Paleoseismic Investigations in the Selenga River Delta (Lake Baikal)

The article analyzes possible mechanisms behind the formation of Proval Bay, Lake Baikal, as a result of the Tsagan earthquake (12 January, 1862, \M = 7.5 +/- 0.3) and estimates the recurrence rate of destructive earthquakes in the Selenga River delta based on investigations of seismic deformations in mine workings. Earlier unpublished results of seismogeological and geophysical studies are presented. Geological and geophysical studies of seismic dislocations in the epicentral zone of the Tsagan earthquake indicate that the source of this earthquake could have been associated only with the major Morskoy fault. The displacement along this fault, as well as the one along the zone of the branching Del'tovyi (Delta) fault, contributed to the occurrence of a tsunami and to the landslide mechanism behind the formation of Proval Bay. Paleoseismological interpretation of the trench sections and the results of radiocarbon dating show that seismic events similar to the Tsagan earthquake also occurred in 1399-1496 CE, 1256-1401 CE, 887-1150 CE, and 233 BCE-221 CE in the north of the Selenga River delta. This gives an average recurrence rate of destructive earthquakes in the region as once every 500 years. Trenching and geophysical studies have established the features of oblique-thrust slip kinematics in Holocene, Pleistocene, and Neogene-Quaternary sediments along the Delta fault. It is possible that a geodynamic compression regime took place over relatively short-term intervals. Our conclusions do not contradict the geological and geomorphological conditions in the region or the historical information about the dislocations and surface effects of the Tsagan earthquake.