High Resolution Microseismicity in the İzmit Basin-Sea of Marmara from OBS Data

The İzmit Basin, located at the easternmost part of the Sea of Marmara, constitutes a key segment of the North Anatolian Fault system where the main fault strand bifurcates into northern and southern branches. This structurally complex transition zone is characterized by interacting fault segments and distributed deformation, making it a critical area for investigating microseismic activity.  The dataset analyzed in this study was acquired by an ocean-bottom seismometer (OBS) deployment in the İzmit Basin. Eight OBS stations equipped with 4.5 Hz geophones were deployed in September 2023 and recovered in July 2024, providing approximately 10 months of continuous three-component seismic recordings. The instruments recorded data at a sampling rate of 100 samples per second, enabling the detection of local microearthquakes with magnitudes down to approximately M 0.2.

Initial earthquake locations reveal dense microseismic activity distributed along both the northern and southern branches of the North Anatolian Fault, as well as within intervening fault segments and fracture zones connecting these branches. These preliminary patterns highlight active deformation within the basin but exhibit significant scatter in both epicentral location and hypocentral depth, primarily due to velocity-model uncertainties and sediment effects inherent to offshore OBS observations. Reliable earthquake locations, in both horizontal and vertical dimensions, are critical for resolving fault-specific seismicity patterns and for investigating depth-dependent variations in seismic parameters, such as b-values. To improve location accuracy, events were refined using VELEST (Kissling et al., 1994), which iteratively optimizes a one-dimensional P- and S-wave velocity model and relocates earthquakes by minimizing travel-time residuals.

The initial VELEST velocity models display substantial scatter at shallow depths, particularly within the upper 0–5 km, reflecting strong sediment-related velocity uncertainties typical of offshore OBS observations. Following iterative relocation and velocity model optimization, the final VELEST solutions show clear convergence of both P- and S-wave velocities. The most pronounced improvement is observed within the 3–12 km depth range, corresponding to the main seismogenic layer. At depths around 10 km, the optimized models yield Vp values of approximately 5.8–6.2 km/s and Vs values of approximately 3.3–3.6 km/s, consistent with previous studies in the Marmara region and indicating a physically meaningful, data-driven velocity structure. The VELEST-based iterative velocity model optimization substantially reduces velocity uncertainty within the seismogenic depth range (approximately 5–15 km), leading to improved hypocentral depth and overall location reliability. The refined locations delineate coherent patterns of microseismic activity along the fault branches and connecting structures, providing a high-resolution view of seismic deformation in the İzmit Basin.