A stable shield cracks
Before 1962, the Koyna region in Maharashtra, India, was considered seismically stable. It sits on the Deccan Traps, a massive province of volcanic basalt rock formed around 66 million years ago, which forms a part of the old and stable Indian Precambrian shield. The construction of the 103-meter-high Koyna Dam, which began in 1955 and created the enormous Shivajisagar reservoir, changed everything.
Soon after the reservoir began to fill in 1962, minor tremors started. This activity culminated on December 11, 1967 (local time), when a magnitude 6.3 earthquake struck the area. The epicenter was just a few kilometers from the dam. The event caused around 180-200 deaths, injured over 2,200 people, and left thousands homeless as masonry buildings in the nearby town of Koynanagar collapsed. This event is the world's most significant example of Reservoir-Induced Seismicity (RIS), showing that large-scale human engineering could trigger deadly earthquakes.
How water starts an earthquake
The 1967 earthquake was not a coincidence. The immense weight of the water in the Shivajisagar reservoir—with a storage capacity of over 2,800 million cubic meters—was a direct trigger. Scientists identified two primary mechanisms. First, the sheer mass of the water adds stress to the Earth's crust. Second, and more importantly, water percolates deep into the ground through fractures and fissures in the basalt.
This infiltration dramatically increases the "pore fluid pressure" within the rock. The high-pressure water acts like a lubricant on pre-existing, critically-stressed faults deep beneath the surface, reducing the friction that holds them in place. With the friction lowered, the faults can slip, releasing accumulated tectonic strain as an earthquake. The seismic activity at Koyna often correlates with the annual cycle of filling the reservoir during the monsoon and depleting it, a process that alters the stress on the underlying faults.
A global laboratory for induced quakes
The 1967 earthquake made the Koyna-Warna region a natural laboratory for studying how human activities can cause seismic events. The seismicity did not stop after the main shock. The area has since experienced 22 earthquakes with a magnitude of 5.0 or greater and thousands of smaller tremors. This persistent activity makes it the most significant site of reservoir-triggered seismicity globally.
To study the phenomenon, a dense network of seismographs, including borehole seismometers, has been installed, showing the fault systems. Scientists have drilled boreholes up to 3 kilometers deep to directly study the physical properties of the fault zones, measure stress, and understand how water moves through the rock. The data gathered here is used for seismic hazard assessments for large dam projects worldwide.