segment that directly menaces Istanbul. The need to clarify the slip rate and locking state of the offshore faults is motivating current studies and seafloor monitoring projects. An acoustic ranging experiment is ongoing in the framework of EMSO and early results indicate that the Istanbul segment is not creeping at the seafloor at a significant rate. Other recent results include new estimations of the geological slip rate on the main strike slip fault branch and of the amount and timing of extension, assessment of earthquake recurrence intervals and segmentation based on turbidite-homogeneite records, mapping of fluid and gas emissions at the seafloor and of their relationships with active faults and sedimentary structures. Overall, the slip rate on the MMF represents 2/3 to 3/4 of the Eurasia-Anatolia plate motion. Although creep on this fault appears to influence seismogenesis, it is unlikely to take up a large part of the plate motion. A relation is found between the distribution of gas emissions along the faults and occurrence of background seismicity, thought to be an indicator of creep at crustal level. Sediment deformation around a moving fault and seismic shaking favor fluid migration through the sediment. Segments remaining fully locked for long periods of time would thus be comparatively less favorable for fluid expulsion at the seafloor than segments with creep or frequent moderate size earthquakes. Temporal variations of gas emission along the fault and of microseismicity rates both appear as potential indicators of strain rate variations and should be considered in an integrative monitoring strategy.