**7. Local site effects linked to the topography**

**•** The use of noise measurements indicates that, at least in case of rockfall landslides, the existence of different zones characterized by differences in the dominant spectral ratio peaks that can be related to the presence of shallow lithotypes as well as to the existence of the fractures in the rock and active slip surface that allows the slow sliding of the upper landslide

**Figure 20.** a) Cross section along the A-B profile in Fig. 27; b) 2-D diagram obtained combining all the ambient noise measurements along the profile as a function of distance (x axis) and frequency (y axis); c) HVSR results at the record‐

**•** The instrumental observations indicate that the seismic ground motion can be considerably amplified and such amplification has a directional character that appear related with topographic, lithologic and structural features as well as normal mode rock slope vibration.

**•** The results of horizontal-to-vertical spectral ratio measurements indicate that this method

could be useful for the recognition of site response directional phenomena.

body.

ing sites located across the profile.

130 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

The evaluation of the local seismic response when affected by the presence of topographic irregularities is particularly important, from the engineering point of view, mostly since a number of historical villages in Italy are erected on the top of natural reliefs. The influence of the topography on ground motion is linked to the sharpness of the ridge crest (Géli *et al*., 1988; Bard and Riepl-Thomas, 1999). Amplification effects are generally linked to the focalization of seismic waves at topmost part of a hill, due to the existence of diffraction, reflection, and conversion of the incident waves (Bard, 1982). They appear also frequency-dependent so that resonance phenomena occur when the wavelength of the incident wave is comparable to the horizontal dimension of the hill. In addition, significant directional effects, transverse to the major axis of the ridge, are often observed (Spudich *et al.*, 1996).

Several analytical and numerical methods have been developed to study incoming seismic waves when crossing a hill shaped morphology (*e.g.*, LeBrun *et al.*, 1999; Paolucci, 2002). Although experimental studies using earthquake instrumental records are relatively few, the use of earthquake data has shown to be a successful tool for the evaluation of topographic effects, as well as artificial explosions and ambient noise records, processed with the HVNR technique (*e.g.*, Borcherdt, 1970; LeBrun *et al.*, 1999; Poppeliers and Pavlis, 2002; Pagliaroli *et al.*, 2007).

The present study was performed in two reliefs having different morphologic and geologic features aiming to discriminate the topographic from the stratigraphic effects, using experi‐ mental techniques based on earthquake and ambient noise recordings in order to test at the same time the reliability of ambient noise recordings, processed through HVNR techniques, to estimate topographic effects. The first relief investigated is the area of Ortigia (downtown Siracusa, Sicily). It is a hill shaped peninsula, mostly formed by a carbonate sequence, elongated in the N-S direction, reaching a length of about 1,500 m, with a maximum height of 30 m a.s.l., having a transverse section width of about 700 m (Fig. 21). The second test area is the university campus (S. Sofia hill), a ridge located in the northern part of Catania (Fig. 22). The S. Sofia hill has a gentle topography with a flat surface at the top, it is elongated for about 700 m in NW-SE direction with a maximum height of 40 m. Its longitudinal section (B-B' in Fig. 22) is asymmetric, consequently the northwestern side is quite gentle with respect to the southeastern part. On the other hand, the transverse section is more regular and symmetric (A-A' in Fig. 22), with a base having width of about 500 m. This area has a more complex geology. The most frequently cropping out lithotype is basaltic lava that in pre-historical and historical times flowed onto the valleys originally existing in the sedimentary formations, formed by sand and gravel, laying over a marly clays basement.

In both areas several ambient noise measurements were performed, processing data with spectral ratio techniques and evaluating the directional effects as well. Moreover, in the S. Sofia area three permanent stations located respectively on the top of the hill, along the slope and at a reference site, about three kilometers away from the study area (see Fig. 22a, b). The area was monitored for about two years and a set of 44 local and regional seismic events, having a good signal-to-noise ratio was selected and processed using HVSR and SSR techniques.

**Figure 21.** Geolithologic map of Ortigia (downtown Syracuse).

#### **7.1. Results and discussion**

Figure 23 shows a direct comparison of the rotated HVNRs, in the frequency band 1.0-10.0 Hz, and the results of noise polarization analysis for the same recording sites, filtering the signal in the range 1.0-3.0 Hz. Both methodologies agree, indicating, particularly in the frequency range 1.0–3.0 Hz, that maxima of HVNR amplitudes take place at 90–100° and maxima of the horizontal polarization strike in the E-W direction. We also compared field data observations with the theoretical resonance frequency (*f*0) expected for the topographic effects in Ortigia hill. We adopted the relationship *f*0 = *Vs/L* (Bouchon, 1973; Géli *et al.*, 1988), where *L* is the width of the hill (about 700 m) and *Vs* is the shear wave velocity of the limestone outcropping in the peninsula (1,000 m/s). The predicted value, *f*0 = 1.4 Hz, is consistent with the observed spectral ratio peaks, in the range of 1.0–3.0 Hz. In general, the amplification of ground motion connected to the surface topography is directly related to the sharpness of the topography (Bard, 1994). In such instances topographic effects become clearly detectable with experimental and numerical approaches. In our study, the gentle topography and the homogeneous lithology of the Ortigia peninsula make it an ideal and simple case study for investigating topographic effects using ambient noise records. The Ortigia hill has a natural frequency of about 1.4 Hz and shows an E-W preferential direction of vibration. The specific directional effects in ambient noise, well defined both in space and in a narrow frequency band (1.0–3.0 Hz), are signs of a normal mode of vibration of the hill (Roten *et al.*, 2006).

The analysis performed in the other study case, (S. Sofia hill) indicate a more composite situation where both the complexity of surficial geology and the morphology significantly affect local amplification and directional effects. The results of SSR and HVSR are reported in Figure 24. Inspection of the Horizontal Standard Spectral ratio (HSSR) (Fig. 24a) point out that at the station CITT less pronounced spectral ratio peaks are observed with respect to the station POLI. The spectral ratio amplitudes obtained through the HVSR method appear underesti‐ mated in amplitude with respect to those obtained through the HSSR approach, however, especially at POLI, a good agreement between the two methodologies is observed as regards the frequency range of the dominant peaks (Fig. 24b). Moreover, it is interesting to observe that CITT and POLI stations show flat spectral ratio peaks, with HVSR amplitudes going down to values lower than 1 unit at frequencies higher than about 3.0 and 5.0 Hz, respectively. This appears related to the presence of a significant amplification of the vertical component of motion, as can be observed in the V/Vref plots (Fig. 24c). Such effect could be explained in terms of the complexity of the near-surface morphology and the existence of pronounced stratigraphic heterogeneities that, as postulated by many authors (e.g. Raptakis *et al.,* 2000; Bindi *et al.,* 2009), may affect the vertical component of motion. In any case, greater amplifi‐ cations are observed in the HSSR at station POLI, in the frequency range 2.0-5.0 Hz and with smaller amplification values at CITT, in the range 1.0-2.0 Hz. This is different from what is usually expected for a topographic ridge, where major spectral ratio amplifications are

**Figure 22.** a) Geolithologic map of the S. Sofia hill; the insets show cross sections AA' and BB'. (b) location of the per‐

Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities,

Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 133

manent stations with respect to the reference one (UNIV) (modified from Monaco et al., 2000).

Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities, Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 133

**Figure 22.** a) Geolithologic map of the S. Sofia hill; the insets show cross sections AA' and BB'. (b) location of the per‐ manent stations with respect to the reference one (UNIV) (modified from Monaco et al., 2000).

**Figure 21.** Geolithologic map of Ortigia (downtown Syracuse).

132 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

normal mode of vibration of the hill (Roten *et al.*, 2006).

Figure 23 shows a direct comparison of the rotated HVNRs, in the frequency band 1.0-10.0 Hz, and the results of noise polarization analysis for the same recording sites, filtering the signal in the range 1.0-3.0 Hz. Both methodologies agree, indicating, particularly in the frequency range 1.0–3.0 Hz, that maxima of HVNR amplitudes take place at 90–100° and maxima of the horizontal polarization strike in the E-W direction. We also compared field data observations with the theoretical resonance frequency (*f*0) expected for the topographic effects in Ortigia hill. We adopted the relationship *f*0 = *Vs/L* (Bouchon, 1973; Géli *et al.*, 1988), where *L* is the width of the hill (about 700 m) and *Vs* is the shear wave velocity of the limestone outcropping in the peninsula (1,000 m/s). The predicted value, *f*0 = 1.4 Hz, is consistent with the observed spectral ratio peaks, in the range of 1.0–3.0 Hz. In general, the amplification of ground motion connected to the surface topography is directly related to the sharpness of the topography (Bard, 1994). In such instances topographic effects become clearly detectable with experimental and numerical approaches. In our study, the gentle topography and the homogeneous lithology of the Ortigia peninsula make it an ideal and simple case study for investigating topographic effects using ambient noise records. The Ortigia hill has a natural frequency of about 1.4 Hz and shows an E-W preferential direction of vibration. The specific directional effects in ambient noise, well defined both in space and in a narrow frequency band (1.0–3.0 Hz), are signs of a

**7.1. Results and discussion**

The analysis performed in the other study case, (S. Sofia hill) indicate a more composite situation where both the complexity of surficial geology and the morphology significantly affect local amplification and directional effects. The results of SSR and HVSR are reported in Figure 24. Inspection of the Horizontal Standard Spectral ratio (HSSR) (Fig. 24a) point out that at the station CITT less pronounced spectral ratio peaks are observed with respect to the station POLI. The spectral ratio amplitudes obtained through the HVSR method appear underesti‐ mated in amplitude with respect to those obtained through the HSSR approach, however, especially at POLI, a good agreement between the two methodologies is observed as regards the frequency range of the dominant peaks (Fig. 24b). Moreover, it is interesting to observe that CITT and POLI stations show flat spectral ratio peaks, with HVSR amplitudes going down to values lower than 1 unit at frequencies higher than about 3.0 and 5.0 Hz, respectively. This appears related to the presence of a significant amplification of the vertical component of motion, as can be observed in the V/Vref plots (Fig. 24c). Such effect could be explained in terms of the complexity of the near-surface morphology and the existence of pronounced stratigraphic heterogeneities that, as postulated by many authors (e.g. Raptakis *et al.,* 2000; Bindi *et al.,* 2009), may affect the vertical component of motion. In any case, greater amplifi‐ cations are observed in the HSSR at station POLI, in the frequency range 2.0-5.0 Hz and with smaller amplification values at CITT, in the range 1.0-2.0 Hz. This is different from what is usually expected for a topographic ridge, where major spectral ratio amplifications are

remembered that POLI is set on sedimentary terrains whereas CITT is located on a compact lava flow and such lithotype generally does not shows significant spectral ratio peaks as already observed by several authors (Lombardo and Rigano, 2007; Panzera et al, 2011b).

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Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 135

**Figure 24.** Spectral ratios (HSSR a), HVSR b) and VSSRc)) at permanent stations CITT and POLI; d) HVSR of the NS and

Ambient noise was recorded, in different lithotypes, along the slopes of the hill at decreasing height from the top (Fig. 22a), to identify possible topographical effects. The HVNRs show flat and, at times, deamplificated spectral ratios in several recording sites (see examples #9, #11, #16, #22 reported in Fig. 25) where the local geology consists in a sequence of thick (10-20 m) massive lava flows that overlie the sand and coarse gravel sediments lying on the marly clay basement. Conversely, when the sedimentary terrains outcrop (see examples #3, #4, #21, #23 reported in Fig. 25), significant spectral ratio peaks, are observed. Results obtained by the HVNR confirm the findings from earthquake data analysis and set into evidence that we are dealing with a geological setting more complex than a simple 1-D layered structure, for which the noise spectral ratio method was originally proposed. The presence of lava flows at the surface imply the existence of possible velocity inversions that give origin to H/V spectral amplitude lower than one unit (Castellaro and Mulargia, 2009; Di Giacomo *et al.,* 2005) and the

The existence of directional amplification was investigated using both earthquake and ambient noise data. Directional analysis and polarization of the horizontal components of motion show less pronounced directional effect at CITT with respect to POLI station, where clear polariza‐ tion effects at about 40° appear. The results of polarization analysis are depicted in Figure 26. The hodograms obtained from noise measurements show that the polarization azimuths are similar to those obtained by processing the earthquake data. It appears indeed confirmed that at the top of the studied hill (#9, #12, #14 and #28) the pattern of polarization directions is similar

existence of amplification in the vertical component of the ground motion.

EW components of motion at the reference site UNIV.

**Figure 23.** Contours of the geometric mean of the spectral ratios as a function of frequency (*x* axis) and direction of motion (*y* axis) and polarization rose diagrams calculated in the ranges 1–3 Hz.

supposed to take place at the top rather than along the slopes of the hill. This behavior, in our opinion, could be related to the gentle slope of the S. Sofia hill. In such conditions the reflection angle between the direction perpendicular to the free surface topography and the upward propagating wavefront is smaller than in the case of a steep slope. Therefore, the focusing effects at the crest are shadowed by laterally propagating waves (Boore, 1973) and it is reasonable to observe only moderate amplifications at the top of the hill. Moreover, it must be remembered that POLI is set on sedimentary terrains whereas CITT is located on a compact lava flow and such lithotype generally does not shows significant spectral ratio peaks as already observed by several authors (Lombardo and Rigano, 2007; Panzera et al, 2011b).

**Figure 24.** Spectral ratios (HSSR a), HVSR b) and VSSRc)) at permanent stations CITT and POLI; d) HVSR of the NS and EW components of motion at the reference site UNIV.

Ambient noise was recorded, in different lithotypes, along the slopes of the hill at decreasing height from the top (Fig. 22a), to identify possible topographical effects. The HVNRs show flat and, at times, deamplificated spectral ratios in several recording sites (see examples #9, #11, #16, #22 reported in Fig. 25) where the local geology consists in a sequence of thick (10-20 m) massive lava flows that overlie the sand and coarse gravel sediments lying on the marly clay basement. Conversely, when the sedimentary terrains outcrop (see examples #3, #4, #21, #23 reported in Fig. 25), significant spectral ratio peaks, are observed. Results obtained by the HVNR confirm the findings from earthquake data analysis and set into evidence that we are dealing with a geological setting more complex than a simple 1-D layered structure, for which the noise spectral ratio method was originally proposed. The presence of lava flows at the surface imply the existence of possible velocity inversions that give origin to H/V spectral amplitude lower than one unit (Castellaro and Mulargia, 2009; Di Giacomo *et al.,* 2005) and the existence of amplification in the vertical component of the ground motion.

The existence of directional amplification was investigated using both earthquake and ambient noise data. Directional analysis and polarization of the horizontal components of motion show less pronounced directional effect at CITT with respect to POLI station, where clear polariza‐ tion effects at about 40° appear. The results of polarization analysis are depicted in Figure 26. The hodograms obtained from noise measurements show that the polarization azimuths are similar to those obtained by processing the earthquake data. It appears indeed confirmed that at the top of the studied hill (#9, #12, #14 and #28) the pattern of polarization directions is similar

supposed to take place at the top rather than along the slopes of the hill. This behavior, in our opinion, could be related to the gentle slope of the S. Sofia hill. In such conditions the reflection angle between the direction perpendicular to the free surface topography and the upward propagating wavefront is smaller than in the case of a steep slope. Therefore, the focusing effects at the crest are shadowed by laterally propagating waves (Boore, 1973) and it is reasonable to observe only moderate amplifications at the top of the hill. Moreover, it must be

**Figure 23.** Contours of the geometric mean of the spectral ratios as a function of frequency (*x* axis) and direction of

motion (*y* axis) and polarization rose diagrams calculated in the ranges 1–3 Hz.

134 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**Figure 25.** Examples of HVNR results at representative recording sites located on lava flows (a) and on sedimentary terrains (b); solid black lines refer to the average H/V spectra, dotted grey and black lines refers to NS/V and EW/V spectra, respectively.

to what is observed at the station CITT. On the other hand, the noise rose diagrams obtained at the other recording sites point out polarization azimuths that seem to be in agreement with the slope directions of the hill flanks. Only few sites (#17, #21, #22, #25) make an exception to such trend, showing a directional variability that could be linked to the local shallow lithologic features. The investigation on the characteristics of the site response at the S. Sofia hill, therefore set into evidence that the complexity of the near-surface geology, as well as the morphology strongly influence the local amplification of the ground motion and the directional effects. Findings of the present study confirm that major amplification effects do indeed take place on the sedimentary terrains which outcrop along the flanks of the hill. On the contrary, on the lava flows, a significant amplification of the vertical component of motion, is observed as a consequence of velocity inversion effects.

The results coming out from investigations performed in the two test areas allow us to draw the following general considerations about topographic effects:

degree-of-freedom damped oscillator (Gallipoli *et al*.2009). In such instances the vertical component of motion travels through the building without amplification, whereas the

**Figure 26.** Ground motion polarization from noise measurements in the S. Sofia hill area; hodograms with a grey

Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities,

Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 137

background refers to results coming from the analysis of earthquakes recorded at CITT and POLI stations.

**•** As a practical implication of the present study it can be observed that the topographic effects cannot be easily evaluated especially when subsurface morphology and lithologic features

The present study has tested the use of ambient noise recordings as a speedy technique for evaluating the local seismic response in several instances where either lithologic and/or morphologic and structural features can significantly affect the response of shallow geologic formations to a seismic input. Our findings further support the reliability of the use of ambient

horizontal components undergo a significant amplification.

are predominant.

**8. Concluding remarks**


Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities, Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 137

**Figure 26.** Ground motion polarization from noise measurements in the S. Sofia hill area; hodograms with a grey background refers to results coming from the analysis of earthquakes recorded at CITT and POLI stations.

degree-of-freedom damped oscillator (Gallipoli *et al*.2009). In such instances the vertical component of motion travels through the building without amplification, whereas the horizontal components undergo a significant amplification.

**•** As a practical implication of the present study it can be observed that the topographic effects cannot be easily evaluated especially when subsurface morphology and lithologic features are predominant.
