Sensitivity Analysis of Soil Site Response Modelling in Seismic ... - ITC

Figure 2-7 The microtremors are placed to an alluvial site, a Rock site and at the top of a hill. The recordings
are received and processed. Then, the amplification spectrum is plotted. The differences between
the sites are explained using the ratios and comparisons of the H/V components. This method refers to
Standard Spectral Ratio of site effect estimations (Duval, 1994)
Various sets of experimental data confirmed that these ratios are much more stable than the raw noise
spectra. Above all, Lacave and Bard added that several theoretical investigations supported these observations
(Lacave, Bard et al. 2002). They showed that synthetic strong motion records obtained with
randomly distributed near surface sources lead to horizontal-vertical ratios sharply peaked around the
fundamental S-wave frequency, whenever the surface layers exhibit a sharp difference with the underlying
stiffer formations. Meaning that the formations with high differing properties; unit weights,
shear wave velocities etc have been estimated successfully from the H/V ratios. On the other hand the
studies formerly mentioned also concluded; the amplitude of this peak is not well correlated with the S
wave amplification at the site’s resonant frequency. But more sensitive to Poisson’s ratio near the surface,
which is the ratio of the transverse strain to the longitudinal extension strain (n). Tensile deformation
(its ability to support a load without breaking; the material can be stretch) is considered positive
and compressive deformation is considered negative. The definition of Poisson's ratio contains a
minus sign so that normal materials have a positive ratio (Figure 2-8).
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Sensitivity Analysis Of Soil Site Response Modelling In Seismic Microzonation For Lalitpur, Nepal