Quake that Redefined a River’s Path
A significant study has unveiled how a major earthquake 2,500 years ago caused one of the world’s largest rivers, the Ganges, to abruptly change its course. Published in Nature Communications on June 17, 2024, this research highlights the seismic impact on river systems and raises concerns about future vulnerabilities in densely populated regions such as Bangladesh.
Earthquake-Driven River Avulsions
The study reveals that the earthquake-induced avulsion of the Ganges River was a significant geological event. Co-author Michael Steckler, a geophysicist at Lamont-Doherty Earth Observatory, emphasized the unprecedented scale of this avulsion. Unlike the slow, progressive course changes typically observed in river deltas, this event caused an almost instantaneous shift in the river’s path, highlighting the powerful influence of seismic activity on landscape reorganization.
Lead author Liz Chamberlain, from Wageningen University in the Netherlands, pointed out that while river avulsions are common, the direct link to seismic activity, especially in such a large river system, had not been previously confirmed. The discovery of sedimentary archives indicating an abrupt avulsion of the Ganges underscores the potential for significant landscape changes triggered by earthquakes.
Evidence of Abrupt Avulsion
The researchers identified large sand dikes more than 180 kilometers from the seismogenic source region, suggesting a major earthquake likely caused the avulsion. These sand dikes, formed by pressurized sand being forced upward through layers of mud, are telltale signs of seismic activity. Chemical analyses and luminescence dating of sediment samples confirmed that the avulsion and the seismic events occurred around 2,500 years ago.
Further investigation revealed that the Ganges River’s reorganization broadened the seismic risk in the Bengal delta. The study’s findings emphasize the role of seismic events as geomorphic agents in tectonically active lowlands, highlighting the cascading hazards posed by earthquake-triggered ground liquefaction and avulsions in populated river basins.
Vulnerability of Populated River Basins
The study’s implications are far-reaching, especially for densely populated areas like Bangladesh. Earthquake-triggered avulsions pose catastrophic risks to these regions, exacerbated by climate change and human activities. The 2016 study led by Steckler showed that seismic zones in the region are currently building stress, capable of producing earthquakes comparable to the one 2,500 years ago. The last significant event in 1762 caused a deadly tsunami that traveled up the river to Dhaka, indicating the potential for widespread devastation.
Mechanisms and Hazards
The mechanisms governing river avulsions include both natural sediment deposition and allogenic forces like earthquakes. In the case of the Ganges, the earthquake’s impact was immense, triggering subsurface liquefaction and an abrupt reorganization of the river system. Historical examples, such as the seismic activity along the Mississippi River and the Rann of Kutch, demonstrate the rapid topographical changes that can result from such events.
Climate change and human impacts further compound these risks. Enhanced flooding and increased stream power due to climate change, combined with urbanization and industrialization, extend flood damages to lowland areas. Engineering solutions that constrain waterways can inadvertently increase flood risks, making populated regions even more vulnerable to cascading hazards.
Global Perspective on Seismic Risk
The Ganges is not alone in facing such hazards. River basins in tectonically active settings worldwide, including China’s Yellow River and Myanmar’s Irrawaddy, are susceptible to similar landscape changes from earthquake-triggered events. The study’s findings underscore the need for a global perspective on seismic risks and the potential for unrecognized hazards in sedimentary basins.
The research employed stratigraphy and sedimentology techniques, including hand coring and tube-well drilling, to analyze sediment samples. Luminescence dating provided accurate timelines for the seismic events and avulsions, validating the study’s conclusions. These methods, combined with detailed sediment transport analysis and dose rate calculations, offer a robust framework for understanding the geological history of river systems.
Looking forward, the study calls for continued research into the impact of seismic events on river-channel networks. Understanding the interplay between natural forces and human activities is crucial for developing effective strategies to mitigate the risks posed by earthquakes in densely populated regions.
This landmark study sheds light on the profound impact of earthquakes on river systems, using the Ganges River as a case study. The findings not only debunk previous misconceptions but also highlight the need for rigorous, evidence-based approaches to managing seismic risks. As climate change and human activities continue to increase vulnerabilities, the importance of understanding and preparing for such natural events cannot be overstated. By recognizing the powerful role of seismic activity in shaping landscapes, policymakers and scientists can better protect vulnerable populations and ensure the resilience of critical river systems worldwide.
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Here is the summary of the Actual study:
Impact of Earthquakes on River-Channel Networks:
– Earthquakes can trigger significant landscape changes, including river avulsions.
– Evidence of a major Ganges River avulsion at ~2.5 ka highlights the seismic impact.
Evidence of Abrupt Avulsion and Earthquake:
– Discovery of sedimentary archives indicates an abrupt avulsion of the Ganges River.
– Large sand dikes >180 km from the seismogenic source region suggest a major paleo-earthquake.
Reorganization of River-Channel Networks:
– The reorganization of the Ganges River in the Bengal delta broadens seismic risk.
– Recognition of seismic events as geomorphic agents in tectonically active lowlands.
Seismic Risk to Populated River Basins:
– Earthquake-triggered ground liquefaction and avulsion pose catastrophic risks to populated river basins.
– Climate change and human impacts increase vulnerability to cascading hazards.
Mechanisms Governing Avulsion:
– Avulsion sustains coastal landscapes by redistributing sediment deposition over time.
– Allogenic forces like earthquakes may contribute to avulsions in seismically active basins.
Seismic Hazards in Lowland Basins:
– Lowland river-channel networks face seismic risks amplified by various factors.
– Potential for river channel impacts due to earthquakes, even in distal regions.
Paleoearthquake Evidence in Bengal Basin:
– Abandoned channel scar and sand dikes indicate a major paleo-earthquake and avulsion.
– Luminescence dating supports the coeval occurrence of the avulsion and seismic events.
Bengal Basin Paleochannel Archive:
– Bengal basin’s paleochannel evidence showcases the seismic history.
– Holocene delta sediments sourced by Ganges and Brahmaputra rivers contribute to landscape changes.
River System Activity:
– A large, aggrading river system was active around 2.6 ka.
– Sand transport and overbank deposition abruptly ceased around 2.5 ka.
Overbank Sedimentation Rate:
– Prior to abandonment, the average overbank sedimentation rate was fast at 1.9 ± 0.8 cm/yr.
– Consistent with rates from other floodplain settings in the delta.
Channel Abandonment:
– Widespread cessation of sand transport along the paleochannel at 2.5 ka.
– Shallow-buried sands on mid-channel bar top show a coeval age of 2.62 ± 0.16 ka.
Paleochannel Features:
– Paleochannel represented the mainstem pathway of the Ganges River.
– 1.0–1.7 km wide with cross-sectional area consistent with the modern Ganges channel.
Sand-Dike Features:
– Two large, linear sand dikes ruptured the floodplain indicating major liquefaction of sand units.
– Consist of well-sorted fine sands typical of river-channel sands.
Dike Structure:
– Main dikes are 10–15 cm wide at base and may widen to 30–40 cm at top.
– Complex structure with subordinate dikes forming interconnected network.
Sand-Dike Origin:
– High-energy disturbance consistent with seismic origin.
– Fracturing and sand injection rule out riverbank slumping.
Emplacement Timing:
– Timing of sand dike emplacement constrained by OSL ages of capping floodplain muds.
– Floodplain deposits yield burial ages of 2.63 ± 0.15 ka and 2.58 ± 0.15 ka.
Sand Dikes and their Characteristics:
– The sand dikes found in the Ganges floodplain are similar in scale and architecture to those associated with the 1811–1812 New Madrid earthquakes near the Mississippi River, indicating a seismic origin.
– The dikes also show similar surface expressions to those reported following the 1897 Shillong Massif earthquake in the Bengal basin.
Subordinate Seismite Features:
– Other subordinate deformation features, such as disturbed bedding and convolute patterns in the sediments, support a seismic origin.
– Mud-filled pipes and micro-fractures, along with mixing of convolute sand layers, indicate active compression of sediments by tree-root motion and suggest that the earthquake occurred during the wet season.
Event Reconstruction and Earthquake Magnitude:
– The preservation of large, under-filled channel scars suggests an abrupt avulsion involving a major reorganization of the Ganges River system.
– Empirical relationships between earthquake magnitude, distance, and width of resulting sand dikes indicate a minimum earthquake magnitude of M 7.5–8.0.
Earthquake Magnitude Reconstructions:
– Reconstructions based on distance and dike width suggest that the ~2.5 ka earthquake was likely in the range of M 7.0–8.0.
– The width of the sand dikes implies a minimum M ~6.5 event, but the characteristics of the dikes suggest a likelihood of M >7 events.
Impact of the Earthquake:
– The earthquake’s impact was immense, triggering far-reaching subsurface liquefaction and an abrupt avulsion of the mainstem Ganges River.
– The relationship between these hazards suggests earthquake-generated ground roll and shallow-sediment liquefaction as plausible mechanisms for channel avulsion.
Impact of Earthquakes on Landscapes:
– Rapid topographical changes were documented along the Mississippi River and Rann of Kutch region due to historical earthquakes.
– Seismic activity can transform coastal lowlands into saline lakes with environmental impacts.
Factors Contributing to Landscape Alterations:
– Historical records and archaeological correlations have limitations in discerning the contributions of river flooding and tectonic subsidence.
– Seismic events can abruptly alter delta-plain elevations and river gradients.
Environmental Impacts of Seismic Events:
– Earthquakes during the monsoon season can trigger landslides, bank failures, and changes in river morphology.
– Flooding and water loading can lead to lithospheric compression and landscape changes.
Role of Climate Change in Landscape Transformations:
– Climate change is increasing flood risks globally due to enhanced flooding and increased stream power.
– Human activities like urbanization and development practices compound flood risks in lowland areas.
Man-made Vulnerabilities to Cascading Hazards:
– Engineering solutions that constrain waterways can increase flood risk in river systems.
– Urbanization, industrialization, and population pressure extend flood damages to lowland areas.
Study on Cascading Hazards and Seismic Risks:
– Earthquake-driven river-channel avulsions pose a severe, unrecognized threat to seismically active lowlands.
– Widespread liquefaction and river flooding could result from earthquake-triggered avulsions.
Global Implications of Seismic Risk:
– Sedimentary basins in tectonically active settings globally may face unrecognized hazards due to seismic risk.
– Various river basins worldwide are susceptible to landscape changes from earthquake-triggered events.
Research Methods on Sedimentology and Stratigraphy:
– Stratigraphy and sedimentology were studied using hand coring and tube-well drilling techniques.
– Sediment samples were analyzed for grain-size distribution and bulk element concentrations.
Luminescence Dating:
– Luminescence dating was conducted on the silt fraction using specific protocols.
– Multiple regenerative dose protocols were employed, validating measurement sequences.
Dose Rate Calculation:
– Dose rates were determined using standard procedures and corrected for water content.
– A water content correction was applied to estimate time-averaged water content.
Sand Transport Analysis:
– Active sand transport in the paleochannel was determined using multiple samples.
– Overbank aggradation rate was calculated using luminescence dating.
Event Magnitude:
– Magnitude of the paleo-earthquake was estimated based on prior studies.
– Location and area of dredge-fill construction sites were quantified over two decades.
– Rashmi Kumari
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For original research paper, click the link: