23 April 2025 Marmara Sea (Mw 6.3), Türkiye earthquake: mainshock, aftershock, and ground observations

On 23 April 2025, a Mw 6.3 earthquake struck the Sea of Marmara near the Kumburgaz segment of the North Anatolian Fault (NAF), triggering over 500 aftershocks within 15 days. This study presents a rapid assessment of the event through aftershock relocation using double-difference technique, full moment tensor inversion of the mainshock, and ground motion analysis. The mainshock exhibited a strike-slip mechanism at a depth of 6 km with a significant non-double-couple component (40%). The aftershocks mostly occurred east of the mainshock, primarily within 10 km depth. Shakemaps derived from ground motion recordings highlight peak ground accelerations exceeding 210 cm/s2 east of the mainshock in western Istanbul and Modified Mercalli Intensities reaching level 6. The ground motion prediction equation developed for the region slightly underestimated the peak ground motions in short-period pseudo-spectral acceleration (PSA) and peak ground acceleration (PGA). Comparison with Turkish seismic design codes revealed that short-period PSA reached code limits in some stations, raising concerns for structural resilience especially in older buildings in those areas.