Microearthquakes and Seismicity in the Marmara Sea: An Analysis Using OBS Data

The Sea of Marmara, located along the North Anatolian Fault (NAF) system, represents a geologically significant region where the fault’s bifurcation into northern and southern branches plays a key role in shaping the region's tectonic framework.  The NAF, acting as a primary tectonic driver, has not only shaped the basin's morphology but also controlledits seismicity, making the region a key area for studying active tectonic processes and earthquake hazards. The İzmit Gulf, located at the easternmost part of the Marmara Sea, is of particular tectonic significance as it marks the location where the NAF splits into its northern and southern branches. This bifurcation creates a structurally complex and seismically active environment characterized by fault interactions and the transfer of stress, offering a natural laboratory for investigating the mechanisms of fault segmentation, branching, and seismic activity in this tectonically intricate region. These insights are essential for understanding the broader dynamics of the NAF system and for assessing seismic risks in northwestern Turkey.
Given the seismic risk and tectonic complexity of the Marmara Sea region, in September 2023, eight Ocean Bottom Seismometers (OBS) equipped with 4.5 Hz geophones were deployed across the Sea of Marmara, including the İzmit Gulf, at depths ranging from 145 to 1269 meters. These instruments were operated for 10 months, recording seismic data with a high sampling rate of 100 samples per second (sps). The data were manually analyzed through visual inspection to pick P- and S-wave arrivals enabling precise calculation of earthquake locations, depths, and magnitudes. Preliminary analysis of 3 month data we identified 45 micro-earthquakes that were not present in land-based seismic catalogs, demonstrating the enhanced detection capability of the OBS array. The OBS network achieved a minimum detectable earthquake magnitude of 0.3, significantly improving the resolution of seismic monitoring in the region. These micro-earthquakes were primarily clustered along the northern and southern branches of the NAF with distinct waveform characteristics suggesting localized fault activity and varying focal depths. 
The findings also revealed variations in seismic clustering patterns between the northern and southern fault branches. Seismic activity along the northern branch was observed to have distinct waveform characteristics and shallower focal depths compared to the southern branch, indicating differences in fault behavior and stress accumulation processes. These insights into fault dynamics underline the importance of high-resolution OBS data in characterizing microseismic events and understanding fault interactions within this tectonically complex region. In future stages of the study, AI-based automatic modules will be utilized to process the same dataset and their performance will be compared with manual analysis to evaluate their relative advantages. This approach is expected to streamline data processing and improve the accuracy and efficiency of microseismic event detection.
The findings demonstrate the critical importance of OBS technology in advancing our understanding of the tectonic and seismic behavior of the Marmara Sea region. Continuous, real-time monitoring of the Marmara Sea is essential for capturing microseismic events and detecting early signs of larger seismic activity.