An accidental seismometer
The Shardara Reservoir, a vast artificial lake on the Syr Darya River, is an unintentional and sensitive earthquake detector. This huge body of water, with a surface area of around 783 square kilometers and a volume of over 5 cubic kilometers, is prone to a phenomenon called a seiche. A seiche is a standing wave that oscillates in an enclosed or partially enclosed body of water, much like water sloshing back and forth in a bathtub. While wind or changes in atmospheric pressure can cause seiches, the most intriguing oscillations at Shardara are triggered by seismic activity thousands of kilometers away.
Hydrodynamic models of the reservoir reveal that it has a natural period of oscillation (or a resonant frequency) of approximately two hours. When long-period seismic surface waves from a distant earthquake pass through the region, they cause the ground to tilt almost imperceptibly. This slight, slow tilting is enough to disturb the massive volume of water in the reservoir. If the frequency of the seismic waves matches the reservoir's natural two-hour period, a resonance effect occurs, amplifying the initial disturbance into a measurable, long-lasting standing wave.
Responding to remote tremors
The standing waves generated in Shardara are not violent or destructive; they show a clear signal of a distant seismic event. The phenomenon makes the entire reservoir a natural seismometer. The specific type of seismic waves responsible are called Rayleigh waves, which are surface waves that cause the ground to move in an elliptical, rolling motion. These waves can travel vast distances from an earthquake's epicenter.
Events more than 1,000 kilometers away can initiate these seiches. This is possible because the energy from a powerful earthquake spreads over enormous distances. While the ground movement at Shardara is far too small for a person to feel, the enormous, contained body of water acts as an amplifier, showing the ground movement. The study of these "seismic seiches" gives data to seismologists. The term was first used in 1955 to describe oscillations in Norwegian lakes caused by the 1950 Assam earthquake in India. The 1964 Alaska earthquake, a particularly powerful event, caused seiches across North America and even in swimming pools in Puerto Rico. The oscillations in the Shardara Reservoir are a modern example of this fascinating geophysical connection.