Robustness of Focal Mechanism Solutions for Strong Earthquakes in the Southern Baikal Region (Mw ≥ 4.9, 2020–2023) Depending on Methods and Crustal Velocity Models

We consider the influence of different methods and crustal velocity models on the focal mechanisms of strong earthquakes (Mw ≥ 4.9) in the Southern Baikal Region in 2020–2023. For each seismic event, source parameters were estimated using three approaches: from P-wave first-motion polarities (FM method), by inverting waveforms from regional broadband seismic stations (ISOLA method), and from the joint inversion of surface wave amplitude spectra, recorded at remote broadband stations, and P-wave first-motion polarities (SWP method). Depending on the location of the earthquake epicenter, its source parameters were estimated using a corresponding set of crustal velocity models. In total, seven velocity models obtained by different authors were considered. The results showed that, within a single method, different crustal velocity models do not significantly affect the stability of the focal mechanism solutions (the Kagan angle Ф, an angle in 3D space between two focal mechanisms, is generally less than 10°). This is because the wavefield recorded at regional (∆ > 100 km) and teleseismic distances, with wavelengths significantly exceeding the rupture length, is not sensitive to the details of the velocity structure of the geological medium. When comparing solutions obtained by different methods, a slight variation in the estimated parameters is also observed (most Ф-values are less than 30°). Thus, the example of strong earthquakes in the Southern Baikal Region (Mw ≥ 4.9, 2020–2023) demonstrates that robust estimates of source parameters can be obtained using simplified crustal velocity models. This study also confirms the potential of tensor solutions based on the joint use of teleseismic and regional data. In this case, the hypocentral depth and energy characteristics are determined along with the focal mechanism solution.