**4.3 The in/adequacy of** *VS30***,** *VSbedrock* **and site classification of building codes**

Many authors [34, 35] have argued that the *V*S30 is not an adequate proxy to characterize the amplification potential of a site, especially in the type of geological situation being considered here. Since the low-velocity layer is found at a depth which is usually not considered in the *V*S30 calculation, these authors have suggested the use of the travel-time average shear-wave velocity down to the bedrock (*V*Sbedrock) as an alternative.

To assess the reliability of the *V*S30 and *V*Sbedrock parameters and site classification schemes as proxies for site effects, a number of different shear-wave velocity profiles, all having the same *V*S30 (670 m/s +/� 15 m/s) and *V*Sbedrock (625 m/s +/� 15 m/s) were randomly constructed. According to the *V*S30 values, these profiles classify as EC8 Class B sites. Within the profiles, the shear-wave velocity and thickness of each layer (UCL, BC, UCL) were constrained to be within the ranges of values measured in the study, shown in **Table 1**. The numerical analysis was conducted again for each of these profiles and the resulting spectra for 4 such profiles, together with the Type 1 EC8 spectra for the different site classes, are presented in **Figure 15**.

Significant differences can be seen between the different response spectra at a wide period range. In particular, the PGA varies from 0.68 *g* to 0.1 *g* and the maximum spectral acceleration varies from 0.2 *g* to almost 0.35 *g*. As regards the EC8 design spectra, profile 2 is the only profile whose response is comparable with the EC8 site class B design spectrum. The spectrum of profile 4 can be compared

*FPGA* <sup>¼</sup> *PGAoutput PGAinput*

> *FA* <sup>¼</sup> *SAoutput SAinput*

*FV* <sup>¼</sup> *SVoutput SVinput*

*SA* ¼

*SV* ¼

called the predominant period in Section 4.1 above); and

*SA* and *SV* are obtained as follows:

*Earthquakes - From Tectonics to Buildings*

maximum.

**Figure 13.**

**154**

where the input values are obtained from the mean spectrum in **Figure 10** while the output values are obtained from the mean output spectra shown in **Figure 11**.

> Ð <sup>1</sup>*:*5*TA* <sup>0</sup>*:*5*TA Sa dT TA*

where *TA* is the period at which the spectral acceleration (*Sa*) is maximum (also

Ð <sup>1</sup>*:*2*TV* <sup>0</sup>*:*8*TVSv dT* 0*:*4*TV*

with *TV* representing the period at which the velocity response spectrum (*Sv*) is

It can be noted that TA is generally smaller than TV, and that the integral in SA

The FPGA and FA values are in the majority of the cases both larger than 1 which implies that amplification is expected. However in a few cases values less than 1 were obtained, indicating that these sites are capable of deamplifying the input ground motion. FPGA values are less than 1 at sites which are characterised by a UCL thickness greater than 48 m and are thus classified as class A according to the EC8

is generally determined over a range that includes shorter periods. In fact [33] deduce that FA is more relevant at shorter periods (< 0.5 s) while FV may be related to the behaviour at longer periods, and thus more relevant in the case of taller buildings. The values of FPGA, FA and FV are mapped in **Figure 13**.

*Maps showing the three amplification factors at the studied sites: (a) FPGA; (b) FA and (c) FV [27].*

;

;

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 earth-

*Assessing Seismic Site Response at Areas Characterized by a Thick Buried Low-Velocity Layer*

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 *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

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

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.

This work was supported by the Endeavour Scholarship scheme financed out of

Government of Malta national fund and also formed part of the SIMIT project (Integrated Italy-Malta Cross-Border System of Civil Protection) (B1-2.19/11) partfinanced by the European Union under the ItaliaMalta Cross-Border Cooperation

Programme, 2007–2013. The authors are grateful to Dr. D. Albarello and

Dr. E.Lunedei for the use of the ESAC and joint inversion codes.

as the impedance contrast also contribute to high FV amplification values.

the characteristics of a low-velocity layer in the stratigraphy.

quake engineering community.

*DOI: http://dx.doi.org/10.5772/intechopen.95277*

stratigraphic sequence is present.

**Acknowledgements**

**157**

**Figure 14.**

*Graphs showing the variation of the amplification factors with VS profile characteristics [27].*

#### **Figure 15.**

*Left: The hypothetical VS profiles used for the test. Right: The resulting 5% damped response spectra of the used profiles and the EC8 design spectra.*

with the site class A spectrum, profile 3 with site class C, while profile 1 can be seen to exceed the response of a class E site. These results imply that both the *VS30* and *VSbedrock* proxies are not the ideal parameters to use for site response approximations and neither is the use of rigid classes represented by one design spectrum. On the contrary, this continues to support the idea that site-specific response analysis is required, especially for sites characterised by a buried low-velocity layer.
