Classifying satelite-derived topographic slope as a proxy to in-situ measurement based VS30 for selective parts of north east India

Assam and Tripura are two very important states in North-East (NE) India under the ‘Look East’ policy of Government of India. Both the states face significant liquefaction risks due to high seismic activity and the presence of poorly graded silty sand (SP-SM) to filled-up soil (typically consisting of loose, uncompacted materials used for land reclamation or construction purposes). While past studies have assessed bedrock-level seismic hazards, surface-level seismic hazard estimation requires site-specific stiffness of near surface medium (often measured in terms of average shear wave velocity in top 30 m or VS30 data). Determining VS30 is challenging in rugged terrains of Assam and Tripura. This study presents a novel classification between VS30 and topographic slope for the region using 256 borelogs and Shuttle Radar Topography Mission (SRTM) based Digital Elevation Model (DEM) data. Unlike existing global models, which lack representation from NE India, the proposed classification improves Site Class (SC) mapping accuracy, as validated through in-situ tests in Agartala and Guwahati. The findings provide a region-specific VS30- topographic slope classification, enabling more accurate site classification towards improved surface seismic hazard assessment of above two states.