**5. Conclusion**

Low-velocity layers are known to have the ability to amplify and lengthen the duration of earthquake ground motion. However, their effect when found buried

### *Assessing Seismic Site Response at Areas Characterized by a Thick Buried Low-Velocity Layer DOI: http://dx.doi.org/10.5772/intechopen.95277*

between two geologically harder layers is not well documented. The fact that the Maltese islands are characterized by a thick buried low-velocity layer in almost half of their span provides a good opportunity to study the effect of this type of stratigraphic sequence in a deeper manner. One of the main motivations for this study was the long-standing lack of knowledge within the local scientific and engineering community of how the present, highly densified building stock would fare in the case of a major earthquake in the region. However, the results from this study also provide an important contribution to the international seismological and earthquake engineering community.

A comprehensive investigation was carried out by obtaining *V*<sup>S</sup> profiles at various sites around the islands which were then used as an input to the equivalentlinear site response analysis programme SHAKE2000. The H/V, ESAC and genetic inversion algorithm were used for obtaining *V*<sup>S</sup> profiles at 20 sites and they have been shown to perform very well, particularly in resolving both the presence and the characteristics of a low-velocity layer in the stratigraphy.

The *VS* profiles together with dynamic soil properties and design acceleration time histories were then used as inputs to SHAKE2000 to provide the acceleration response spectra at each site. Significant differences were obtained between the EC8 design spectra and some of the site response spectra at various periods. In particular, the response spectrum of the majority of the sites significantly exceeds the EC8 spectra at the plateau, and longer periods of the design spectrum for the site class based on VS30. The predominant period and fundamental frequencies obtained from the transfer function coincide with resonance frequencies of typical 2–10 storey buildings, which are becoming increasingly common in areas where such stratigraphic sequence is present.

From the calculated amplification factors (FPGA, FA and FV), it has been observed that sites with a thin capping of UCL above the clay layer exhibit high amplification factors. However, it was also noted that the other properties of the *VS* profiles such as the impedance contrast also contribute to high FV amplification values.

Finally, the inadequacy of using *VS*<sup>30</sup> or *V*Sbedrock to generalize the behaviour of different sites using one design response spectrum was shown by creating layered structures having the same *VS30* and *VSbedrock* but a different *V*<sup>S</sup> profile. The resulting response spectra varied even by a factor of 2 or more at certain periods.

Whereas a site with outcropping clay is normally perceived as vulnerable within the local construction industry, sites on outcropping UCL may often be regarded as "rock sites" without adequate consideration of the effect of the underlying clay. The results here highlight the importance of carrying out site-specific response investigations in areas with buried low-velocity layers, and the need for care in the use of *VS30* as a proxy for site amplification. The use of non-invasive ambient noise measurements allows a more cost-effective way of obtaining knowledge about the deeper VS structure, which may influence earthquake response at the site.
