Why in News?

  • Maharashtra has experienced a series of small earthquakes over the last fortnight.
  • Around 29 tremors were reported:
    • 24 in and around Nashik
    • 4 in Palghar
    • 1 in Nagpur
  • The strongest tremor was recorded in Nagpur on 8 August, with a magnitude of 4.3.
  • Most of the earthquakes were shallow, occurring at depths of around 5–8 km.
  • Since there was no clear mainshock and several earthquakes of comparable magnitude occurred over a period of time, seismologists describe this pattern as an earthquake swarm.

WHAT IS AN EARTHQUAKE?

  • An earthquake is the sudden movement of rocks along a fault or zone of weakness in the Earth's crust.
  • Rocks can remain locked because of friction even when forces are acting on them.
  • As stress continues to build, the rocks may eventually slip and release stored energy as seismic waves.

MAJOR TYPES OF SEISMIC WAVES

  • P Waves: Compress and stretch rocks in the direction in which the waves travel.
  • S Waves: Move rocks sideways through a shearing motion.
  • Surface Waves: Travel along the Earth's surface.
  • The combined movement of these waves produces the shaking experienced at the surface.
MAHARASHTRA EARTHQUAKE SWARM explained for UPSC

WHAT IS AN EARTHQUAKE SWARM?

  • In a normal earthquake sequence, a major mainshock is usually followed by smaller aftershocks.
  • In an earthquake swarm, there is no clearly identifiable mainshock.
  • Instead, several earthquakes of relatively comparable magnitude occur over days or even longer periods.
  • Their magnitudes can fluctuate during the swarm.
Important Point
  • An earthquake swarm does not necessarily mean that a larger earthquake is about to occur.
  • Different earthquake swarms can have different causes.
Examples
  • Palghar, Maharashtra: Experienced a major earthquake swarm beginning in November 2018, with thousands of earthquakes continuing for more than a year.
  • More short-duration seismic activity has also been recorded in the region in subsequent years.
  • Campi Flegrei, Italy: A major seismic swarm occurred in mid-2024. On 20 May, more than 150 earthquakes were recorded within a few hours, including a magnitude 4.4 earthquake, the strongest in the region in four decades.

WHY DOES MAHARASHTRA EXPERIENCE EARTHQUAKE SWARMS?

1. Intraplate Location
  • Maharashtra lies far from a major tectonic plate boundary.
  • Therefore, many earthquakes occurring in the State are classified as intraplate earthquakes.
  • Such earthquakes occur within a tectonic plate, rather than at its boundary.
2. Deccan Traps
  • Much of Maharashtra lies on the Deccan Plateau.
  • Its bedrock mainly consists of extensive basaltic lava flows, known as the Deccan Traps.
  • These formations were created around 66 million years ago when huge quantities of lava erupted through fissures in western-central India.
  • Thousands of individual lava flows accumulated over time, in some places reaching hundreds of metres in thickness.
  • After solidification, these basalt layers developed fractures and other weaknesses.
  • Such structures can transmit accumulated tectonic stress in ways similar to faults.
3. Continuing Movement of the Indian Plate
  • The Indian Plate continues to move and deform.
  • This movement generates and accumulates stresses even in the interior of the plate.
  • Since the Deccan Plateau lies within the Indian Plate, these stresses can activate existing fractures and structures.
4. Local Geological Structures
  • Studies around Nashik have identified geological structures such as:
    • Shear zones
    • Fractures
  • These structures can contribute to local seismic activity.

WHAT ROLE DOES WATER PLAY IN EARTHQUAKE?

Rainfall-Triggered Seismicity
  • Water can move deeper underground through:
    • Soil
    • Fractures
    • Permeable rocks
  • If water reaches a network of cracks connected to a fault, it can increase pore pressure.
  • Increased pore pressure can reduce the stability of rocks and contribute to fault movement.
  • When rainfall-induced water movement triggers earthquakes, the phenomenon is called rainfall-triggered seismicity.

EVIDENCE FROM PALGHAR

  • Researchers studying the 2018 Palghar earthquake swarm using satellite data found evidence of land subsidence.
  • They suggested that rainwater entering the subsurface could have helped trigger faults.
  • A detailed study of more than 21,000 earthquakes recorded between January 2019 and October 2020 found that seismic activity increased following rainfall.
  • The study suggested that water migrated through cracks and fissures, increasing pore pressure and helping drive the earthquake swarm.

BUT RAINFALL IS NOT THE ONLY EXPLANATION

  • Other studies have given different views about the importance of rainfall.
  • Possible contributing factors include:
    • Normal faulting
    • Aseismic deformation
    • Tectonic stresses
    • Existing geological structures and fractures
Aseismic deformation refers to gradual movement or deformation of the Earth's crust that does not produce a detectable earthquake Koyna: Reservoir-Triggered Seismicity
  • Maharashtra's Koyna-Warna reservoir region is a famous example of reservoir-triggered seismicity.
  • Scientists have observed earthquakes in the region for decades.
1967 Koyna Earthquake
  • On 10 December 1967, a magnitude 6.3 earthquake occurred southwest of the Koyna Dam.
  • It caused extensive damage in Koynanagar.
  • Houses were destroyed and more than 200 people were killed.
  • Such a major earthquake was unprecedented in the region.
Link with the Reservoir
  • There was reportedly no recorded seismicity in the region before construction of the dam began in 1956.
  • The Koyna reservoir was filled in 1962.
  • Seismic activity began after the reservoir was filled and could be detected at the seismological station in Pune, around 120 km away.
  • Minor seismic activity eventually culminated in the 1967 Koyna earthquake.