Home – Home – Jornals – Russian Geology and Geophysics 2006 number 9

We investigated signature of Middle Pleistocene soft sediment deformation in lacustrine deposits of the Kok-Moinok basin (a bay of Lake Issyk-Kul', western Issyk-Kul' region) and its implications for past seismicity. Convolute structures are exposed in interfingering shallow lacustrine, beach and fluvial sediments. Other deformational structures we encountered exist as marl intrusions in coarse pebbly sand whereby the latter progresses upward making a sort of pillars. We used seven field criteria for relating soft sediment deformation to paleoseismic triggering which provide strong evidence for the seismic origin of the structures: (1) proximity to active seismic zones; (2) potentially liquefiable composition of sediments which are soft metastable sand and low-cohesion clay; (3) similarity to structures formed experimentally and those produced by natural seismic shaking; (4) absence of detectable effects of slope instability (gravity sliding); (5) stratigraphic position of deformation units between undeformed sediments; (6) lateral extent and regional abundance of deformational structures correlated over large areas; (7) cyclic repetition of structures in the section. We infer from empirical relationships between earthquake magnitude and the maximum epicentral distance to liquefaction sites that deformation was most likely associated with events in the highly seismic active epicentral zone north of Lake Issyk-Kul'. The earthquake-induced deformational structures record high seismic activity already in the Middle Pleistocene.