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Based on the amplitude spectra of surface waves, the source parameters of the strong Turkish earthquakes on February 6, 2023 (Mw = 7.8 and Mw = 7.7) were calculated in two approximations: an instantaneous point source and an elliptical shear dislocation. As a result, fault planes were identified, data were obtained on the scalar seismic moment, moment magnitude, focal mechanism, and source depth of the considered seismic events, and the integral parameters characterizing the fault geometry and its development in time were estimated. It is shown that the sources of the earthquakes under study were formed under the action of the regional stress field and their focal mechanisms were sinistral strike-slips with a strike direction close to the strike of the East Anatolian fault zone for the first event and close to the strike of the Sürgü-Çardak fault system for the second one. For the first earthquake, our estimates of the rupture duration and its length (t = 52.5 s, L = 180 km) probably refer not to the entire rupture, but only to its main phase, confined to the northeastern segments of the East Anatolian Fault and characterized by maximum displacements and values of the released seismic moment. The values of t = 30 s and L = 180 km that we obtained for the second earthquake fully characterize the entire rupture. [ABSTRACT FROM AUTHOR]

We study in detail the M w 5.4 September 6, 2021 Tofalaria earthquake occurred in a mountain area of the Eastern Sayan which is characterized by a low level of seismic activity. An interest in the seismic event is caused, on the one hand, by poor knowledge about stress-strain field of the crust in the considered region, and, on the other hand, by its relation to the NW segment of the ancient Main Sayan fault – a structural boundary between the Sayan-Baikal fold belt and the tectonically stable Siberian platform. Seismic moment tensors and hypocentral depths of the mainshock and its largest aftershock (M w 4.6) are inverted from intermediate-period surface wave amplitude spectra calculated at the stations located at teleseismic distances. Integral source parameters of the mainshock, characterizing its spatio-temporal development, are also estimated and the fault plane is determined. Epicenters of 31 aftershocks with M ≥ 1.8, occurred up to the end of 2021, are constrained from body waves recorded at regional seismic stations. The obtained results show that the Tofalaria earthquake occurred under the influence of the SW-NE compression, which is observed in Western Mongolia. Focal mechanism of the largest aftershock and the NE elongation of the aftershock epicentral field (22 km) indicate stress redistribution after the mainshock in a local crustal volume, bordered by small-scale faults. • Source parameters and aftershock process of the Tofalaria earthquake (Eastern Sayan) are studied. • The Tofalaria earthquake occurred under the influence of the SW-NE compression. • The Tofalaria earthquake caused stress redistribution in a local crustal volume, bordered by small-scale faults. [ABSTRACT FROM AUTHOR]

We study the M w 5.6 Bystraya earthquake, occurred on September 21, 2020 at the SW flank of the Baikal rift zone in the area, in which only few small seismic events have been detected during the whole period of instrumental observations. Source parameters of the mainshock and its largest aftershock (M w 4.7) are inverted from intermediate-period surface wave amplitude spectra. Hypocentral depths and seismic moment tensors in a double-couple approximation are calculated for both earthquakes, while integral source characteristics, describing its spatio-temporal development, are estimated only for the mainshock. For the Bystraya earthquake, directivity effects (rupture direction and velocity) are also modelled. The obtained results show that the study seismic event can be associated with activation of the SE segment of the Main Sayan fault. The faults of the mainshock and its largest aftershock are formed under the influence of the NW-SE extension and NE-SW compression. The predominance of left-lateral strike-slip motions agrees well with previous studies at the SW flank of the Baikal rift zone. A small thrust-fault component could reflect the inversion uplift of some blocks of the Bystraya basin bottom as well as the Bystrinskaya Sopka massif. The dipping of the earthquake fault plane could indicate flattening of the considered fault segment at some depth or the existence of differently oriented structural discontinuities inside the fault zone of a finite width. The long rupture of the Bystraya earthquake could be connected with its almost pure strike-slip focal mechanism and orientation of the rupture plane approximately along the Main Sayan fault strike. [Display omitted] • Source parameters of the Bystraya earthquake (SW flank of the BRZ) are studied. • The event is associated with activation of the SE segment of the Main Sayan fault. • The obtained results indicate the inversion uplift of some blocks of the Bystraya basin bottom. • Geometry and kinematics of the SE segment of the Main Sayan fault are discussed. [ABSTRACT FROM AUTHOR]