Observations of Seismic Activity Using Fiber Optic DAS in the Sea of Marmara

The Istanbul Natural Gas Distribution Company has started monitoring seismic activity within the Sea of Marmara using a fiber-optic (F/O) cable integrated with a Distributed Acoustic Sensing (DAS) system in order to mitigate secondary disasters that may occur after earthquakes and to protect critical infrastructures, such as pipelines. The monitored F/O cable, originally designed for telecommunications purposes, extends over a length of 60 kilometers beneath the Sea of Marmara. In 2022 October, this cable is integrated with a DAS system through an interrogator unit, installed at Tavşantepe Metro Station. The system consists of an analyzer that allows detection up to 40 kilometers, operates with a spatial channel spacing of 10 meters, in total 3910 channels, and a sampling rate of 200 Hz, enabling high-resolution seismic data acquisition. The cable’s route follows several critical regions: it enters the Sea of Marmara, traverses Büyükada, runs behind the Princes' Islands parallel to the Marmara Fault, intersects the fault at multiple locations, and ultimately terminates on land at Ambarlı. This strategic placement provides extensive coverage for monitoring seismic activity along this geologically active region.

Since the beginning of 2023, more than 500 earthquakes, with magnitudes ranging from 0.7 to 7.8, have been recorded using the F/O cable. Our analysis reveals that the quality of recorded seismic signals is strongly influenced by two factors: the incidence angle of wave on the cable and the cable's coupling with the ground. Poor coupling reduces the energy transfer from the ground to the cable, leading to weaker or distorted signals, while unfavorable incidence angles of wave, affect the strain response detected by the DAS system. These findings highlight the importance of optimizing cable placement and ensuring effective coupling for reliable seismic monitoring.

The developed algorithms have enabled the real-time automatic detection of earthquakes occurring within and around the Sea of Marmara using the F/O cable, and the initial results have been promising. The first real-time detection is accomplished for the M3.9 Çanakkale earthquake occurred on 19 November 2024 at 07:46:15 UTC. The F/O cable detects the earthquake 33 seconds following its occurrence, and the system sent an automatic detection notification approximately 1 second later after detection.

As part of our project, at the beginning of January 2025, a vessel-based survey is conducted to determine the submarine position of the F/O cable passing beneath the Sea of Marmara. This study contributes to improving the application of DAS in submarine seismic observation and highlights potential challenges in data acquisition from F/O cables.