SEISMIC HAZARD ASSESSMENT OF UTTARAKHAND

Abstract

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 has been carried out for the state of Uttarakhand, which is one of the most seismically active states of India. For the purpose of conducting seismic hazard analysis, thirty six seismotectonic sources in and around Uttarakhand were identified, based on tectonics and seismicity of the region. Maximum magnitude was assigned to each source assuming that 1/3 of the total fault length would rupture in an earthquake. Using an appropriate attenuation model the peak horizontal accelerations, peak vertical accelerations and ratio of vertical to horizontal accelerations at 180 grid points, at 0.25° intervals, were computed using the shortest distance from the zone of energy release. The estimated peak horizontal accelerations in the state of Uttarakhand vary between 0.1 g and 0.50 g, whereas peak vertical accelerations fall in the range 0.05 g to 0.38 g. The ratio of vertical to horizontal accelerations varies from 0.53 to 0.76, with average value of the ratio as 0.66. In the western, central and eastern parts of Uttarakhand the peak horizontal accelerations vary from 0.3 g to 0.5 g. In the Dehradun valley the peak horizontal accelerations are in the range of 0.3 g to 0.4 g. The central part of the state, covering southern part of Chamoli district, western part of Almora district, northern part of Nainital and southern part of a Pouri Gahrwal district and half of Champawat district have higher levels of hazard. The computed peak horizontal accelerations in the present study are almost in agreement with the observed peak horizontal accelerations in the case of Uttarkashi and Chamoli earthquakes.