GROUND MOTION ESTIMATION USING SITE RESPONSE ANALYSIS FOR DEEP SEDIMENTARY DEPOSITS

Considering as a simplified procedure, the ground motion of the soft deposits is empirically estimated by multiplying the bedrock motion with the site amplification factor that is estimated from the average shear wave velocity of the top 30 m (V_s³⁰). This amplification factor does not consider the response of the soft deposits that exist below 30 m depth. In the study area, the depth of the soft deposits above the bedrock (V_s³⁰ ≥ 760 m/s) is more than 200 m. The soft deposits are composed of sand, silt, and clay of the Holocene to Pliocene age. The state of boundary between the soft sedimentary deposits and bedrock is not of a geological type but rather a gradational type. The impedance contrast is not very high along this boundary and the sediments above and below the boundary are not consolidated. Therefore, the ground motion estimation using empirically estimated amplification factor is not appropriate for deep and soft deposits. One-dimensional site response analysis can be used for deep sedimentary deposits. The properties of the soft deposits are not linear. Consequently, the ground motion estimation using one-dimensional linear and equivalent-linear response analyses become inaccurate at high strain. The nonlinear site response analysis has been performed to estimate the ground motion of deep and soft sedimentary deposits. It is observed that the bedrock motion is deamplifying at short spectral periods and amplifying at long spectral periods with the increasing depth of soft sedimentary deposits. In contrast, the bedrock motion is always amplified in case of the V_s³⁰-based site response analysis. It has been identified that large magnitude seismic sources (M_w > 8.0) are located more than 100 km away from the study area, therefore, the peak ground acceleration and spectral acceleration of short period seismic waves will be attenuated and spectral acceleration of long period seismic waves will be amplified in the study areas due to deep and soft deposits. The long period seismic waves of the far-field earthquakes will match with the natural periods of the high-rise buildings, and resonance will occur. Consequently, the damage to high-rise buildings will be increased. The high-rise buildings on the deep and soft sedimentary deposits of the study area are potentially vulnerable to far-field large earthquakes.