Site Characterization and Assessment of Probabilistic Seismic Hazard in Northeast India Region

Northeast India region presenting the most complex neotectonic assemblage is one of the world’s deadliest seismic territory being struck time and again by devastating earthquakes like the 1897 Shillong earthquake of M_w 8.1, 1934 Bihar-Nepal earthquake of M_w 8.1, 1950 Assam earthquake of M_w 8.7, and 1988 Burma-India border earthquake of M_w 7.2 being triggered from the Shillong, Eastern Himalaya, Mishmi tectonic block and Eastern Boundary zones. Ground motion of an impending earthquake in the Northeast India region is amplified due to trapping up of incident energy in the overburden soft sediments/soils thus necessitating site classification and its characterization to understand Seismic Hazard potential of the region. Shear wave velocity (V_s³⁰) is estimated from empirical relation obtained through nonlinear regression analysis of geology, geomorphology, slope and landform in conforming to NEHRP and UBC nomenclature which together with measured (V_s³⁰)  and liquefaction susceptibility assessment  classifies the region into Site Class A, B, C1, C2, C3, C4, D1, D2, D3, D4, E and F. 1-D nonlinear/equivalent linear site response analysis performed using DEEPSOIL package estimates spectral  site amplification of  4.28 in E/F), 3.64 in D4, 2.95 in D3), 2.91 in D2, 2.80 in D1, 2.71 in C4, 2.29 in C3, 2.16 in C2, 1.98 in C1 and 1.53 in B at corresponding predominant frequencies of 0.76Hz (in E/F), 1.05Hz (in D4), 1.1Hz (in D3), 2.21Hz (in D2), 2.95Hz (in D1), 3.0Hz (in C4), 3.37Hz (in C3), 3.45Hz (in C2), 5.41Hz (in C1) and 4.42Hz (in B) along with the absolute site amplification factor 2.1  in E/F, 1.93 in D4, 1.9 in D3, 1.85 in D2, 1.81 in D1, 1.78 in C4, 1.71 in C3, 1.68 in C2, 1.6 in C1 and 1.56 in B respectively. Surface Consistent Probabilistic Seismic Hazard Assessment of this region for 10% probability of exceedance in 50 years with a return period of 475 years considered both polygonal and tectonic seismogenic sources, wherein the entire region predicted Peak Ground Acceleration (PGA) variation within 0.34-1.88g placing Dispur in the ‘Severe’ hazard regime (PGA:1.5-1.88g) while  Kohima, Shillong, Itanagar and Imphal are in the ‘Moderate’ to ‘High’ hazard (PGA:0.73-1.12g), but Agartala, Aizawal and Gangtok in the ‘Low’ hazard (PGA:0.34-0.73g) domain correlating well with the isoseismal distributions of the great historical earthquakes impeded in this region. The assessment is expected to be useful for updating the urban development plan, developing design principles for future earthquake-resistant structures.

Keywords: Northeast India; Shear Wave Velocity; Site Class; Peak Ground Acceleration; Site Characterization.