Seismic site characteristics of shallow sediments in the Bangkok Metropolitan Region, and their inherent relations

Abstract

This paper presents the site characteristics of soft deposits in Bangkok Metropolitan Region, and their inherent relations, which were explored using microtremor observations. The predominant period (TP) and shear wave velocity (VS) profiles from the surface to a 100-m depth at 150 sites over an area of approximately 150 × 200 km were explored using the Horizontal-to-Vertical (H/V) spectral ratio method and the Spatial Autocorrelation (SPAC) method, respectively. Spatial variations of TP and average VS are presented in maps and discussed with the distribution of soft clay. The results show that the shallow structure is rather uniform in the central area, having low VS, long TP, and thick soft clay. VS increases, while TP tends to decrease gradually towards the north and east and more sharply towards the west. Using a number of the observed site characteristics, relations among the average VS at different depths and TP were examined. Regression analysis revealed high correlations and almost linear relations among them, suggesting that VS at deep strata could be simply estimated from the shallow VS or TP. Finally, a simple scheme to estimate the VS profile using only the phase velocity at specific wavelengths is proposed.