New empirical relationship between resonance frequency and thickness of sediment using ambient noise measurements and joint-fit-inversion of the Rayleigh wave dispersion curve for Kangra Valley (NW Himalaya), India

Northwest Himalaya is one of the seismically active regions that had experienced a number of major and moderate earthquakes in the past. The 4th April 1905 Kangra earthquake (Ms 7.8) is one amongst those which had caused a devastating effect to the built environment, as a consequence of amplification factor due to varied impedance contrast between the overlying alluvial soil and underlying bedrock. Preventive actions aimed at minimizing the economic losses caused by such events require estimation of dynamic characteristics of the surface soil and its behaviour under strong motion excitation. Under this initiative, the Kangra Valley, which falls in seismic zone IV and V, was extensively studied to elucidate the effects of earthquake excitation on the built environment due to the soil amplification. The Kangra Valley is characterized by the presence of hills and valleys, thus has a very high variation in the sediment thickness above bedrock which is an important parameter for seismic hazard assessment. This study aims to establish a new empirical relationship from the experimental data for estimating sediment thicknesses of the Kangra Valley. Under this initiative the resonance frequency has been calculated from ambient noise measurement from 200 sites of the Kangra Valley. This extensive survey allowed the estimation of resonance frequencies (4–20 Hz) in the region and identified the areas prone to site amplification. Furthermore, joint-fit-inversion modelling of horizontal to vertical spectral ratio (HVSR) and dispersion curves obtained from ambient noise measurement and multichannel simulation with one receiver (MSOR) data acquisition, respectively, led to 1-D shear wave velocity (Vs) profile. New empirical relationships were established for Kangra Valley to estimate the thickness of overburden using non-linear regression analysis between resonance frequency (f₀) obtained by HVSR technique and pseudo-depth (H) from the study area. The equation provides an effective tool to detect sediment thicknesses in the area with similar geological setup. The methods described in the study apply a new approach to derive new empirical relationship in hilly terrain and the area with sparse borehole record.