An Overdue Tectonic Shift
On July 11, 1889, at 3:14 PM local time, the ground in the northern Tien Shan mountains began to move with catastrophic force. This event, known as the Chilik earthquake, was one of the largest continental earthquakes ever recorded, with an estimated moment magnitude of 8.0 to 8.3. The shaking was so intense that people and animals were reportedly thrown off their feet. While the city of Verny (modern-day Almaty) sustained heavy damage, the deepest impact occurred across the landscape itself. The earthquake triggered massive landslides and, most remarkably, deformed the ground so severely that it diverted the course of three rivers, including shifting a section of the Ili River by two kilometers.
The quake was the result of a complex, multi-segment rupture along the Chilik-Kemin fault zone, a major structure that manages the immense pressure of the Indian plate colliding with the Eurasian plate. This tectonic collision is the same process that formed, and continues to build, the Tien Shan mountains and the Himalayas. The 1889 event produced a visible surface rupture extending for at least 175 kilometers, with some estimates suggesting up to 300 km. In places, the ground was displaced by as much as 10 meters. Eyewitnesses reported ground cracks up to five meters wide and a kilometer long.
Reading the Scars in the Earth
For decades, the exact faults responsible for such a large earthquake were a puzzle, as the ruptures were not immediately obvious. Modern science provided the tools to solve it. Using satellite imagery and performing fieldwork, scientists identified a series of scarps, steep banks created by fault movement, which matched the scale of the 1889 event. To understand the history of this fault, researchers engage in paleoseismology, the study of past earthquakes. This involves digging trenches directly across the fault scarps to get a cross-section of the earth's layers.
Inside these trenches, geologists can see layers of soil and sediment that have been broken and shifted by past seismic events. By using techniques like radiocarbon dating on organic material found within these displaced layers, they can build a timeline of when the fault has ruptured. Studies on the Saty fault, one of the segments that ruptured in 1889, revealed a startling history. Before the 1889 event, the fault had not ruptured for at least 5,000 years. Other faults in the region show a more regular pattern. Paleoseismic trenching suggests that major earthquakes occur on this fault system every 400 to 600 years. Given that the last major rupture was in 1889, the fault is accumulating strain, and the next large earthquake is considered statistically overdue. This poses a significant hazard to the nearby city of Almaty, of approximately two million people.