Koyna-Warna Seismic Zone

The Koyna-Warna Seismic Zone is one of the most prominent intraplate seismic regions in India, located in the Satara district of Maharashtra, within the Western Ghats. It is globally recognised as a classic example of reservoir-triggered seismicity (RTS)—a phenomenon where the impoundment of large reservoirs induces earthquakes in the surrounding region. The area derives its name from the Koyna and Warna reservoirs, both constructed on tributaries of the Krishna River. Since the 1960s, the region has been an active zone of moderate to strong earthquakes, attracting significant scientific research and monitoring efforts.

Geographic and Geological Setting

The Koyna-Warna Seismic Zone lies near 17°N latitude and 73°45′E longitude, approximately 200 kilometres south of Pune. It is part of the Deccan Traps volcanic province, a vast basaltic plateau formed by extensive volcanic eruptions during the Late Cretaceous period, about 65 million years ago.
The geological structure of the region is characterised by:

• Layered basaltic formations of varying thickness.
• Fractured and jointed rock systems providing zones of weakness for stress accumulation.
• Underlying crystalline basement rocks, including granites and gneisses.
• A network of north-south and east-west trending faults, some of which are reactivated due to stress changes associated with reservoir impoundment.

The Koyna and Warna rivers form deep valleys with steep escarpments, where the construction of large dams altered the regional hydrological and stress regimes, contributing to induced seismic activity.

Reservoir-Triggered Seismicity (RTS)

The Koyna-Warna region is internationally renowned for being the most studied case of reservoir-induced seismicity. The impoundment of the Koyna Dam (completed in 1962) on the Koyna River and the subsequent filling of its reservoir coincided with a sudden increase in earthquake frequency in the region.
The process of reservoir-triggered seismicity occurs when water impounded in large reservoirs increases pore pressure in the underlying rocks. This reduces the frictional resistance along pre-existing faults, potentially causing slippage and releasing stored tectonic stress. The magnitude of such induced earthquakes depends on reservoir load, hydrogeological conditions, and regional tectonic stress.

Historical Seismicity and Major Earthquakes

The Koyna region has experienced hundreds of measurable earthquakes since the filling of the reservoir, with magnitudes ranging from microseismic events to destructive shocks.
Key seismic events include:

• December 12, 1967: The most significant earthquake in the region, with a magnitude of 6.3, causing extensive damage in and around Koynanagar and resulting in over 180 fatalities. This event remains one of the largest recorded reservoir-induced earthquakes globally.
• 1973–1974: Increased seismic activity following the impoundment of the nearby Warna Reservoir, located south of Koyna.
• 1993–2005: Recurring moderate earthquakes of magnitudes between 4.0 and 5.0, indicating ongoing stress adjustments in the crust.
• Recent Years: Continued microseismic activity has been recorded, demonstrating that the region remains seismically active even decades after the initial impoundment.

The earthquakes occur predominantly at shallow depths of 2 to 10 kilometres, consistent with stress changes in the upper crust due to water infiltration and reservoir loading.

Scientific Studies and Monitoring

The Koyna-Warna region is one of the most instrumentally monitored seismic zones in India. The National Centre for Seismology (NCS) and the Geological Survey of India (GSI), along with the National Geophysical Research Institute (NGRI), have conducted extensive research on the region’s seismic behaviour.
Key scientific initiatives include:

• Dense Seismograph Networks: Over 60 seismometers and accelerographs monitor microseismic activity in real time.
• Borehole Seismology Project: India’s first Deep Borehole Observatory for earthquake studies was established at Koyna to study in-situ stress, pore pressure, and fault behaviour at depths exceeding 3 kilometres.
• Hydrological Studies: Correlation analyses between reservoir water levels and earthquake occurrences to understand stress variations.
• Geophysical Imaging: Use of seismic tomography and magnetotelluric surveys to map subsurface structures and fault zones.

These studies have contributed significantly to global understanding of induced seismicity, crustal stress dynamics, and the role of fluids in fault activation.

Tectonic and Hydrogeological Mechanism

The Koyna-Warna Seismic Zone lies within the stable continental region (SCR) of the Indian Peninsular Shield, where tectonic activity is generally low. However, the presence of ancient fault systems within the Deccan basalts provides potential planes of weakness.
The main mechanisms contributing to seismicity include:

1. Reservoir Loading: The weight of water in the Koyna and Warna reservoirs adds stress to the crust.
2. Pore Pressure Increase: Water infiltrates rock fractures, increasing pore pressure and reducing frictional strength on faults.
3. Elastic Stress Redistribution: The periodic filling and drawdown of reservoir levels cause cyclic stress changes.
4. Tectonic Reactivation: Regional compressive stresses related to the ongoing northward drift of the Indian Plate may interact with local stress fields.

The interaction between these factors results in a complex seismic pattern characterised by shallow, recurrent earthquakes.

Socio-Economic and Infrastructural Impact

The 1967 earthquake and subsequent tremors caused widespread structural damage in Koynanagar, including cracks in the Koyna Dam and nearby infrastructure. Although the dam survived the event, it was reinforced with prestressed concrete to enhance stability.
Ongoing seismicity has prompted the government to adopt special engineering standards and continuous monitoring for dam safety in the region. Local populations have been relocated from high-risk zones, and disaster preparedness measures have been enhanced.

Research Significance

The Koyna-Warna Seismic Zone serves as a natural laboratory for studying induced seismicity, fault mechanics, and crustal stress evolution. Findings from this region have contributed to:

• Improved seismic hazard assessment for other reservoir projects worldwide.
• Development of models explaining crustal deformation and fluid-induced fault activation.
• Advances in earthquake prediction research and seismic risk management in stable continental interiors.

The establishment of India’s Borehole Seismological Observatory marks a pioneering step in global earthquake research, enabling direct observation of fault processes at seismogenic depths.

Environmental and Geotechnical Aspects

The interaction between geological and hydrological systems in the Koyna-Warna region also influences environmental stability. The steep terrain, heavy monsoonal rainfall, and fractured basaltic bedrock contribute to landslides and soil instability, especially after strong tremors. Groundwater dynamics and reservoir sedimentation are also closely studied to understand their relationship with seismic activity.

Current Status and Future Outlook

The Koyna-Warna Seismic Zone remains moderately active, with frequent low-magnitude earthquakes recorded each year. Continuous monitoring and research efforts are helping to refine the understanding of induced seismic processes and mitigate associated risks.