SEISMIC HAZARD ASSESSMENT OF HIMACHAL PRADRESH & UTTARAKHAND

Abstract

The study is made to determine the probabilities for the return periods of earthquakes in the area in and around Himachal Pradesh bounded by 29° N — 33° N latitude and 75° E — 81° E longitude. Annual Extreme Values Method of Gumbel (1958) is used by selecting the earthquakes equal or greater than M = 4 that occurred for the time interval 1963-2017. Also, the methodology used to estimate the magnitudes of future earthquakes is by statistical method developed by Richter and Gutenberg (1942) in which all the earthquakes of the past of any size may be taken into calculations. According to this method proposed by Gumbel, the distribution of the annual maximum earthquake magnitudes is given which yielded that the probability of an earthquake occurrence of magnitude = 7.5 in 100 years is 74 percent and its return period is 77 years. Seismic hazard analysis involves the quantitative estimation of ground shaking hazard at a particular site or for a particular region. In the present study Deterministic Seismic Hazard Analysis (DSHA) has been carried out for the states of Himachal Pradesh & Uttarakhand. The study region is one of the most seismically active regions of India and there are numerous major seismic faults present in this region. Eighty nine seismotectonic sources in and around Himachal Pradesh & Uttarakhand were identified. Using an appropriate attenuation model the peak horizontal accelerations, peak vertical accelerations and ratios of peak vertical to horizontal accelerations were computed. For this purpose the study region was divided into grids of 0.5° by 0.5°. The estimated peak horizontal accelerations vary from 0.02g to 0.60g and peak vertical accelerations vary from 0.01g to 0.47g. The ratios of vertical to horizontal accelerations vary from 0.27 to 0.78. The PGA contour maps prepared for the region show that larger Peak Ground Accelerations are present in the region where there is a higher density of larger faults and vice versa.