The 1950 M w = 6.9 Mondy earthquake in southern East Siberia and associated deformations: facts and uncertainties

Abstract

We investigated the surface deformations of one of the strongest earthquakes in the Baikal rift zone occurred on April 04, 1950. This event gave an impulse to the development of seismological and seismotectonic researches in southern East Siberia though the event remained poorly studied. We established that the source of the M w = 6.9 Mondy earthquake manifested on the surface as a rupture zone 5.4 km long and at least 0.6–1.7 km wide. The rupture zone comprises northern and southern segments trending WNW–ESE (nearly E–W) and NW–SE, respectively. The structural and geological observations testify to left-lateral slip along nearly E–W trending ruptures that is consistent with the recent solution of focal mechanism and previous geomorphologic observations. The open question concerns the vertical component of displacement as the direct records in the excavations show both normal and reverse offsets. The Mondy earthquake initiated the active development of thermokarst, which is expressed on the surface by subsidences and sinkholes spatially associated with the seismogenic ruptures. The melting of sporadic permafrost in the fracture zones and temporary streams contribute to a good state of preservation of the surface ruptures. Numerous neptunian dikes are revealed on the GPR profiles. Their formation is associated with both filling the blind seismogenic fractures and fall of sediments in the sinkholes. Some drawdowns associated with a subsidence as a result of ejected liquefied sand. Outside of the 1950 earthquake, at least two other rupturing palaeoevents occurred in the study area after 4628–5212 BP and 2968–3241 BP. The obtained results provided a basis for further investigations in the epicentral area of the 1950 earthquake in order to study the dynamics of the postseismic exogenous processes, search new surface ruptures east of the village of Mondy and constrain the recurrence interval of earthquakes along the Mondy fault.