Liquefaction Potential Assessment of Northeast India Region: Its earthquake and deterministic scenario

Northeast India is the most seismically active region being located in Seismic Zone-V and experienced liquefaction phenomenon triggered by large earthquakes with maximum MM Intensity of X. The 1950 Assam earthquake of M_w 8.7, 1897 Shillong earthquake of M_w 8.1, 1869 Cachar earthquake of M_w 7.4 and 1988 India-Burma border earthquake of M_w 7.2 reportedly induced scattered liquefaction phenomenon with the surface exposure of sand boils, ground subsidence and lateral spreading in the Northeast India region. Having a shallow groundwater condition in major populated areas of the region located on the alluvium-rich Bramhaputra river system with deltaic plains, lacustrine swamp and marsh geomorphological conditions, Northeast India region presents a strong case for systematic liquefaction potential modelling using modern multivariate techniques. In the present investigation, we delivered synthesised bedrock ground motion for the aforementioned earthquakes using finite fault stochastic simulation followed by 1-D non-linear/equivalent linear site response analysis using DEEPSOIL module for Site Amplification and Peak Ground Acceleration assessment at the surface. Factor of Safety (FOS), Liquefaction Potential Index (LPI), Probability of Liquefaction (P_L), and Liquefaction Risk Index (I_R) are estimated to make a more subtle understanding of the severity of liquefaction under the impact of earthquake loading and also to predict deterministic liquefaction scenario in the event of a surface-consistent probabilistic seismic hazard condition at 10% probability of exceedance in 50 years with a return period of 475 years. From the results, it is observed that, ‘Severe’ (LPI>15)  liquefaction susceptible zone exists around the cities of Guwahati and Digboi in Assam, while Silchar and Jorhat are lying in ‘High’ (5<LPI≤15) liquefaction potential zone. Imphal, Agartala, and Itanagar are the other major cities that fall under the ‘moderate’ liquefaction potential (0<LPI≤5) zone. The entire Northeast India region has been classified into ‘Severe’, ‘High’, ‘Moderate’ and ‘Non-liquefiable’ zones based on LPI distribution while the Liquefaction Risk map classified the terrain into ‘Low (I_R≤20)', ‘High (20<I_R≤30)’ and ‘Extremely High (I_R>30)’  Risk zones. The results of this investigation are very useful to identify liquefaction susceptible areas, as well as for future development and planning of cities against liquefaction failure.

 Keywords: Northeast India, Liquefaction, Factor of Safety, Liquefaction Potential Index, Liquefaction Risk Index, Landslide Susceptibility.