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 (Mw 7.2) and 1999 Izmit
(Mw 7.4) major devastating earthquakes and thus poses a significant seismic hazard potential
for the megacity Istanbul. The 26 September 2019 Mw 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 the present study, we examine source properties of the mainshock 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 mainshock. The detected aftershock activity appears to focus on the east side of the
mainshock 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 Mw5.7 for the 26 September 2019 mainshock.
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 mainshock 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
Northern Anatolian Fault Zone.