The October 18 and 25, 2017 Earthquakes in Western Transbaikalia: Confirming the Present-Day Activity of Local Faults

This paper considers two earthquakes which occurred on October 18, 2017 and October 25, 2017 in western Transbaikalia. In spite of the moderate energy level of both events, they can be treated as significant for the study area, because such earthquakes have been recorded there relatively rarely compared with the adjacent high-seismicity areas in the Baikal Rift Zone. The mechanisms of both earthquakes based on surface wave amplitude spectra showed that these events occurred under a dominating east–west near-horizontal compression and an inclined or nearly vertical NW‒SE tension, which is typical for western Transbaikalia. For both earthquakes, we also computed source parameters: scalar seismic moment M₀ = 5.0 × 10¹⁵ N m, moment magnitude M_w = 4.4, and source depth h = 7 km for the October 18, 2017 event; M₀ = 3.5 × 10¹⁵ N m, M_w = 4.3, and h = 29 km for the October 25, 2017 event. These earthquakes have caused noticeable macroseismic effects in the near-field; the maximum observed shaking intensity was IV–V (MSK-64) during the October 18, 2017 earthquake and V during the October 25, 2017 earthquake. These data were the basis for our analysis of present-day activity of faults in the study area. The results may be helpful for more accurate assessment of earthquake hazard and seismic risk in western Transbaikalia.