ITU research team consisting of Prof. Dr. Tuncay Taymaz (Faculty of Mines, Department of Geophysics Engineering and Institute of Disaster Management) from the Department of Geophysics Engineering, Faculty of Mining, Assoc. Prof. Dr. Tuna Eken, (Faculty of Mines, Department of Geophysics Engineering), Prof. Dr. Seda Yolsal-Çevikbilen (Faculty of Mines, Department of Geophysics Engineering), and research assistants Ceyhun Erman (Faculty of Mines, Department of Geophysics Engineering) and Berkan Özkan (Faculty of Mines, Department of Geophysics Engineering) presented the first and most detailed scientific study to date on the 23 April 2025 Mw 6.3 Silivri–Kumburgaz earthquake.
Combining advanced modeling techniques and precise aftershock analysis, the study reveals that this medium-sized earthquake may be an important “wake-up signal” for a dangerous section of the Main Marmara Fault (MMF), one of the major branches of the North Anatolian Fault (NAF) located south of Istanbul, which has been silent for approximately 260 years. that this medium-sized earthquake could be an important “wake-up signal.”
The research results were published in the Journal of Seismology, one of the leading journals in seismology. During the study, the ITU team was joined by Prof. Dr. T. Serkan Irmak (Kocaeli University, Geophysical Engineering), Dr. Buse Turunçtur (CSIRO, Australia), and Dr. Metin Kahraman (Erzincan Binali Yıldırım University, Earthquake Technologies Institute) also contributed to the study with their analyses.
The long-feared fault is now moving again
The April 23 earthquake occurred on the Marmara Main Fault, one of the most critical fault segments of the North Anatolian Fault that runs beneath the Sea of Marmara. This segment has long been considered one of Turkey's most dangerous seismic gaps due to its proximity to Istanbul, its lack of a major earthquake since 1766, its structurally locked state, and the significant stress it has accumulated in the region. The research team determined that the earthquake occurred precisely in the transition zone between this locked region and a partially sliding (slowly moving) fault segment. Worldwide, such transition zones are known as sensitive points where both medium-sized earthquakes are focused and large earthquakes are triggered.
The figure shows the tectonic and bathymetric map of the Marmara Sea region, highlighting the main traces of the North Anatolian Fault Zone (NAFZ) using red lines and arrows indicating right-lateral (dextral) motion. The map displays the epicenters of three major earthquakes: the 1912 Şarköy–Mürefte earthquake, the 1999 İzmit earthquake, and the 2019 Silivri earthquake, each represented with stars of different colors and their focal mechanisms. The 2025 Mw 6.3 Silivri–Kumburgaz earthquake is also shown with a yellow star and focal mechanism. The map further illustrates the fault behavior along the Marmara segment using the classification of Becker et al. (2023), with red shading for locked zones, green for creeping zones, and yellow for transition regions (Eken et al., 2025).
What the April 23, 2025 Mw 6.3 Silivri–Kumburgaz earthquake actually did
Advanced modeling in the study revealed that the tremor occurred in a two-stage slip at two separate foci called “asperities,” which had accumulated more stress than their surroundings. This explains why many people felt two strong tremors in succession during the earthquake. As the rupture progressed toward the relatively easier-to-slip region to the west, it rapidly weakened as it approached the completely locked Kumburgaz fault segment to the east. This locked segment is estimated to be the most dangerous point that scientists have long pointed to for a possible Mw 7+ Marmara earthquake.
The figure presents detailed rupture characteristics obtained through joint modeling of teleseismic and strong ground motion recordings of the 23 April 2025 Mw 6.3 Silivri–Kumburgaz earthquake (Eken et al., 2025).
The earthquake that triggered the fault: What do the aftershocks tell us?
Researchers used a high-precision algorithm called HypoDD to relocate 590 aftershocks and obtained a striking result: Within the first 24 hours after the main shock, the aftershocks moved approximately 20 kilometers eastward, directly toward the locked Kumburgaz fault segment. This rapid migration indicates that the main shock transferred significant stress towards the locked zone. However, the aftershocks abruptly stopped when they reached an area with a known high crustal velocity (high Vs) anomaly. This corresponds to a fault segment composed of very strong crustal rocks deep underground that accumulate stress for long periods without fracturing. The research also shows that the aftershocks clustered around the immediate vicinity of the rupture rather than the main rupture zone, which is a classic behavior often seen in moderate-sized earthquakes that could be precursors to a major earthquake.
The figure displays the spatiotemporal distribution of aftershock activity following the 23 April 2025 Mw 6.3 Silivri–Kumburgaz earthquake (Eken et al., 2025).
The figure illustrates how S-wave velocity structures and earthquake locations vary beneath the Marmara Sea. Panel (a) presents a regional topographic map showing seismic activity from the KOERI catalog and this study, marked with grey and red circles, while the epicenter and hypocenter of the mainshock are indicated with a white star. Panel (b) shows a north-northwest to south-southeast cross-section (CC’) illustrating S-wave velocity variations beneath the Kumburgaz Basin. Panel (c) shows the southwest–northeast profile (BB’), highlighting velocity anomalies, and related seismic activity. Panel (d) presents an east-west section (AA’), showing shear-wave variations along major geological structures. The velocity models were prepared by Turunçtur et al. (2023), and the black line in each section represents the locking depth estimated by Schmittbuhl et al. (2016) (Eken et al., 2025).
Is this a sign of a larger earthquake to come?
While scientists avoid making definitive predictions, the study's findings once again strongly highlight the risk of a major earthquake long anticipated in the Marmara region. The 2025 earthquake may have broken the transition zone, transferring stress eastward; this appears to have further increased the already high tension on the Kumburgaz fault segment. Previous studies estimated the probability of an earthquake of magnitude Mw 7 or greater occurring on this fault segment in the coming decades to be between 35% and 70%. The new findings indicate that the 2025 earthquake is part of a chain of stress transfer that began with the 1999 Izmit earthquake and continues westward, advancing this process. In short, the April 23, 2025 earthquake is thought to be not just an isolated event but part of a larger geodynamic process.
Why is this study important for the scientific community and Istanbul?
The study brings together numerous advanced techniques, ranging from teleseismic (distant area/region) wave modeling to regional moment tensor solutions, finite-fault slip modeling, and high-resolution seismic velocity imaging, to provide a comprehensive picture of the behavior of the Marmara Main Fault. The results provide important clues about where a major earthquake could start beneath the Sea of Marmara and how stress is transferred along the fault. This information is not only scientifically important but also vital for Istanbul, home to more than 20 million people, because urban transformation, disaster management, and risk reduction strategies can be shaped by such data.
More information about this study can be found at the following web link:
https://link.springer.com/article/10.1007/s10950-025-10342-8
Related Article: Eken, T., Taymaz, T., Yolsal-Çevikbilen, S., Irmak, T.S., Erman, C., Özkan, B., Turunçtur, B., Kahraman, M. (2025). Source and Rupture Properties of the 23 April 2025 Mw 6.3 Silivri High–Kumburgaz Basin Earthquake Threatening İstanbul, NW Türkiye. Journal of Seismology, Vol. 29(5), https://doi.org/10.1007/s10950-025-10342-8.