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

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 (T_P) and shear wave velocity (V_S) 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 T_P and average V_S 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 V_S, long T_P, and thick soft clay. V_S increases, while T_P 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 V_S at different depths and T_P were examined. Regression analysis revealed high correlations and almost linear relations among them, suggesting that V_S at deep strata could be simply estimated from the shallow V_S or T_P. Finally, a simple scheme to estimate the V_S profile using only the phase velocity at specific wavelengths is proposed.