A Foundation of Unstable Dust
The Fergana Valley, a large and fertile basin in Central Asia, rests upon thick layers of a peculiar type of sediment called loess. Loess is essentially wind-blown dust, composed of silt-sized grains of minerals like quartz and feldspar, loosely held together by weak carbonate cement. This material, ground into a fine powder by ancient glaciers in the surrounding Tien Shan and Alay mountains, was carried by wind and deposited across the valley floor over millennia. In some parts of Central Asia, these loess deposits can reach thicknesses of over 70 meters.
The structure of loess makes it hazardous. It is highly porous, with up to 40% of its volume being empty space, and is riddled with fine vertical tubes. While it can form stable, steep cliffs when dry, its structural integrity collapses when saturated with water and subjected to seismic shaking. The loosely packed, angular grains lose contact with each other, and the soil suddenly behaves like a dense fluid in a process known as liquefaction.
The Andijan Event of 1902
On December 16, 1902, a powerful earthquake struck the region near the city of Andijan. The quake registered a surface-wave magnitude of 6.4 and a maximum intensity of IX (Violent) on the Mercalli scale. The shallow focus of the earthquake, at a depth of about 9 kilometers, amplified the shaking on the surface. In the water-saturated loess deposits beneath Andijan, the intense vibrations triggered widespread liquefaction.
The consequences were catastrophic. The ground lost its strength, causing buildings to tilt and sink into the earth. In some documented cases, structures sank as much as 3 meters. The old city, with its traditional adobe and brick buildings, was almost completely leveled. Eyewitnesses reported that the shaking was so violent people were unable to stand. It was the deadliest earthquake in Uzbekistan's 20th-century history, claiming the lives of over 4,500 people and destroying more than 40,000 homes in the region. The Fergana Valley is a seismically active area, with the UN identifying it as a highly vulnerable zone due to the combination of seismic risk and dense population.