3. VS30 and fundamental period

constructions at the Port/Coastal Area, and/or a high degree of consolidation due to the constant presence of heavy weight (containers) that are located at these sites for shipping purposes. Note that the DFA predicted very well the H/V ratios in such circumstances as well, for both, the fundamental period and the overall shape of the transfer function. It is noted that this consolidated layer at the top of the Port Area behaves as a low pass filter and does not have an influence in the fundamental period of motion of the whole soil system; such feature was corroborated

performing the DFA without the stiff top layer at Mucurapo Secondary School (see Figure 10b). Sea Lots site at the South East of POS (see Figure 1 at site 6X) is characterized by the lowest VS of 50 m/s for all array sites that correspond to a swamp area overlaid by stiff deposits. Such low values of VS have been observed in

Figure 11 depicts the comparison between the 1-D SH wave amplification employing the Vs profiles obtained by the GAs and the H/V observed spectral ratios. We also adopted for the surface sediments above the bedrock a low-quality

Comparison of H/V spectral ratios and 1-D SH wave transfer function; case (1) only up wave amplification and case (2) up + down amplification with refraction and reflection in bedrock. (a) Queen's Park Savannah array (1X), (b) Port Area (5X), (c) Mucurapo Secondary School (3X), (d) Sea Lots (8X), (e) Nelson Mandela Park (2X), (f) Woodford Square (8X), (g) Federation Park (4X), (h) St. James hospital (9X), and (i) St. Benedict's Children's home (7X). The fundamental period of soil T is indicated by the arrows.

sedimentary stratigraphy of natural intertidal flats [26].

Natural Hazards - Risk, Exposure, Response, and Resilience

Figure 11.

70

2.2 H/V ratios and 1-D theoretical transfer function for SH-waves

An important parameter in the modification of seismic waves propagating toward the surface is the composition of the near-surface soil layers. In different building codes around the world, the average shear wave velocity of the upper 30 m (VS30) has been adopted to characterize the response of seismic waves to the influence of near-surface strata.

In first instance, we compared the VS30 obtained from our microtremors array observation and the ones estimated by the empirical formulas of Matsuoka et al. [27] employing 2000 sites in Japan based on geomorphological units.

We calculated the VS30 from our microtremor results using the following formula:

$$V\_S \mathbf{30} = \frac{\mathbf{30}}{\sum\_{i=1}^{N} \frac{h\_i}{V\_i}} \tag{10}$$

where hi and Vi denote the thickness (in meters) and the shear wave velocity of the ith layer; N is the total number of soil layers respectively.

We classified the sites (1X–5X, 8X) as a Gravelly Terrace, Sea Lots (site 6X) as a Reclaimed Land, and St. Dominic's Children's Home to the East (7X) and St. James Hospital to the West (9X) as Mountain Foot Slope sites (see Figure 1). The empirical formulas to estimate VS30 (m/s) for the Gravelly Terrace (Eq. (11)), the Reclaimed Land (Eq. (12)), and the Mountain Foot Slope (Eq. (13)) yield:

$$\log \text{VSS30} = 2.493 + 0.072 \log Ev + 0.027 \log Sp} - 0.164 \log Dm \pm 0.122(\sigma) \tag{11}$$

$$
\log \text{VS} \mathfrak{B} = 2.37 \mathfrak{B} - 0.124 \log \text{D}m \pm 0.12 \mathfrak{B}(\sigma) \tag{12}
$$

$$\log \text{VSA} = 2.602 \pm 0.092(\sigma) \tag{13}$$

where Ev is the elevation (m), Sp refers to the Tangent of Slope\*1000, Dm yields the distance (km) from mountain or hill, and σ denotes the standard deviation. We took Dm as the shortest distance to the Northern Range or the Laventille Metalimestone foothills (Figure 1). The results are presented in Figure 12. In general the estimated VS30 from the empirical formulas of Matsuoka et al. [27] estimates well the velocities obtained by the GA's from our array measurements in the range of �σ (standard deviation). We also compared the VS30 of our microtremors array profiles with the ones estimated by Allen and Wald [28] using the topographic slope as a proxy of site conditions employing the USGS Web Server (earthquake.usgs.gov/hazards/apps/vs30/). We retrieved the correspondent predicted VS30 at the location of each microtremors array. The most noticeable difference is observed for the mountain foot slope in St. James (site 9X). However, we did not find a good correlation when comparing with soil types proposed by

#### Figure 12.

Comparison of VS30 (m/s) retrieved from our microtremors array and the empirical formulas of Matsuoka [27] and the method of Allen and Wald [28]. The shadowed area represents the classification of Zhao and Xu [29] based on fundamental period of soil and VS30.

Zhao and Xu [29] based on NEHRP classes on VS30 (shadowed areas in Figure 12). This leads to suggest to characterize the soil at POS by the fundamental period rather than the VS30 [30]; Zhao and Xu [29] suggest also that site period is a better parameter for characterizing soil conditions, in very deep or very soft sediments.

#### 4. Preliminary assessment of liquefaction susceptibility

Nakamura [31] proposed a technique to investigate the liquefaction susceptibility based on microtremor measurements, namely, the vulnerability index Kg for the surface ground, as follows:

$$K\_{\rm g} = \frac{{A\_{\rm g}}^2}{F\_{\rm g}} \tag{14}$$

of Saint James in the Coastal area yields a possibility of high liquefaction potential. Evidence of soil subsidence is already present in some structures near the Port Area as the Lighting House Tower (Figure 14). Next to it, the Eric Williams Complex known as the Twin Towers which are one of the tallest buildings in POS (92 m height) has been constructed in the 1980s incorporating piles on their foundations; the eyewitness during the construction process affirmed that some piles sank totally during their driving process due to extremely soft soil conditions found at that time in the coastal area. Several new high-rise buildings including hotels, a water front, high-income class dwellings, amenity centers, and the Port itself are located in this high liquefaction susceptibility area. Ironically, Sea Lots located to the West is characterized by a very low-income social class; it is also a prone area of high potential of liquefaction. It is noted that the study of Kraft [32] employing the methodology of Holzer et al. [33] yielded similar conclusions for POS and another

Preliminary liquefaction hazard map for Port of Spain City, employing Nakamura index Kg, (see Eq. (14)).

Estimation of Shear Wave Velocity Profiles Employing Genetic Algorithms and the Diffuse Field…

DOI: http://dx.doi.org/10.5772/intechopen.85129

cities of Trinidad employing regional geological map conditions.

Zones yielding a Kg above 20 are suggested to a high liquefaction susceptibility.

Shear wave velocity VS profiles were determined by performing nine

microtremors array surveys in Port of Spain (POS), Trinidad, employing the spatial autocorrelation SPAC method and genetic algorithms (GAs); the results yielded VS between 50 and 2000 m/s at POS. The ellipticity pattern for the first mode of Rayleigh waves explains the resulting predominant peak in the H/V Nakamura ratios for all array sites. We validated the soil profiles retrieved by the SPAC and GAs'schemes comparing the synthetics' horizontal-to-vertical spectral (H/V) ratios

5. Conclusions

73

Figure 13.

where Ag is the amplification factor referenced to the engineering bedrock and Fg is the predominant frequency of vibration of the soil profile (the inverse of the period); both values can be taken from the horizontal-to-vertical spectral ratio (H/V) of microtremors; Ag is considered to be the H/V ratio at the predominant frequency. Values of Kg greater than 20 are considered likely to liquefy. The authors computed the liquefaction potential using Eq. (14) at each point and develop an iso-liquefaction potential map interpolating the Kg value of the 1181 single mobile microtremors data employed in Salazar et al. [1] (Figure 13). The results are very concerning regarding this hazard because the water table in POS can be found just at the surface, the soil conditions then are saturated sands and gravels, and sometimes poorly consolidated reclaimed land has been placed specially in the coastal areas. The areas with a high liquefaction susceptibility are The Port, Sea Lots, some parts of Woodbrook, a small spot in Cocorite (where in fact reclaimed land exists), and some small areas in the Queen's Park Savannah and Federation Park. Also South Estimation of Shear Wave Velocity Profiles Employing Genetic Algorithms and the Diffuse Field… DOI: http://dx.doi.org/10.5772/intechopen.85129

Figure 13.

Zhao and Xu [29] based on NEHRP classes on VS30 (shadowed areas in Figure 12). This leads to suggest to characterize the soil at POS by the fundamental period rather than the VS30 [30]; Zhao and Xu [29] suggest also that site period is a better parameter for characterizing soil conditions, in very deep or very soft sediments.

Comparison of VS30 (m/s) retrieved from our microtremors array and the empirical formulas of Matsuoka [27] and the method of Allen and Wald [28]. The shadowed area represents the classification of Zhao and Xu

Nakamura [31] proposed a technique to investigate the liquefaction susceptibility based on microtremor measurements, namely, the vulnerability index Kg for the

> 2 Fg

(14)

Kg <sup>¼</sup> Ag

where Ag is the amplification factor referenced to the engineering bedrock and Fg is the predominant frequency of vibration of the soil profile (the inverse of the period); both values can be taken from the horizontal-to-vertical spectral ratio (H/V) of microtremors; Ag is considered to be the H/V ratio at the predominant frequency. Values of Kg greater than 20 are considered likely to liquefy. The authors computed the liquefaction potential using Eq. (14) at each point and develop an iso-liquefaction potential map interpolating the Kg value of the 1181 single mobile microtremors data employed in Salazar et al. [1] (Figure 13). The results are very concerning regarding this hazard because the water table in POS can be found just at the surface, the soil conditions then are saturated sands and gravels, and sometimes poorly consolidated reclaimed land has been placed specially in the coastal areas. The areas with a high liquefaction susceptibility are The Port, Sea Lots, some parts of Woodbrook, a small spot in Cocorite (where in fact reclaimed land exists), and some small areas in the Queen's Park Savannah and Federation Park. Also South

4. Preliminary assessment of liquefaction susceptibility

surface ground, as follows:

[29] based on fundamental period of soil and VS30.

Natural Hazards - Risk, Exposure, Response, and Resilience

Figure 12.

72

Preliminary liquefaction hazard map for Port of Spain City, employing Nakamura index Kg, (see Eq. (14)). Zones yielding a Kg above 20 are suggested to a high liquefaction susceptibility.

of Saint James in the Coastal area yields a possibility of high liquefaction potential. Evidence of soil subsidence is already present in some structures near the Port Area as the Lighting House Tower (Figure 14). Next to it, the Eric Williams Complex known as the Twin Towers which are one of the tallest buildings in POS (92 m height) has been constructed in the 1980s incorporating piles on their foundations; the eyewitness during the construction process affirmed that some piles sank totally during their driving process due to extremely soft soil conditions found at that time in the coastal area. Several new high-rise buildings including hotels, a water front, high-income class dwellings, amenity centers, and the Port itself are located in this high liquefaction susceptibility area. Ironically, Sea Lots located to the West is characterized by a very low-income social class; it is also a prone area of high potential of liquefaction. It is noted that the study of Kraft [32] employing the methodology of Holzer et al. [33] yielded similar conclusions for POS and another cities of Trinidad employing regional geological map conditions.
