Effects of mineral composition and elastic properties on amplification factor of soil samples from Changwat Samutprakan

Abstract

Soft sediments underlying Samut Prakan has long been postulated to cause ground motion amplification during earthquake events. To remediate the potential hazard, the amplification factor must be considered for ground motion assessment in earthquake-prone areas. This study thus focuses on modeling the amplification factor by calculating shear wave velocity from mineral composition and elastic properties of soil samples from Bang Phli (BP) and Bang Sao Thong (BS) districts. The composition, volume fractions, and crystallographic preferred orientation of minerals in the soil samples were quantified from synchrotron x-ray diffraction method. Soil samples generally contain high clay content, approximately 60-80 vol%, including illite-mica, montmorillonite, and dickite. The quantitative results were incorporated with elastic constants of constituent minerals and density of soil samples to calculate shear wave velocities, using mathematical averaging approaches in BEARTEX software. A total porosity of 21.5% was further used in shear wave approximation, suggesting that shear wave velocities within 20 m depth in BP and BS areas are approximately 90-150 m/s and 70-130 m/s, respectively. Calculated shear wave velocities, soil profiles, and earthquake waveforms were then incorporated in a 1D amplification model using DEEPSOIL program, to estimate the peak ground acceleration at the surface of the study areas. To represent potential earthquake sources that may cause damage to Samut Prakan, three different seismic events were chosen from active fault zones, which are Phayao Fault, Three Pagoda Fault Zone, and Sagaing Fault Zone. The earthquake records were scaled to peak acceleration of 0.1 g. The amplification factors range from 1.7 to 2.1 for BP and 1.2 to 1.5 for BS. In conclusion, amplification factor can be effectively estimated from shear wave velocity obtained from soil composition and elastic properties. Our results further suggest that soft sediments in the study areas have the ability to substantially amplify earthquake ground motions.