Seismotectonics and Seismicity in the Area of the Tsagan Earthquake (1862, M7.5, the Selenga Delta, Baikal)

The paper analyzes the results of shallow geophysical studies and seismicity of the Selenga delta and adjacent areas of the South Baikal depression, where the catastrophic Tsagan earthquake of 1862 with M7.5 occurred. This study area is characterized by the formation of epicentral zones, substituting each other in an echelon manner, as well as seismic “silence” of the eastern segment of the Deltaic fault, to which the dislocations of the Tsagan earthquake are confined. The distribution of seismicity, as well as the similarity of coseismic effects during the Tsagan and Middle Baikal (1959, M6.8) earthquakes, allow us to assume that the shock of 1862 was caused by a fault slip within the water area, either along the flank Beregovoy fault or along the intra-depression Middle Baikal fault, which length allows an earthquake with M > 7 to occur. The activation of one of these faults in 1862 led to the subsidence of the northeastern block of the Delta trough with the opening of ruptures along its perimeter, including along the Delta fault. The consequence of the activation of the environment after strong events and the clustering of strong shocks in time is a large number of weak shocks, which is reflected in the increased value of the recurrence graph slope (γ = −0.54 ± 0.01). According to geophysical data, the range of vertical movements during the last cycle of seismotectonic activation reaches 35 m.