Describe Reservoir Induced Seismicity (RIS) with a particular reference to Koyna earthquake.

Understanding Reservoir Induced Seismicity (RIS)

RIS is a complex phenomenon influenced by several factors. It isn’t simply the weight of the water that causes earthquakes. The primary mechanisms involved are:

• Pore Fluid Pressure Increase: Water infiltrating into the subsurface increases pore fluid pressure, reducing the effective normal stress on faults. This lowers the shear strength of the fault, making it easier to slip.
• Stress Transfer: The weight of the reservoir and the altered stress distribution can transfer stress to nearby faults, potentially triggering their reactivation.
• Capillary Forces: Changes in water saturation can induce capillary forces that contribute to fault slip.
• Sediment Loading: The deposition of sediments at the reservoir bottom can add to the stress on underlying faults.

The Koyna Earthquake: A Case Study

The Koyna region, located in the Deccan Trap basalt province of Maharashtra, India, experienced a significant increase in seismic activity following the construction of the Koyna Dam, completed in 1962.

Geological Setting

The Koyna region is characterized by a dense network of pre-existing faults and fractures within the basalt layers. These faults are generally considered to be ancient, formed during the rifting associated with the Deccan Trap volcanism. The Koyna Dam is built across the Koyna River, impounding a large reservoir.

Sequence of Events

The first felt earthquake occurred in December 1963, approximately a year after the reservoir began to fill. This was followed by a series of earthquakes, culminating in the devastating Koyna earthquake of December 11, 1967, which measured 6.5 on the Richter scale. The earthquake caused widespread damage and resulted in approximately 180 fatalities. Seismic activity has continued in the region, albeit at a lower frequency, to the present day.

Analysis and Causative Factors

Extensive research has been conducted to understand the causes of the Koyna earthquakes. Key findings include:

• Fault Reactivation: The earthquakes are believed to be caused by the reactivation of pre-existing faults, specifically the Vena fault system.
• Pore Pressure Diffusion: The increase in pore fluid pressure due to water infiltration from the reservoir diffused through the fractured basalt, reaching the faults and triggering slip.
• Deep-Seated Source: Unlike many other RIS events, the Koyna earthquakes originate from a relatively deep source (8-12 km), suggesting that the pore pressure changes are affecting faults at considerable depths.
• Long-Term Activity: The continued seismic activity for over five decades suggests that the reservoir is still influencing the stress state in the region.

The Koyna case is unique because the seismicity continues even after the reservoir has reached a stable water level. This suggests that the process of pore pressure diffusion and fault reactivation is ongoing and may be sustained for a long period.

Mitigation Strategies

Mitigating RIS is challenging, as it involves managing the complex interplay of geological factors. Some potential strategies include:

• Careful Site Selection: Avoiding areas with known active faults or dense fracture networks during reservoir construction.
• Controlled Reservoir Filling: Gradual filling of the reservoir to allow for stress adjustment and monitoring of seismic activity.
• Pore Pressure Management: Implementing measures to control pore pressure, such as drainage systems or injection of fluids. (Though this is often impractical)
• Seismic Monitoring: Establishing a robust seismic monitoring network to detect and track earthquake activity.
• Early Warning Systems: Developing early warning systems to provide timely alerts in the event of a significant earthquake.

However, in the case of Koyna, given the long-term nature of the seismicity, mitigation options are limited. Focus is primarily on preparedness and disaster management.