Nonlinear Soil Amplification Models for a Moderately Active Seismic Zone in India

The dynamic stiffness and damping of the soil material, depth of the soil profile, impedance between the soil and the underlying bedrock and soil nonlinearity are the factors influencing the local site response. The important parameter in assessing the site response is the “amplification factor,” which is usually correlated to Shear Wave velocity in the top 30 m (Vs₍₃₀₎)_. Though using V_S(30) as an index for amplification is simple and robust, it is not recommended for site-specific applications. In the present study, two distinct soil types i.e. “Sand” and “Clay” with the same value of V_S(30) demonstrated variable amplification characteristics. Hence, distinct site amplification models were derived for the two soil types considering the intensity of the input bedrock motion as the primary independent variable. The borehole data from nearly 50 locations in North Kerala, an intraplate region in the Southern part of India was collected. The ground response was simulated in 1-dimension considering equivalent linear behavior of soils on the SHAKE 2000 platform. The ground motions used in the simulation were scaled to the target spectrum obtained from the regional seismic hazard assessment. The average spectral amplification observed is 5 for “Clay” and 3.5 for “Sand” in the study region. The soil profiles categorized as “sand” exhibits nonlinear behavior. “Clay” deposits reveal sustained amplification at longer periods and hence, can significantly influence ground response during longer duration ground shaking. The empirical amplification equations developed from the study can be used to modify the generic ground motion prediction models to region-specific applications.