Urban-Scale Seismic Imaging Using Ambient Noise and Dark Fiber Distributed Acoustic Sensing in Istanbul

Urban areas are highly vulnerable to the impacts of geohazards due to their dense populations and complex infrastructure, with potentially severe consequences for human life and economic stability. Improving our knowledge of near-surface and shallow subsurface structures in urban environments is therefore essential for effective seismic hazard assessment and risk mitigation. However, conventional geophysical surveys in cities are often limited by logistical constraints, including strong anthropogenic activity, restricted access, legal limitations, and risks associated with instrument deployment. In this context, repurposing existing telecommunication optical fibers (so-called dark fibers) as dense seismic sensing arrays using Distributed Acoustic Sensing (DAS) offers a powerful alternative for urban subsurface investigations. This approach enables continuous, high-resolution seismic monitoring without the need for extensive field instrumentation.

The megacity of Istanbul (Turkey) is located in one of the most tectonically active regions worldwide and is exposed to significant seismic hazard. Since May 2024, we have been continuously recording passive seismic data using Distributed Acoustic Sensing (DAS) along an amphibious fiber-optic cable, is deployed in the urban district of Kartal (eastern region of Istanbul) and immediately offshore. In this study, we focus on the 3 km-long urban segments of the fiber. We analyze ambient seismic noise generated by various anthropogenic sources, such as train and vehicle traffic and other urban activities, and evaluate their suitability for high-frequency, DAS-based passive seismic interferometry in a complex and heterogeneous urban setting.

We develop and adapt processing strategies for ambient-noise interferometry that address the challenges of dense urban environments and DAS array geometries, including the identification of suitable fiber sections, channels, and source-receiver configurations, as well as preprocessing schemes designed for strongly anthropogenic noise.The objective is to retrieve high-resolution, urban-scale subsurface velocity models that improve our understanding of shallow structures and material properties relevant to seismic hazard. Ultimately, this work aims to establish efficient methodologies for imaging the urban subsurface using existing infrastructure, contributing to improved geohazard assessment and supporting sustainable urban development in seismically active regions.