A research team from Istanbul Technical University, led by Prof. Dr. Tuncay Taymaz (ITU Disaster Management Institute and ITU Faculty of Mines, Department of Geophysical Engineering), and consisting of Assoc. Prof. Dr. Tuna Eken, Prof. Dr. Seda Yolsal-Çevikbilen, and Research Assistants Ceyhun Erman and Berkan Özkan from the ITU Faculty of Mines, Department of Geophysical Engineering – Seismology, Seismotectonics and Geodynamics research group, presents the first and most detailed scientific analysis to date of the 23 April 2025 Mw 6.3 Silivri–Kumburgaz earthquake.

Combining advanced modeling techniques with precise aftershock analyses, the study reveals that this moderate-sized earthquake may be an important “wake-up signal” on a dangerous section of the Main Marmara Fault (MMF), located south of Istanbul and known to have remained silent for approximately 260 years.

The research findings were published in Journal of Seismology, one of the established journals in the field of seismology. During the study, additional contributions were made to the ITU team by Prof. T. Serkan Irmak (Kocaeli University, Department of Geophysical Engineering), Dr. Buse Turunçtur (CSIRO, Australia), and Assist. Prof. Dr. Metin Kahraman (Erzincan Binali Yıldırım University, Earthquake Technologies Institute).



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).

A Long-Feared Fault Segment Is Becoming Active Again

The 23 April earthquake occurred on one of the most critical segments of the North Anatolian Fault—the Main Marmara Fault beneath the Marmara Sea. This segment has long been considered one of the most dangerous seismic gaps in Türkiye due to its proximity to Istanbul, the absence of a major earthquake since 1766, its locked structural condition, and the large amount of accumulated stress. The research team determined that the earthquake occurred precisely within the transition zone between the locked region and the partially creeping segment. Globally, such transition zones are known to host moderate earthquakes while also serving as sensitive regions where major earthquakes can be triggered.

What Exactly Did the 23 April 2025 Mw 6.3 Silivri–Kumburgaz Earthquake Do?

Advanced modeling from the study shows that the shaking occurred through a two-stage rupture involving two separate asperities—areas that accumulate more stress than their surroundings. This explains why many people reported feeling two distinct strong shaking phases during the earthquake. As the rupture propagated westward into a more compliant zone, it weakened as it approached the fully locked Kumburgaz segment to the east. This locked section has long been identified by scientists as the most likely area to generate a future 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).

A Fault Awakened: What Do the Aftershocks Tell Us?

Researchers relocated 590 aftershocks using the high-precision HypoDD algorithm and uncovered a striking result: within the first 24 hours after the mainshock, the aftershocks migrated approximately 20 kilometers eastward, directly toward the locked Kumburgaz segment. This rapid migration indicates a clear stress transfer from the mainshock to the locked region. However, the aftershocks abruptly stopped upon reaching an area characterized by a known high-velocity (high Vs) crustal anomaly. This corresponds to a deep, strong rock block that can accumulate stress for long periods without breaking.

The study also found that the aftershocks did not cluster within the main rupture zone, but rather around its immediate surroundings—classical behavior often observed in moderate earthquakes that may precede larger events.

The figure displays the spatiotemporal distribution of aftershock activity following the 23 April 2025 Mw 6.3 Silivri–Kumburgaz earthquake (Eken et al., 2025).


Another 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?

While scientists avoid making definitive predictions, the results strongly underscore the long-recognized major earthquake risk in the Marmara region. The 2025 earthquake may have transferred stress eastward by rupturing the transition zone—possibly increasing the already elevated stress on the Kumburgaz segment. Previous research has estimated a 35% to 70% probability of an Mw 7 or larger earthquake occurring on this segment within the coming decades. The new findings indicate that the 2025 earthquake is part of an ongoing chain of stress transfer initiated by the 1999 İzmit earthquake and continuing westward. In short, the 23 April 2025 earthquake is not merely an isolated event but appears to be part of a broader geodynamic process.

Why Is This Study Important for the Scientific Community and Istanbul?

By integrating numerous advanced techniques—from teleseismic wave modeling and regional moment tensor solutions to finite-fault slip modeling and high-resolution seismic velocity imaging—the study provides a comprehensive view of the behavior of the Main Marmara Fault. The resulting insights offer guidance on where a major Marmara earthquake may initiate and how stress is transferred along the fault. Such information is critically important for Istanbul, home to more than 20 million people, and shapes strategies for urban transformation, disaster management, and seismic risk reduction.

< PUBLICATION >

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).

Journal of Seismology, Vol. 29(5), https://doi.org/10.1007/s10950-025-10342-8.

To read the full article : springer.com/article/10.1007/s10950-025-10342-8