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

Abstract—The paper analyzes the results of shallow geophysical studies and seismicity of the Selenga Delta and adjacent areas of the South Baikal Basin, where the destructive Tsagan earthquake of 1862 with M7.5 occurred. This study area is characterized by the formation of epicentral zones, which overlap each other in a en echelon–like manner, as well as the seismic “quiescence” of the eastern segment of the Delta Fault, to which the dislocations of the Tsagan earthquake are confined. The distribution of seismicity and similarity of coseismic effects during the Tsagan and Middle Baikal (1959, M6.8) earthquakes, allow us to suggest that the 1862 shock was caused by displacement along a fault within the water area, either along the lateral Coastal Fault or along the intradepression Middle Baikal Fault, the length of which allowed an earthquake with M > 7 to occur. The activation of one of these faults in 1862 led to subsidence of the northeastern block of the Delta Trough with the opening of faults along its perimeter, including along the Delta Fault. The consequence of activation of the medium after strong events and clustering of strong shocks in time is a large number of weak shocks, reflected by the increased value of the slope of the recurrence graph (γ = –0.54 ± 0.01). According to geophysical data, the range of vertical movements during the last seismotectonic activation cycle reached 35 m.