10. Site-Specific Seismic Hazard Assessment of Gorakhpur City, Uttar Pradesh, India: A Holistic Approach

A comprehensive strategy incorporating inputs from geological, geotechnical, and geophysical studies was carried out to account for the local site effects on earthquake ground motion at various sites in and around Gorakhpur City, Uttar Pradesh, India, falling in Seismic Zone-IV of Seismic Zoning Map of India (BIS 2002, 2016). Gorakhpur City, one of the biggest and rapidly developing cities is located on the middle Gangetic Plain, which comprises fluvial sediments from the Older Alluvium (Middle to Late Pleistocene age) and the Newer Alluvium (Holocene age). The M 7.8 (Mw), 2015 Gorkha/Nepal Earthquake, located around 200 km northeast of Gorakhpur, has jolted the city at > VI seismic intensity, with vertical and horizontal peak ground acceleration (PGA) of 0.25733 g and 0.385995 g, respectively. The horizontal-to-vertical-spectral-ratio (HVSR) approach of ambient noise survey and microtremor study is used to evaluate the seismic site amplification of Gorakhpur city. The fundamental soil frequency and HVSR peak amplitude maps demonstrate peak soil amplification in the range from 0.7 to 3.5. The scatter plot between predominant frequency and peak amplification shows that the bulk of sites in Gorakhpur City (65.42%) fall into the medium damage potential zone, followed by the low damage potential zone (31.53%), and the high to extremely high damage potential zone (3.05%). In accordance, with the multichannel analysis of surface waves (MASW) and Standard Penetration Test (SPT) studies, the average shear wave velocity at different locations in and around Gorakhpur city varies between 202.5 m/s and 357.4 m/s, which corresponds to soil type D (stiff soil) of the site classes of the NEHRP Code Provisions (1997). In Gorakhpur, the prevalence of high liquefaction domains can be seen at low-lying places where the groundwater is shallow (= < 10 m depth), as evidenced by liquefaction susceptibility safety factor values for different locations. The Analytical Hierarchy Process and GIS-based integration were applied to produce a final seismic hazard microzonation map of Gorakhpur City. The southern part of Gorakhpur city, particularly the low-lying area, where newer alluvium is exposed, around Ramgarh Tal/lake, has the highest seismic hazard index. A few isolated high seismic microzone patches have been identified in the northern region in Chilua Tal/Lake, Sonbarsa, and Balapar regions. The present study is beneficial for seismic disaster preparedness, urban planning, and the preservation of cultural heritage in the city while designing future major engineering projects, multi-story buildings, etc. to withstand the incoming seismic ground motion caused by the large earthquakes in the region.