Scientists have identified seismogenic liquefaction features such as multiple sand dykes and sand sills in the active Kopili fault (KF) zone in the northeastern region (NER). This fault has experienced large earthquakes in 1869 and 1943, and the study suggests that paleoseismic investigations can help trace the history of earthquakes and prepare for the future.
The occurrence of past great earthquakes, for which no historical or instrumental records exist, can be identified through geological, geomorphological, fluvial signatures, and radiocarbon dating. Liquefaction, or the transformation of granular material from a solid to a liquefied state due to increased pore water pressure, is a crucial secondary evidence of earthquakes. It occurs primarily in soft sedimentary sequences, especially interbedded sand and silt or clay. The resulting structures include sand dykes, sand blows, sand veins, pseudonodules, convolute bedding, and load structures. This information is important for designing infrastructure to withstand future large earthquakes.
To mitigate future earthquakes in the KF, scientists from the Indian Institute of Geomagnetism (IIG) identified seismogenic liquefaction features at three trench sites in the floodplain deposits of the Kolong River near KF. These features include multiple sand dykes and sand sills which are direct responses to past seismic activity-induced liquefaction of saturated sediment.
A total of seven samples from marker horizons were processed using optically stimulated luminescence (OSL) dating technique to constrain the chronology of liquefaction features.
The OSL age constraints indicate two earthquakes induced liquefaction in the vicinity of the KF during the past around 480 years. This information will help in interpreting the 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 recognition of liquefaction features in the absence of surface rupture.
Publication link: https://doi.org/10.1007/s11069-023-06200-w
Figure 1: Geological location map of the study trench sites along Kolong River, KF, showing major structures (T1-Trench 1 at Namgaon site, T2-Trench 2 at Nampani site and T3- Trench 3 at Satargaon site). * denotes epicentres of earthquakes that occurred in the Kopili fault region are plotted on the map. Photograph showing observations from Namgaon Site, Trench 1, indicate two episodes of earthquake-induced liquefaction at Trench 1 since perhaps AD 1692.