Seismic site classification of Itanagar city, India, considering spatial variation of shear wave velocity obtained using extensive active MASW survey

Abstract

This study presents an advanced seismic site characterization for Itanagar, Arunachal Pradesh, situated within India's highly seismically active Himalayan belt. The research addresses critical limitations associated with traditional V_s30-based seismic site classifications, highlighting the necessity of incorporating detailed spatial variations of subsurface shear wave velocity (SWV) profiles. Employing Multichannel Analysis of Surface Waves (MASW), SWV data were collected at 22 selected locations, covering diverse geological terrains throughout the city. The MASW methodology involved field data acquisition, dispersion analysis, and inversion to obtain accurate SWV profiles up to 30 m depth. The analysis revealed significant heterogeneity in soil stratification, challenging the reliability of V_s30-based site classifications, particularly for sites exhibiting notable subsurface layer variability. The study demonstrates that relying solely on V_s30 values can lead to underestimation or overestimation of seismic response, especially in complex geological environments. To address this, the research introduces an alternative approach of computing average SWV values over discrete 5 m depth intervals, providing a clearer and more realistic depiction of subsurface conditions. This innovative approach better captures soil stiffness variations and their implications for seismic wave amplification. Spatial distributions of SWV were visualized through contour maps, effectively delineating areas with potentially higher seismic amplification risks. Results indicate predominant site classes of C and D under NEHRP guidelines, characterized by medium to dense soils and soft rocks. This refined classification methodology advocates for more accurate site-specific seismic evaluations, emphasizing the importance of detailed subsurface characterization.