Site-specific Next Generation Ground Motion Prediction Models for Darjeeling-Sikkim Himalaya using Strong Motion Seismometry

In the perspective of Probabilistic Seismic Hazard Assessment, the ground motion parameters at a site of interest are evaluated by using a ground motion prediction equation that relates a specific strong motion parameter of ground shaking to one or more seismic attributes. In this study, we deliver site-specific Next Generation Prediction (NGP) Models for Darjeeling-Sikkim terrain located in the eastern Himalayan seismogenic province implicating a maximum credible earthquake of magnitude M(w)8.3. The NGP models have been developed for three dominant tectonic domains viz. normal, strike-slip and thrust faulting mechanism of earthquake nucleation as per site classes A, B, C & D for different station elevation ranges for Peak Ground Acceleration (PGA) and 5%-damped Pseudo Spectral Acceleration (PSA) at 0.2 and 1.0 sec time period as a function of magnitude, fault rupture distance and site condition. The Extended Finite Fault simulation approach EXSIM is used for ground motion synthesis with the source parameters extracted from the recorded and historical earthquakes reported in the territory. In order to strengthen the ground motion data base, the seismic events of small to moderate magnitude with signal-to-background noise ratio >= 3 recorded by Darjeeling-Sikkim Strong Motion Array of IIT Kharagpur have been amalgamated with the simulated ones for a wide magnitude range of M(w)3.5 to 8.3 at 140 locations at a grid spacing of 0.1 degrees x 0.1 degrees. Altogether 42 ground motion prediction equations have been worked out through a nonlinear regression process of strong ground motion data versus magnitude, distance, and other predictive variables using two classical ground motion attenuation models which predicted 14 coefficients for each of the derived equations depicting zero clustered residuals. The derived NGP models have been used in a logic tree framework for probabilistic seismic hazard assessment of the Darjeeling-Sikkim Himalaya depicting PGA distribution for 10% probability of exceedance in 50 years at surface level varying from 0.293g to 0.807g. The major urban centers viz. Gangtok, Mangan, Singtham, Melli, Jorethang, Uttare and Darjeeling are seen to have enhanced hazard level to the tune of 0.60 - 0.750g placing those to probable Seismic Zone V with the suggested zone factor of 0.75g.