Fiber-optic sensing for subsurface investigation in coastal areas at risk: Istanbul and the Sea of Marmara

Coastal areas are among the most densely populated areas on Earth, with 50% to 70% of the population projected to live in these regions in the next 50-100 years. Many large cities are located along tectonically active coastal areas, and the combination of increasing population, sea-level rise, and extreme weather events expose coastal regions to significant geohazard risk. Therefore, detailed characterization of the structure, physical properties and dynamics of the shallow subsurface in coastal urban areas is critical for geohazard assessment and mitigation. However, this task remains challenging, mostly due to limited access to the subsurface for the deployment of conventional sensors. In this context, Distributed Acoustic Sensing (DAS) deployed on existing, unused (“dark”) telecommunication networks offers an unprecedented opportunity to efficiently investigate subsurface seismic structure at high spatial and temporal resolution over tens of kilometers.

In this study, we establish an amphibious fiber-optic sensing testbed to investigate the subsurface structure and dynamics of the megacity of Istanbul (Türkiye) and the eastern Marmara Sea, one of Europe's highest earthquake risk areas. Istanbul is located approximately 20 km north of the North Anatolia Fault Zone (NAFZ), one of the World's most active faults. Since 2015, the GFZ Helmholtz Centre for Geosciences is operating the Geophysical Observatory at the Northern Anatolian Fault (GONAF) in collaboration with the Turkish Disaster and Emergency Management Presidency (AFAD). The observatory consists of 10 boreholes equipped with seismometer strings and partly with strainmeters, providing key information on seismicity and deformation processes in the Marmara Sea. Despite this efforts, high-resolution imaging of the NAFZ, and continuous recording of near-fault seismicity, aseismic deformation and slow-slip events remains challenging. Detailed data on near-city fault complexity, potential hidden faults directly underneath the urban area, and the spatial variability of subsurface material properties at high resolution is also still lacking. By integrating fiber-optics sensing, we expand and enhance the observatory by simultaneously providing critical data on offshore fault structure and seismicity and enabling efficient investigation of structure and seismic hazard along the coast.

Since May 2024, continuous passive seismic data have been recorded along two dark fibers in eastern Istanbul: a 17 km-long cable crossing the coastal district of Kartal, and a 34 km-long cable immediately offshore, connecting the coast with the Princess Islands. Both natural (i.e. ocean waves) and anthropogenic (traffic) seismic noise, as well as local and regional earthquakes have been captured by both fibers, enabling the characterization of the testbed and its potential and limitations. We apply ambient seismic noise interferometry approaches across multiple spatial scales and frequency bands for multi-resolution imaging, and explore the potential for temporal monitoring of subsurface variations associated with earthquake processes and environmental changes. We also assess the capabilities of the testbed to detect near-fault seismic events and improve seismicity catalogs. Ultimately, our study will provide a framework to leverage dark fibers in densely populated coastal areas for efficient subsurface imaging and near-fault monitoring, with significant potential to improve geohazard assessment.