ESTIMATION OF SEISMIC HAZARD PARAMETERS FOR UTTARAKHAND REGION

Abstract

Seismic hazard is a natural phenomenon generated by an earthquake that causes rupture of faults, ground shaking or soil liquefaction. It incorporates the estimation ground shaking in quantitative form at a particular region. Due to the continues collision of Eurasian and Indian plate within the last 100 years results that the Himalayan region has experienced moderate to severe ground shaking. Uttarakhand, lies in 28.7°N-31.1°N latitude and 77.6°E-81°E longitude, is a seismically active region, comprising with great earthquake of Chamoli (March 1999, Mw=6.6) and Uttarkashi (October 1991, Mw=6.8). The State inherent high river valley and hydropower projects. By keeping this in mind, knowledge of ground motions is necessary for the various locality. In this study, Gumbel type-I (1958) extreme value theory and Probabilistic Seismic Hazard Analysis (PSHA) have been prosecuted for the Uttarakhand region. There are numerous major seismic faults and thrusts present in the study area that's why study region is one of the most seismically active regions of India. Seismic Hazard computation is performed using CRISIS 2015 software. For this purpose, the study area has been divided into a grid size of 0.2° x 0.2°. The input parameters are seismicity parameters and attenuation models. Seismic hazard is computed in terms of `a' & `b' values, Probabilities of exceedance of a given magnitude, Return periods, PGA for 20%, 10% and 2% probability of exceedance in 50 years which are equivalent to return periods of 225, 475 and 2475 years respectively. The entire set of earthquake catalog used for the study region covers the period from 1976-2018 i.e. 44 years are collected from international agency like United State Geological Survey (USGS) with regional magnitude Mw 3.5. The analysis using Gumbel extreme value theory stipulates that the most probable largest annual earthquake is close to 4.6 and the most probable earthquake that may occur in an interval of 50 years is 6.6. The value of hazard parameters i.e. `a' and `b' are 3.81, 0.83 and 4.59, 0.99 which were calculated using Gumbel's method and PSHA respectively. For the calculation of hazard parameters the region is divided in four seismic source zones i.e.UK- I, UK-II, UK-III, UK-IV. The values of hazard parameters and PGA are calculated only for three zones because zone UK-IV has not su cient data. Table 4.7 depicting information about area covered by individual seismic source zone, maximum magnitude, magnitude of completeness, rate of exceedance, return periods, `a' and `b' values for all three source zones. Seismotectonic zone UK-III is most seismically active zone with returning period 59.954 years followed by zone III UK-II and UK-I having returning periods of 69.183 years and 60.256 years for magnitude of 6.0 respectively. PGA contour maps have been prepared for 2%, 10% and 20% probability of exceedance in 50 years. UHS at various sites for return periods of 225, 475 and 2475 years have been plotted. PSHA has been committed and the result is presented in terms of PGA contour maps for various return periods for each district of Uttarakhand by considering two 2013 NGA attenuation models i.e.Abrahamson et al. (2013) and Boore (2013). Estimated PGA values for return periods of 225 years varies from 0.04g to 0.21g and 0.02g to 0.23g using attenuation model of Abrahamson et al. (2013) and Boore (2013) respectively. For this return period, Central part of state covering Chamoli, Rudraprayag, Tehri Garhwal districts having higher hazard level due to presence of Main Central Thrust and Almora Fault system and associated seismicity. For region located to Udham Singh Nagar and Haridwar districts hazard level is less. .