Evaluation of site effects by developing an empirical relation between fundamental frequency and thickness of sediments for Shimla city, India

Abstract

This study focuses on developing a relation between fundamental frequency (fo) and the depth of sediments above the bedrock. The fundamental frequency of the site ranges from 1.25 to 27.19 Hz, whereas the depth of sediments above the bedrock varies from 2 to 145 m. The eastern Shimla region has a fundamental frequency variation from 3.6 to 5.5 Hz, whereas the western and central parts have a fundamental frequency variation from 1.24 to 5.5 Hz and high frequency (>20 Hz) at isolated locations. The seismic data has also been recorded in active mode using MSOR (multichannel simulation with one receiver) to derive the dispersion curve, enabling the derivation of a 1-D, shear wave velocity (Vs) model using a joint fit modelling procedure. The 1-D profile shows variation in Vs from 180 m/s at the top to 760 m/s at bedrock. As per the National Earthquake Hazards Reduction Program (NEHRP) classification, the sites show that almost 40% of the city area fall under stiff soil and 60% fall under very stiff soil categories. Since most of the study region is covered with forest and sloping terrain, an empirical relationship (H = 154.36fo^–1.451; R² = 0.91) has been established for assessing the thickness of unconsolidated sediments. Furthermore, the thickness of sediments is determined by forward modelling by keeping the constant velocity at bedrock (Vb) 800 m/s. Additionally, the spatial map for sediment thickness is generated using the interpolation approach in combination with the forward modelling and regression analysis techniques. The derived relationship will provide major input for the area sharing similar geographical variability, where long active seismic profiles are not possible.