A New Empirical Relation Converting Magnitude Applied in Bulgarian Seismological Practice $(\rm M_P)$ to Moment Magnitude $(\rm M_W)$

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Abstract
Earthquakes are some of the deadliest natural disasters affecting human environment. The kinematic and dynamic parameters of earthquakes are the basis of almost all research in the Earth sciences. The results of theoretical and applied seismology are used to explain the main social and economic problems associated with earthquakes and their consequences. Knowledge of the earthquake source parameters enables us to understand better the recent tectonic setting and earthquake generation processes. One usable method for estimating source parameters of earthquake is the spectral analysis of seismic waveforms. The earthquake source parameter – seismic moment, M0, has the largest practical and scientific application. From M0 seismic moment magnitude MW (that is a measure of the energy released by an earthquake) can be computed. In the present study, we aim to improve the scaling between MP (local magnitude applied in Bulgarian seismological routine practice) and MW. We extend the MW estimates to lower magnitudes via a spectral fitting method. Applying P-wave displacement spectra we estimate MW for 127 small to moderate earthquakes with magnitude ranging between 2.0–5.3 which occurred in Bulgaria and surroundings from 2008 to 2024. The considered earthquakes are recorded at stations of Bulgarian Seismological Network – NOTSSI. As a result, we propose a new empirical scaling relation between local magnitude MP and seismic moment magnitude MW. Additionally, 148 MW estimates from the seismological centres – ISC, EMSC and Harvard are used. The well-defined relation between MW and MP can be accurately used for compiling homogeneous, with respect to magnitude, earthquake catalogue for Bulgaria.

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