Spatio-temporal variations of seismic coupling along a transform fault: the western North Anatolian Fault Zone

The Main Marmara Fault (MMF) forms a major segment of the North Anatolian Fault Zone (NAFZ) in northwestern Türkiye. The MMF represents a seismic gap with currently high seismic hazard and associated risk for the Istanbul metropolitan area. Here we estimate the seismic coupling defined as the ratio of the seismic strain rate to the tectonic strain rate, for the MMF and adjacent NAFZ segments. This ratio indicates the fraction of total strain accumulated with time that is released seismically. We compare the results of seismic strain rates and coupling estimated from earthquakes included in historical and instrumental catalogs, which allows us to identify fault segments that represent a considerable seismic threat during the current seismic cycle. We find that along the main fault traces hosting the large events, seismic strain rates from the historical catalog are of the same order as the tectonic strain rates. In contrast, coupling estimates based on seismic data from the instrumental catalog covering also off-fault areas, are up to 100 times smaller, highlighting that most of the seismic energy is released in large earthquakes with recurrence times longer than the time covered by the instrumental catalog. Within the Sea of Marmara, a significant portion (48 per cent) of shear strain from the instrumental catalog is currently being accommodated by seismic deformation. Significant variations of the seismic coupling are observed before and after the 1999 M > 7 Izmit earthquake, highlighting the different contribution of aseismic slip over different portions of the seismic cycle. A comparison of the temporal evolution of the 1999 Izmit and Düzce postseismic deformation with seismic strain rates shows that the largest seismic strain rates coincide with the largest post-seismic deformation.