Liquefaction-induced flow slide in the collapsible loess deposit in Soviet Tajik

In the suburb of Dushanbe, Tajikistan Republic of the Union of the Soviet Socialist Republics, an earthquake of magnitude 5.5 took place on January 23, 1989. In this event, extensive liquefaction developed in the loess deposit of aeolian origin in the gently sloping hilly terrain and led to a series of catastrophic landslides accompanied by a large-scale mud flow. In contrast to the hitherto known cases of liquefaction which have usually occurred in water-sedimented sand deposits, the liquefaction in Tajik was unique and novel in that it occurred unexpectedly in a wind lain deposit of silt in a semi-arid region. The reasons for such liquefaction are thought to be the collapsible nature of highly porous loessial silt which had been wetted by irrigation water over the past years. The complete collapse of the loess structure due to the additional action of the seismic shaking appears to have led to the catastrophic landslide. In addition, the silt-sized soil constituting the loess was of low plasticity and, hence, could easily slump and flow through a distance as long as 2.0 km.