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