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Context

Recently, researchers from the Indian Institute of Geomagnetism (IIG) have identified seismogenic liquefaction features in the active Kopili Fault (KF) zone.

About

• The Kopili fault zone runs 300 kilometers northwest-southeast.
• It stretches from the western section of Manipur to the border with Bhutan, Arunachal Pradesh, and Assam.
• According to the National Centre for Seismology (NCS), the tremors in Assam result from the Kopili Fault Zone.
• A fault is a zone of fractures or a fracture between two pieces of rock.
• Faults enable the blocks to move about one another.
• This shift could happen quickly, like an earthquake. It could also happen gradually, as in creep.
• Seismically Active: This zone is a seismically active area and falls into the highest Seismic Hazard Zone V.
• The zone is associated with collisional tectonics because of the Indian Plate subducting beneath the Eurasian Plate.
• Subduction is a geological process in which one crustal plate is forced below the edge of another.
• Characteristics: Kopili fault zone and its neighboring areas are characterized by alluvial soils.
• These alluvial soils have a higher potential of trapping seismic waves.
• Thus, making the region the most earthquake-prone zone in North East India.
• Earlier Earthquakes: The Kopili fault zone has witnessed many seismic activities. This includes the 1869 earthquake (7.8 magnitude) and the 1943 earthquake (7.3 magnitude).

How paleoseismic investigations can help trace and understand the history of earthquakes?

• Seismogenic liquefaction features like multiple sand dykes and sand sills have been identified by scientists in an active fault in the northeastern region (NER), called Kopili fault (KF) zone which is known to have experienced large earthquakes in 1869 and 1943.
• The study indicates that paleoseismic investigations can help trace and understand the history of earthquakes and help us prepare for the future.
• The occurrence of great earthquakes in the past, for which no historical or instrumental records are available, can be identified in the form of geological, geomorphological, fluvial signatures and radiocarbon dating, to arrive at the most crucial aspect of recurrence period of great/major earthquakes in the region.
• Liquefaction or transformation of a granular material from a solid to liquefied state due to increased pore water pressure is crucial secondary evidence of earthquakes.
• It occurs mostly in soft sedimentary sequences especially interbedded sand and silt or clay.
• The structures resulting from liquefaction include sand dykes, sand blows, sand veins, pseudonodules, convolute bedding, load structure, and so on.
• This is important for designing bridges and big buildings to withstand future large earthquakes.
• To mitigate future occurrences of earthquakes in the KF, scientists from Indian Institute of Geomagnetism (IIG), an autonomous institute of the Department of Science and Technology (DST), identified seismogenic liquefaction features at three trench sites in the floodplain deposits of Kolong River, near KF.
• The liquefaction features include multiple sand dykes and sand sills and are direct response to liquefaction of saturated sediment induced during past seismic activity.
• A total of seven samples from marker horizons have been processed to constrain the chronology of liquefaction features using a technique called optically stimulated luminescence (OSL) dating technique.
• The OSL age constraints indicate two earthquake-induced liquefaction in the vicinity of the KF during the past around 480 years.
• These details in turn will help in the interpretation of long-term rupture history of faults and intraplate seismicity.
• The study published in Natural Hazards demonstrates that paleoseismic investigations can provide useful information on past earthquakes through the recognition of liquefaction features in the absence of surface rupture.

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