An Instantaneous Landform
In the early morning hours of May 17, 1976, a magnitude 7.0 earthquake violently shook the region near Gazli, Uzbekistan. The ground rupture from this event created an immediate and dramatic change in the landscape: a fault scarp. This feature, a small cliff up to two meters high, appeared in an instant where one side of the Earth's crust was thrust upwards relative to the other. This event was not isolated; it was part of a doublet, with another magnitude 7.0 earthquake having struck the same area just over a month earlier on April 8, 1976.
The Kitab-Shakhrisabz fault scarp is a surface expression of the much larger Gissar-Kokshaal fault system. This network of faults accommodates the immense pressure generated by the ongoing collision between the Indian and Eurasian tectonic plates. The 1976 earthquake was a thrust-faulting event, meaning older rock was pushed up and over younger rock. The rupture began at a depth of about 15 kilometers and propagated upwards, creating the visible scarp and causing intense ground shaking. The average dislocation, or slip, along the fault during the May 17 event was calculated to be around 3.3 meters.
Reading a Fault's Buried History
The scarp created in 1976 is part of a single earthquake; it is a part of a very long story of repeated seismic activity. The field of paleoseismology allows scientists to read this history by digging trenches directly across the fault line. When a fault ruptures and creates a scarp, it buries the existing ground surface, including the topsoil and any organic material within it. Over centuries, a new layer of soil develops on top of the buried surface. The next major earthquake repeats the process, creating a new scarp and burying the new soil layer.
By excavating trenches, geologists expose these stacked layers of ancient, buried soils, known as paleosols. They can then use radiocarbon dating on the carbon-based materials found within each distinct soil horizon. This technique measures the decay of the radioactive isotope Carbon-14 to determine the age of the organic matter. Analysis of trenches across the Kitab-Shakhrisabz fault has showed a clear pattern of repeated ruptures. The data show that a major earthquake, similar to the one in 1976, occurs on this specific fault segment with a recurrence interval of 800 to 1,200 years. This forecasts seismic hazard in the region.