Microtremor measurements in the India's holy city, Varanasi for assessment of site characteristics

Abstract

Microtremor measurements are used for preliminary site effect of the Quaternary sediments of Varanasi in Indo-Gangatic plain, which is being planned as a Smart City of India. We evaluated site effects using single and arrays Microtremor measurements at various sites in the Varanasi City, Uttar Pradesh, India. At City, H/V spectral ratios using microtremor measurements indicate fundamental frequency range 0.37–0.63 Hz, corresponds amplification 5 to 14 times. This indicates that the deep thickness of the upper soft soil at about several hundreds of meters. Such huge amount of sediments deposition sources could be associated with fluvial rivers system in the area and sediment drain from Himalaya. The average peak frequency (~0.5) may reflect the Quaternary-Tertiary boundary. Microtremor array measurements at two sites were taken using circular arrays (30 and 60 m) which consists of three recording stations on the circumference of circle and one in the centre of circle. The Phase velocity dispersion of Rayleigh wave is calculated from array using SPAC method, and a 1-D shear wave velocity structure is determined by means of inversion processes. The top layers up to 50 m show shear velocities between 280 and 320 m/s and the velocities of underneath layer vary between 1500 and 1600 m/s till 100 m of depth. These results from the present study would be useful for future planning purposes as well as risk management and to reduce severity of earthquake effects in the study area.