Distributed Acoustic Sensing Observations of the 23 April 2025 Mw 6.2 Marmara Earthquake in Northwestern Türkiye and Its Comparison with Borehole Seismometer Data

Summary The 23 April 2025 Mw 6.2 Marmara earthquake near İstanbul in northwestern Türkiye provided a rare opportunity to evaluate the capability of distributed acoustic sensing (DAS) for near-fault seismology in a densely populated and tectonically active region. The DAS monitoring complements the borehole-based Geophysical Observatory at the North Anatolian Fault (GONAF) expanding near-fault observatory towards high-resolution monitoring of deformation along the Main Marmara Fault (MMF) in the vicinity of Istanbul. In this experiment, an existing telecommunication dark fiber within the national optical network was repurposed as a long-offset submarine seismic array without any dedicated underwater installation. This fiber-optic cable, which extends underwater along the southern coast of Istanbul and follows the northern boundary of the Princes Islands, successfully recorded the Mw 6.2 Marmara earthquake. In this study, we analyzed the resulting dataset, which provides an aperture of approximately 34 km with 8 m channel spacing and a 250 Hz sampling rate, and includes sections extending both parallel and perpendicular to the fault. The DAS strain rate recordings of the Mw 6.2 mainshock exhibit clear P- and S- wave arrivals across thousands of channels with high signal coherence. After amplitude and phase calibration, the recordings were converted into equivalent particle velocity using a gauge length based transfer function and compared with nearby GONAF borehole seismic waveform data. Spectral analyses show that the DAS array preserves seismic energy up to ~25 Hz, with spectral shape and amplitude distributions consistent with those of the borehole recordings. Curvelet, f–k, and time domain slowness based conversion methods were employed for the transformation process. The submarine DAS array interrogated by an OptoDAS system enables high resolution spatial sampling of strain derived from the recorded strain-rate data. Systematic variations in apparent velocity (Vp ≈ 2.38 km/s, Vs ≈ 2.30 km/s) and amplitude decay are observed, which could possibly represent shallow sediment heterogeneity along the fiber. Short-wavelength fluctuations in strain are present, which may reflect local structural complexity as well as cable-related factors such as orientation and burial conditions. Overall, these results demonstrate that existing dark-fiber infrastructure can be used as long-aperture seismic arrays for capturing local seismic wavefields providing high-density measurements useful for monitoring near-fault strain variations. This study shows the long-offset DAS recording of a moderate size earthquake in Türkiye and one of the few submarine DAS observations worldwide. It demonstrates that dense strain measurements derived from fiber-optic infrastructure can complement the monitoring of offshore active fault segments, offering unprecedented spatial resolution for monitoring dynamic strain, and fault zone deformation in regions of high seismic hazard such as the Sea of Marmara.