Ground Response Analysis with Deep Bedrock Depth in Indo-Gangetic Plains

Local site effects play an important role in causing damage to structures during earthquakes. Thus, one of the aim of seismologists and geotechnical engineers is to characterize the soil for the region prior to seismic hazard assessment. In the present work, an endeavor is made to study the depth of bedrock in Indo-Gangetic Plains from Seismotectonic Atlas of India [6]. A huge variation of bedrock depth ranging from 0 to 4000 m indicates the presence of thick soil cover in the study region. Roorkee city, situated in the foothills of Himalayas has a bedrock depth of around 3000 m and due to the presence of this huge soil cover, the occurrence of any great seismic event will pose a threat to both life and property as properties of propagating waves change as they travel toward the surface. The site characterization is carried out by MASW and Microtremor methods and shear wave velocity profiles are estimated with the thickness of soil. The region has lower values of shear wave velocity and falls in Site Class D. Ground Response Analysis (GRA) of the site is performed using STRATA [9] to Uttarkashi Earthquake (1991) and Chamoli Earthquake (1999) and is compared with the suggested response spectrum in (IS, in (Part 1): 2002 Indian Standard, Criteria for earthquake resistance design of structures, Fifth Revision, Part-I, Bureau of Indian Standard, New Delhi, 1893):2002 (Part 1). The comparison of the response spectrum from two earthquakes inhibits higher variation as provided in the IS Code.