Microtremor measurements were conducted at 25 sites in Thimphu to obtain 1-D shallow shear wave velocity (Vs) profiles. The Vs profiles were obtained from the inversion of the phase velocities using the microtremor Spatial Autocorrelation method. The averaged Vs values in the upper 30 m obtained at 16 sites are categorized under “class C (dense soil and soft rock)” of the NEHRP classification. The horizontal-to-vertical spectral ratios of the observed microtremors were also obtained and compared to the theoretical ones calculated from the inverted Vs profiles. Almost all sites showed a flat characteristic of the spectral ratios between 1 Hz and 15 Hz. The computed site amplifications showed most sites in Thimphu have predominant frequencies above 4 Hz.
We also performed microtremor array surveys at six sites in South Ibaraki, Japan, using the same method and sensors used in Thimphu to corroborate the applied methods and to understand the effects of geology/geomorphology on the results obtained. The results obtained in Thimphu and South Ibaraki showed clear distinctions in the Vs profiles and predominant frequencies. Thimphu City, located on mountainous terrain, showed higher values of Vs30 and dominant frequencies than most sites in South Ibaraki, which is located on a deep sedimentary basin.
Keywords: Microtremors, Thimphu, SPAC method, Vs30, H/V spectral ratio