Site Characterization in the Greater Bangkok Area by Microtremor Observations

The Greater Bangkok area, Thailand, faces potentially substantial risk from distant earthquakes for which the ground motions could be amplified by layers of thick soft clay underlying the region. The seismic regulation and design standards for buildings have been issued recently according to an updated assessment of seismic hazards, but there have been very few studies done of site characterization for evaluation of seismic wave amplification. This research aims to quantitatively investigate site characteristics of subsoils using economical and practical techniques involving microtremor observations. The Horizontal-to-Vertical spectral ratio (H/V) method is used to estimate the predominant period of amplification and the Spatial Autocorrelation (SPAC) method to determine shear wave velocity profiles at 36 sites over an area of approximately 80 km by 80 km. Comparison of velocity profiles with the available results from previous studies shows them to be in good agreement. Site classifications based on the predominant period and the average shear wave velocity are presented and discussed. Relationships between the average shear wave velocity from surface to depth of different levels and the predominant periods are examined for consistencies.