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

Moment magnitude (M_w) is 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 seismic hazard assessment studies (Onur et al., 2020). Yet, a robust estimation of M_w and radiated energy (e.g., apparent stress) over a wide range of magnitudes is difficult, mainly due to the existing strong lateral heterogeneous nature of the crust in various tectonic regimes. Additionally, the extrapolation or linking of short-period magnitudes like M_L to M_w can often lead to significant bias (e.g., Shelly et al., 2022). To address this issue, we employ a coda envelope-based source spectral method, which uses a regional empirical calibration approach by lowering the threshold for reliable M_w and radiated energy estimation. In order to achieve this objective, in this study we analyzed three-component digital waveform recordings of 51 moderate local and regional earthquakes (M_L ≥ 4.0) that occurred from 2013 to 2022 in and around the central North Anatolian Fault Zone (NAFZ), including the 18 April 2024 M_w 5.7 Sulusaray (Tokat) earthquake and the 23 November 2022 M_w 6.1 Gölyaka (Düzce) earthquake, both with two notable aftershocks. Data with 100 Hz sampling rate were collected from 51 broadband stations operated by the Kandilli Observatory and Earthquake Research Institute (KOERI) and the Disaster and Emergency Management Presidency (AFAD). Using the Java-based Coda Calibration Tool (CCT), which applies the empirical approach developed by Mayeda et al. (2003) and efficiently processes seismic coda envelopes (Barno, 2017) for further calibration, we successfully implemented the coda-derived source spectrum method to calculate apparent stress and moment magnitude (M_w) across different regions. Following the calibration with reference events (apparent stress and moment-tensor M_w’s), we plan to extend reliable magnitude estimation to smaller earthquakes (M_L < 4.0), confirming robustness in these predictions. Our results provide a more thorough catalog of seismic events in the NAFZ, thereby contributing to improvement of regional seismic hazard assessments. This approach may also serve as a framework for reliable small-to-moderate earthquake analysis in other tectonically active regions, thus supporting broader seismic risk management efforts.