In Mumbai, the unusual vibrations (like earthquakes), coinciding with high-tide times, were the result of the relocation of tetrapods as part of the ongoing Coastal Road Project (MCRP).

What are tetrapods?

• Tetra pod in Greek means four-legged.
• These are four-legged concrete structures that are placed along coastlines to prevent erosion and water damage.
• Tetrapods were first used in France in the late 1940s to protect the shore from the sea.
• They are typically placed together to form an interlocking but porous barrier that dissipates the power of waves and currents.
• These are large structures, sometimes weighing up to 10 tonnes, and interlocked tetra pods act as a barrier that remains stable against the rocks when buffeted by waves.
• Tetrapods, each weighing about 2 tonnes, were placed along Marine Drive in the late 1990s to break and dissipate waves and maintain the reclaimed shoreline in South Mumbai.

How do we know that the removal of the tetrapods was responsible?

• The BMC has provided vibration monitoring instruments at the site to study the impact of the phenomenon.
• While the corporation has not officially stated that the removal of the tetrapods caused the vibrations, it has agreed to re-install the structures.
• They would be put back over the next two-three days during low tide.

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