Seismic potential of creeping segments of the North Anatolian Fault: insights from seismo-geodetic deployments and deep learning catalogs

Fault zones release tectonic strain through a combination of seismic and aseismic slip. Creeping fault segments may have typically less elastic strain energy accumulated available to rupture in an earthquake compared to largely locked sections. However, where creeping fault segments transition into locked ones, stress rates are the highest as the slip deficit and stored elastic strain show large spatial along-fault gradient.

The North Anatolian Fault Zone (NAFZ) in Türkiye hosts two prominent creeping segments: 1) the Ismetpasa segment on the central part of the NAFZ, which appears devoid of micro-seismicity down to magnitude M=1.8 in the regional catalogs, but hosted the nucleation of two M>7 earthquakes in 1943 and 1944; and 2) the western portion of the submarine Main Marmara Fault, which poses a high seismic risk due to its proximity to the Istanbul metropolitan region. Some of the largest earthquakes of the instrumental era (2019 M5.8 and 2025 M6.2) close to Istanbul nucleated at the eastern edge of this partially creeping segment.

In this study, we combine near-fault dense seismo-geodetic deployments, with deep-learning seismicity catalogs to investigate the role of aseismic deformation in driving the seismicity and controlling the source properties along those creeping segments of the NAFZ. At the Ismetpasa segment, we present the first evidence of significant microseismicity on and up to ~5km off the main NAFZ fault branch (mostly ML < 2) surrounding the creeping patches. This microseismicity is likely driven by the aseismic slip on the main fault plane. We interpret this seismic activity as the signature of a weak, damaged fault zone surrounding the tip of the ruptures of the M>7 1943 and 1944 events.

 In the Marmara region, we show a series of eastward propagating M>5 events and a gradual eastward unlocking of the Main Marmara Fault over the last ~15 years. Seismic activity progresses from creeping toward transitional segments and is currently arriving at the locked Princes Islands segment south of Istanbul, which has the potential to host a M~7 earthquake. These findings highlight the role of aseismic slip in modulating the available shear stress and elastic stored energy, which, in turn, control the nucleation and arrest of large ruptures. Our results also illustrate the importance of monitoring fault systems including multi-disciplinary instrumentation that enables capturing the entire frequency band from slow to fast slip.