What is an Earthquake Swarm?

Context: A state of emergency has been declared on Greece’s Santorini and the nearby islands of Ios, Amorgos, and Anafi after a swarm of undersea earthquakes.

What is an Earthquake Swarm?

• An earthquake swarm refers to a sequence of multiple seismic events of comparable intensity that occur in a small geographic area over a short period of time.
• Unlike earthquake sequences, swarms lack a single large mainshock, and the tremors continue intermittently.
• Swarms are usually short-lived, but they can continue for days, weeks, or sometimes even months.

Causes of Earthquake Swarms

Earthquake swarms can be triggered by various geological and environmental factors, including:

• Volcanic Activity – Magma movement beneath the Earth’s surface can cause swarms, often signaling potential volcanic eruptions.
• Tectonic Stress – Gradual shifts in the Earth’s crust can release stress in clusters rather than a single major quake.
• Hydrothermal and Geothermal Processes – Underground fluids, such as water or gas, can weaken rock formations and trigger seismic activity.
• Human-Induced Activity – Activities like mining, fluid injection (as in geothermal energy production or wastewater disposal), and reservoir-induced seismicity (from large dams) can lead to earthquake swarms.

Effects of Earthquake Swarms

• Structural Damage – While individual quakes in a swarm are often moderate, repeated shaking can weaken buildings and infrastructure.
• Increased Seismic Hazard – Some swarms precede larger earthquakes, though not always.
• Public Concern and Emergency Response – Extended swarms can cause fear and require preparedness efforts, even if they don’t lead to a major quake.

Earthquake Swarms in India

• Peninsular India has experienced earthquake swarms in the past.
• Cause: Water Seepage and Pressure Buildup
  • Heavy rainfall increases the water table, leading to water seeping into underground rock fractures.
  • A 2008 study found that for every 10-meter rise in the water table, the pressure inside rocks increases by 1 atmosphere (atm).
  • This pressure buildup is released in the form of earthquake swarms.

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