An earthquake recorded in sand
In the Surkhandarya region of southern Uzbekistan, a major earthquake from roughly 2,000 years ago left behind dramatic geologic evidence. Beneath the foundations of ancient buildings, archaeologists and geologists found a field of preserved sand volcanoes. These features are the remnants of a powerful seismic event that occurred in this historically active tectonic area. The discovery provides a rare, detailed snapshot of a prehistoric disaster, a field of study known as paleoseismology.
The sand volcanoes are a direct result of soil liquefaction. This process happens when the intense shaking from an earthquake increases the water pressure in saturated, sandy soil. The pressure becomes so high that the soil loses its strength and behaves like a liquid. This slurry of sand and water then erupts to the surface through cracks and fissures in the ground, creating conical mounds that resemble miniature volcanoes. At the Bandikhan site, these eruptions were so forceful that they burst through the floors of existing structures, leaving a permanent record of the event frozen in time. The sand that erupted originated from the saturated alluvial deposits of the nearby Amu Darya river basin.
Clues to a season and a magnitude
The placement of these sand volcanoes directly underneath the architectural remains of an ancient settlement is important for dating the earthquake. The structures provide a clear stratigraphic marker; the earthquake had to have occurred while the settlement was occupied, approximately two millennia ago. The buildings effectively protected the fragile sand structures from erosion, preserving them for modern study.
Analysis of the eruption patterns offers further detail. The shape and extent of the sand boils suggest the ground was relatively dry at the surface when the earthquake hit. This indicates the water table was at its lowest point, which in this region corresponds with the late summer months. Had the quake occurred in the wetter spring season, the liquefaction features would have been more widespread and less defined. This specific detail allows scientists to pinpoint the season of the event with a high degree of confidence.
Based on the size of the sand volcanoes and the geographic extent of the liquefaction field, seismologists estimate the earthquake had a magnitude of 7.0 or greater. The event was powerful enough to cause widespread destruction to the settlements of the time, which were part of the ancient region of Bactria. The Bandikhan site shows the immense power of this ancient quake, providing data on the seismic hazards in this part of Central Asia. Uzbekistan's territory is largely covered by loess soils, which are susceptible to liquefaction and subsidence during seismic events.