Effect of spatial variability in soft soil layers on long-period ground motion amplification

¹ Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand
² Centre of Excellence in Geotechnical and Geoenvironmental Engineering, Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand
³ GreenTech Nexus: Research Center for Sustainable Construction Innovation, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

The Bangkok Basin is characterised by deep alluvial deposits predominantly composed of soft soils, which are highly susceptible to ground shaking induced by distant earthquakes. Previous studies have utilised ground motion parameters, such as shear-wave velocity, together with historical earthquake records, to develop microzonation maps of soil amplification. However, the recent 2025 Mandalay earthquake has highlighted an important gap in the understanding of long-period ground motions, as the observed effects extended well beyond prior expectations. Despite the considerable distance between the epicentre and Bangkok, occupants of high-rise buildings clearly experienced perceptible seismic vibrations. Given the potential for stronger earthquakes to occur at closer distances, it is crucial to investigate the long-period ground response of Bangkok’s soft soil deposits. This study examines the soil amplification factors of Bangkok subsoil under long-period ground motion, using the 2025 Mandalay earthquake as a representative case. One-dimensional site response analyses were performed to simulate the vertical propagation of low-frequency seismic waves from the engineering bedrock through the overlying basin deposits. The results demonstrate that variations in depositional sequences and the thickness of soft clay layers produce distinctive site amplification patterns across the basin.