Source scaling of earthquakes in and around the North Anatolian Fault Zone based on coda-derived source spectra: towards a more accurate and unbiased M_w catalogue

SUMMARY: Moment magnitude (M_w) is a widely accepted magnitude scale as a direct physical measure of the long-period seismic energy released at the foci, and thus its reliable quantification is of great importance for accurate probabilistic seismic hazard assessment studies. Yet, a robust estimation of M_w and radiated energy (E_R) over a wide range of magnitudes is difficult, mainly due to the existing strong lateral heterogeneous nature of the crust. Furthermore, converting short-period magnitudes such as local magnitude (M_L) to M_w can often lead to significant bias. To address this issue, we employ a coda envelope-based source spectral method, which depends on a regional empirical calibration approach that can lower the threshold for reliable M_w and E_R estimates. To achieve this aim, we analysed horizontal component waveforms recorded at broad-band stations operated by the Kandilli Observatory and Earthquake Research Institute and the Disaster and Emergency Management Presidency from 51 selected moderate local and regional earthquakes (M_L ≥ 4.0) that occurred between 2013 and 2024 in and around the central North Anatolian Fault Zone (CNAFZ), including the 2022 November 23 M_w 6.0 Düzce and the 2024 April 18 M_w 5.6 Tokat events. The Java-based Coda Calibration Tool implemented on these waveforms enabled a successful establishment of the coda-derived source spectrum that allowed us to obtain robust estimates of apparent stress (σ_A) and M_w across the CNAFZ. Following the calibration with reference events, we extend reliable magnitude estimation to smaller earthquakes (3.5 ≤ M_L < 4.0), confirming robust predictions. Beyond providing a more thorough event catalogue in the CNAFZ, our results reveal low σ_A levels, comparable to those reported in recent studies in the Marmara Sea, and indicate non-self-similar rupture behaviour that may improve future regional seismic hazard assessments. This approach may also serve as a framework for reliable small-to-moderate earthquake analysis in other tectonically active regions in Türkiye, thereby supporting broader seismic risk management efforts.