Isostasy represents Earth’s lithospheric equilibrium mechanism where crustal blocks float on the underlying asthenosphere, maintaining vertical balance through density adjustments. This principle, combined with understanding reservoir-induced seismicity, forms crucial knowledge for sustainable infrastructure development.

• Isostasy is the state of balance. The gravitational adjustment of the earth’s crust is known as isostasy. It is based on the principle of buoyancy (Archimedes).
• For example: A Ship sinks until it displaces a volume of water equal in weight of the ship and its cargo. Thus an empty ship displaces a smaller volume of water than the same ship when fully loaded.
• The state of equilibrium, resembling flotation, in which segments of the earth crust stand at levels determined by their thickness and density. Isostatic equilibrium is attained by flow of material in the mantle.
• The continental glaciers are a clear example of isostatic adjustment. In Antarctica and Greenland, the weight of the ice has depressed the central part of the land mass below sea level.
• If the ice of Antarctica melted, Antarctica would slowly rise from the water much the same way a ship rises while being unloaded.
• Similarly after the melting of ice, the Hudson Bay and Baltic Sea crust is rebounding at the rate of 5 to 10 metres per 1000 years.

In brief, the concept of isostasy is based on the principle that continental masses ‘Sial’ float on denser substratum ‘Sima’.

Reservoir-Induced Earthquakes

1. Triggering Mechanism: Water loading increases pore pressure in underlying rock formations, reducing effective stress on pre-existing faults. Example: Koyna Dam triggered 22,000+ seismic events since 1967 impoundment.
2. Critical Factors: Reservoir depth >100m and rapid filling create maximum seismic risk through elastic loading. Example: Tehri Dam (260m height) experienced increased seismicity during 2006 first filling phase.
3. Geological Susceptibility: Pre-existing fault systems amplify induced seismicity risk in crystalline basement rocks. Example: Wenchuan earthquake (2008) potentially linked to Zipingpu Reservoir, demonstrating catastrophic consequences.
4. Mitigation Strategies: Controlled filling schedules and real-time monitoring reduce seismic hazards. Example: Sardar Sarovar Dam employs gradual impoundment with continuous seismograph networks for early warning.
5. Global Patterns: Over 100 documented cases worldwide show correlation between large reservoirs and increased seismicity. Example: Lake Mead (USA) triggered 10,000+ earthquakes following Hoover Dam completion.

Understanding isostatic principles and reservoir-induced seismicity guides sustainable development strategies. National Dam Safety Authority (2021) now mandates comprehensive seismic assessments, ensuring infrastructure resilience while maintaining hydropower objectives under climate change adaptation frameworks.