2,500-Year-Old Earthquake Redirected the Ganga River

In the June 2024 paper in Nature Communications, scientists reported that an earthquake of magnitude 7 to 8 caused the Ganga River to dramatically shift its course approximately 2,500 years ago.

Earthquakes

• About: Earthquake refers to the sudden shaking of the earth’s crust. It is highly destructive of all the natural disasters. They occur when there is a rupture in the Earth’s surface, typically along a fault line, leading to seismic waves that propagate through the ground. 
• Causes of Earthquakes: 
  • Tectonic Activities: Earthquakes result from a series of earth movements brought about by a sudden release of energy during the tectonic activities in the earth’s crust.
  • Other causes: It could also be triggered by volcanic eruption, rock falls, landslides, subsidence, particularly in the mining areas, impounding of dams and reservoirs, etc. 
• Measuring Earthquakes: 
  • Seismographs: It is an instrument to detect and record seismic waves produced by earthquakes. 
    • The time, location and intensity of an earthquake from the information recorded by a seismometer. 
    • This record also provides information about the rocks the seismic waves travelled through.

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• • Scaling Earthquake Events: The earthquake events are scaled either according to the magnitude or intensity of the shock.
    • Richter Scale: The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10.
    • Mercalli Scale: The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.

Key Findings from the Research

• Discovery of Sand Dikes: During fieldwork, scientists found two large sand dikes a kilometre east of an ancient river channel.
  • These dikes, formed by liquefaction during an earthquake, showed that sediments flowed like liquid due to seismic activity.
• Evidence of Earthquake
  • Presence of Dikes: The team suspected these dikes indicated a major earthquake event, which was later confirmed.
    • The dikes provided the first evidence that earthquakes can change the course of rivers.
• Study Results
  • Shift of Ganga River: The magnitude of the earthquake caused the Ganga River to shift its course over 2,500 years ago.
  • Presence of Paleochannel: This shift created a well-preserved paleochannel (Ancient riverbed)  in the area, which is now used for rice cultivation.
  • Timing Correlation: The timing of the sand dikes’ formation matched the river’s avulsion, supporting the link between the earthquake and the river’s shift.
• Optically Stimulated Luminescence (OSL) Dating for Timing Analysis: 
  • About: Optically Stimulated Luminescence (OSL) dating is a method used to determine the burial age of sedimentary deposits. It was used to determine the timing of both the earthquake and the river avulsion.
  • Mechanism of OSL Dating: OSL dating estimates how long mineral grains have been buried by measuring the natural radiation stored in them.
    • To estimate the amount of energy stored, the researchers shine blue light on the grains, causing the trapped electrons to escape the crystal structure and return to the atoms. 
    • When they do so, they emit a photon of light.
    • A photomultiplier tube captures the photons and estimates their number, leading to a measure of the trapped energy.
• Unresolved Questions
  • The exact origin of the earthquake remains unknown.
    • Researchers suggest it could have originated in the Indo-Burma mountain ranges or the Shillong hills, where the Indian and Eurasian tectonic plates meet.

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Future Hazards and Preparedness

• Increased Risk from Earthquakes
  • The discovery that large earthquakes can trigger major river avulsions reveals their potentially greater impact than previously understood.
  • River avulsions have caused some of history’s deadliest floods, with severe cascading effects on heavily populated areas like the Ganges-Meghna-Brahmaputra delta.
• Exacerbating Factors
  • Human activities and climate change increase the risk of river avulsions.
  • Factors include rapid subsidence near riverbanks due to widespread embankments and rising sea levels coupled with extreme weather events.

Way Forward

• Need for Future Research
  • Future research should investigate the frequency of quake-driven avulsions and develop forecasting methods for such major earthquakes.
  • Decision-makers and residents in the delta region must prepare for the risks associated with these avulsions.
• Implications for Future Preparedness: The research highlights the need for better earthquake forecasting to prepare for potential river shifts.
  • Experts stress the importance of increasing public awareness and preparedness for such seismic events.
• Regional Cooperation
  • Effective preparedness requires collaboration between India, Bangladesh, and Myanmar in research, monitoring, and support.

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