Critical Evaluation and Estimation of Shear Wave Structure and Horizontal to Vertical Spectral Ratio at Different Sites in Kathmandu Valley

The present study was carried out to evaluate resonant frequency of the ground and to characterize
subsurface ground based on shear wave velocity structure. For this, five sites were selected such as
Pulchowk, Chhauni, Gaushala, Buddhanagar and Bhainsepati. About 20 data were recorded in each
site and then shear wave velocity structure and graph of amplification ratio with their spatial
distribution has been established with the help of software i.e. Seisimager/Seismodule Controller. The
results of both analysis methods were then compared to the amplitude of the Gorkha Earthquake and
borehole data. All these data and study indicates that the Kathmandu Valley sediments are
dependent on the frequency of the seismic waves and the wave velocity is greater in the peripheral
region than in the central part of the Valley. The result had also shown that the presence of silty-sand,
clay and loose gravel soil with low bearing capacity and elastic modulus in most of the sites are
responsible for devastation. It was also noted that apart from few limitations, a non-intrusive
microtremor analysis can be adopted for earthquake site characterization in the Kathmandu Valley
which can be readily applied and expanded upon in future seismic hazard and microzonation efforts
for Kathmandu. The low velocity model for most of the sites is due to presence of soft soil in that area
are responsible for the destruction in most of the sites. The results also have shown that the
parametric values of the ground motion are greater in the peripheral region than in the central part of
the Valley.