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Marmara Region hosts a substantial part of the inhabitants in Turkey, more than 30 per cent of the total population in the cities of Istanbul, Bursa and Kocaeli. This region has experienced several large earthquakes in the past and is still under threat of destructive earthquakes in the future. There, subparallel strands of the North Anatolian Fault (NAF) expand into the region, distributing the earthquake hazards across the whole region. Thus, it is a key issue to investigate how the tectonic process is distributed between these subparallel strands to discriminate their individual earthquake hazards. In this context, we used GPS slip rates to quantify the slip partitioning of the subparallel strands of the fault system. In addition to available slip rates compiled by Bulut et al. and Kreemer et al. we analysed 71 GPS sites (51 continuous and 20 campaign-based) to intensify the GPS network in the region. GPS observations reveal 81.7, 7.7 and 10.7 per cent slip ratios for northern, middle and southern strands. These ratios are also verified by the time series analysis of continuous GPS stations, leading us to the conclusion that the northern strand of the NAF hosts substantially shorter recurrence periods and, therefore, in the long term, the highest earthquake hazard in the Marmara Region. [ABSTRACT FROM AUTHOR]

The Central Marmara Sea region hosts the northwestern branch of the North Anatolian Fault Zone (NAFZ) with its known seismic gap between the 1912 Ganos (M w 7.2) and 1999 Izmit (M w 7.4) major devastating earthquakes and thus poses a significant seismic hazard potential for the megacity Istanbul. The 26 September 2019 M w 5.7 Silivri High-Kumburgaz Basin (central Marmara Sea) earthquake ruptured a thrust fault with a minor strike-slip component at the north of the eastern end of this gap relatively in the shallow depth (h = 8 km) range. Thus, in this study, we examine source properties of the main shock activity and coseismic behaviour of the failure, and the pattern of post-seismic deformation based on the aftershock distribution to have an insight into the role of the subsidiary and main fault structures on the crustal kinematics along this complicated branch of the NAFZ. The relocated epicentres are aligned in the E–W direction and tend to propagate towards the segments to the east of the main shock. The detected aftershock activity appears to focus on the east side of the main shock and almost no seismic activity was observed to the west of the epicentre. Independent investigations from coda-wave fitting, point-source, and finite-fault slip modelling agree on the moment magnitude of M w5.7 for the 26 September 2019 main shock. The kinematic rupture model of this event implied that the main rupture nucleated around the hypocentre, and then propagated bilaterally along the E–W direction but with significant progress towards the east. The distribution of the slip vectors indicates that the rupture evolved on a dextral thrust fault plane. The spatio-temporal behaviour of the overall aftershocks sequence, their focal mechanism solutions and our kinematic slip model clearly shows that the existing secondary structures developed in simple shear dextral deformation are likely responsible for the main shock activity. We conclude that such type of deformation model results in a motion in response to the thrust faulting with strike-slip component with an N89°W (271°) orientation and 33°NE dipping at left stepover transpressional region on the NAFZ. [ABSTRACT FROM AUTHOR]