Next Generation Attenuation Models and Time Independent Probabilistic Seismic Hazard of Darjeeling-Sikkim Himalaya

Abstract

We deliver Next Generation Attenuation (NGA) Model and Time-independent Probabilistic Seismic Hazard Maps of Darjeeling-Sikkim Himalaya based on enriched earthquake catalogue, upgraded tectonic database, seismological database and seismotectonic implications. For consideration of near and far-field source effect, we synthesize a strong motion database by using finite-fault stochastic modeling & frequency wave number (FK) integration. NGA models have been developed for three tectonic types’ in the region viz. normal, strike-slip and thrust faulting. In addition, a regional seismic source characterization is performed using Brune circular crake model. Small to Moderate magnitude earthquakes nucleating at hypocentral depths <70 km in the region recorded by IIT Kharagpur Darjeeling-Sikkim Strong Motion Array (DSSMA) seemed to have Brune stress drop ranging between 0.75 and 147 bars. In compliance with the National Earthquake Hazard Reduction Program (NEHRP) & Federal Emergency Management Agency (FEMA) regulations  a geotechnical site characterization is performed using effective shear-wave (S-wave velocity (Vs30) distribution based on soil taxonomy, composition, grain size, lithology and topographic gradient implicating site classes: A (S-wave velocity, β > 1500 m/s), B (β = 760-1500 m/s), C (β = 360-760 m/s) and D (β = 180-360 m/s). Generic site amplification for each site class is estimated as per elevation from the recorded accelerograms of about 300 seismic events in the terrain that has provided a site amplification variation from 2.5 to 8.5 with the predominant frequency ranging between 4.0 Hz to 18.0 Hz. Probabilistic Seismic Hazard Analysis (PSHA) can be performed using both the time-dependent and the time-independent approaches. The present study follows the time-independent protocol using layered polygonal as well as tectonic sources and a logic tree frame work to consider both the epistemic and aleatory uncertainties in the magnitude, rupture distance and the several combinations of attenuation relations already existing for the terrain and also developed as a part of the present investigation. For seismic hazard analysis, the earthquake source model is composed of layered polygonal seismogenic sources, fault seismogenic sources, several combinations of attenuation relations, seismicity parameters and maximum credible earthquakes for all tectonic types prevalent in the region. The hazard maps in terms of Peak Ground Acceleration (PGA), Pseudo Spectral Acceleration (PSA) at 0.2 s & 1 s for 10% probability of exceedance in 50 years with a return period of 475 years for Darjeeling-Sikkim Himalaya have been generate in GIS platform. The PGA distribution shows a variation between 0.265g and 0.640g. Maximum hazard distribution is associated with the active tectonics viz Main Boundary thrust (MBT), Main Central Thrust (MCT) and Main Frontal Thrust (MFT). The major urban centers like Gangtok, Mangan and Darjeeling depict higher hazard level to the tune of 0.64g. The design response spectra generated from the estimated PGA and PSA at 1.0 s & 0.2 s provide an updated scenario significantly higher than the existing Bureau of Indian Standard (BIS) codal provisions
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