**6. Site effects in landslide zones**

In order to compare the results from recordings in a natural cavity with those from records performed in a cavity having a simpler geometry, ambient noise was also recorded inside, over, and in the neighborhood (≈30 m) of two artificial tunnels. Both tunnels are located close to the Catania urban area, dug at about 4 m from the topographic surface and having a length of about 100 m, but different height. One of them (height of about 4 m) is dug in massive lavas and the other (height of about 7 m) is excavated in altered lavas. HVNRs show that in the smaller tunnel (Fig. 17a), dug in massive lavas, spectral peaks are significantly less pronounced than those observed in the tunnel, dug in altered lavas, having a greater height (Fig. 17b). In this tunnel, H/V spectral peaks, in the frequency range 4.0–7.0 Hz, obtained from measure‐ ments performed over and inside, attain values of about 8 and 4 units, respectively, therefore confirming the observation, aforementioned for the Petralia grotto, that inside the cavity spectral peaks are less pronounced than those observed when measurement is performed over the cavity. It is noteworthy that the spectral ratios from measurements in both tunnels show peaks that however are more pronounced than those observed from H/V performed in and over the study grotto. This is possibly related to the height that in both tunnels is significantly greater than in the studied cavity, considering also that the lava characteristics (altered lavas) of the Petralia grotto area are comparable to those of the lava where the greater artificial tunnel

124 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**Figure 17.** Spectral ratios HVNR from measurements performed in two underground tunnels having different height.

The results so far described are quite complex but nevertheless some interesting considerations

**•** The size of the vertical section and geometry of the cavities seem to play an important role. According to experimental data, it appears evident that only cavities having height greater

**•** Results of spectral ratios at Petralia grotto (having height of about 2.5 m) show spectral ratio peaks less pronounced than those observed in grottoes and tunnels having height of about 4 and 7 m. Such behavior, in our opinion, need to be tested and interpreted with the support of numerical modeling, to investigate also about possible amplifications/deamplification of

the vertical component of motion as highlighted by processing earthquake data.

than about 4 meters show significant H/V spectral peaks.

is dug.

can be inferred:

Landslide phenomena, besides exposing the affected areas to a considerable natural risk, imply the occurrence of significant variations in the local seismic response. To investigate such features, Fekruna Bay, in the area of Xemxija (Fig. 18), was selected. Xemxija is a seaside village and marina on the northeastern part of Malta and it is a very important site for touristic attractions, as well as cultural and historical heritage. The study area spans a couple of square kilometres. More than half of it is intensely built, while the remaining area consists of meadows and agricultural land. The area is characterized by a geology and topography that varies over small spatial scales. Its geomorphologic features are the result of the combined effect of the lithology, tectonics and coastal nature that shaped the region, and such features contribute towards the degree of geological instability of the whole area and particularly to the cliff sections.

The outcropping local geology in the Fekruna bay (Pedley *et al*., 2002) is characterised by the Upper Coralline Limestone (UCL) and the Blue Clay (BC) formations. Underneath the BC, a carbonatic formation, the Globigerina Limestone formation (GL) consisting mainly of loosely aggregated planktonic foraminifers, and a Lower Coralline Limestone formation (LCL), which consists of massive biogenic limestone beds, are present. This geolithologic sequence gives rise, in the north coast of Malta, to lateral spreading phenomena which take place within the brittle and heavily jointed and faulted UCL formation overlying the BC which consists of softer and unconsolidated material (Mantovani *et al.,* 2012). The UCL formation is characterized by a prominent plateau scarp face, whereas BC produces slopes extending from the base of the UCL scarp face to sea level. It is well known that lateral spreading usually takes place in the

We recorded ambient noise at 27 sites using a 3-component seismometer (Tromino, www.tro‐ mino.eu) and directional effects were investigated by rotating the NS and EW components of motion by steps of 10 degrees starting from 0° (north) to 180° (south). Moreover, a direct estimate of the polarization angle by using the covariance matrix method (Jurkevics, 1988) and the time-frequency (TF) polarization method (Burjánek *et al*., 2010; 2012) was achieved.

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Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 127

A dense microtremor measurement survey was carried out focusing attention on the NE part of the bay, in which there is major evidence of slope instability and in which a high level of cliff fracturing is evident. Recording sites were located in order to sample the area as uniformly as possible. Moreover, several recording sites where chosen on a linear deployment for investigating the role of the fractures in the HVNR behaviour. The measurements were performed in three different zones: indeed, we carried out one set of recordings in a stable platform and far away from the cliff edge, a set of measurements in the fractured area along the cliff, and another set on the landslide body (see Fig. 18 for measurement points location). Examples of the spectral ratios obtained in the three different zones are shown in Figure 19a. The results allowed us to identify a region, away from the cliff edge, where the HVNR peaks are around a stable frequency of about 1.5 Hz. These fundamental peaks may be generally associated with the interface separating the BC layer from the underlying carbonates of the Globigerina Limestone formation (GL). Moreover, the presence of the BC layer gives rise to a velocity inversion since it has a lower shear wave velocity with respect to the overlying UCL formation. This causes the HVNR values to drop below 1 unit over a wide frequency range (Di Giacomo et al., 2005). The origin of the resonance peak as linked to the BC/GL interface was confirmed by the results obtained through a 1D modelling, performed by computing the synthetic HVNR curves (Fig. 19b). To compute the synthetic spectral ratios we considered that ambient vibrations wavefield can be represented by the superimposition of random multi modal plane waves moving in all the directions at the surface of a flat 1-D layered visco-elastic solid, as in Herrmann (2002) formulation, extending the modal summation up to the fifth mode. We also applied initial constraints on the thicknesses and elastic parameters of the layers using borehole logs data and we took shear waves velocity values from a separate preliminary study carried out in the same area using the ReMi, MASW and Refraction methods (Panzera *et al.,* 2011a). Similar behaviour to the above is observed in the spectral ratio obtained at sites close to the cliff edge and all around the identified fractures, but with slightly different features at the high-frequency interval. We observe indeed a clear and predominant peak at around 1.5 Hz, which is associated with the interface between BC and GL, and several smaller peaks at higher frequency (> 9.0 Hz). The fact that these peaks are not visible in the unfractured region leads us to postulate that they may be associated with the presence of fractures and of blocks almost detached from the cliff and therefore free to oscillate. Finally, the sites on the rock-fall area show a different HVNR behaviour with respect to the measurements taken on the plateau. In this area it is possible to identify HVNRs showing bimodal dominant peaks at low frequen‐ cy, in the range 1.0-3.0 Hz, as well as pronounced peaks at about 3.0 Hz and at frequency higher than 9.0 Hz. The bimodal peaks at low frequency (1.0-3.0 Hz) can, in our opinion, be associated

**6.1. Results and discussion**

**Figure 18.** Geo-lithologic map of the north-eastern part of the Xemxija bay (modified from MEPA Various Authors, 2004). The insets show the location of the study area and a sketch of lateral spreading effects along the Xemxija coast.

lateral extension of cohesive rock masses lying over a deforming mass of softer material where the controlling basal shear surface is often not well defined.

A preliminary study of the area, focusing our attention on the risk of landsliding and rockfalls, was carried out using the ambient noise HVNR technique in order to characterize the rock masses behavior in the presence of fractures linked to the landslide body. This type of measurements can be done quickly and with a high spatial density, providing a fast tool for setting the dynamic behavior of the rock outcropping. Generally, this spectral ratio exhibits a peak, corresponding to the fundamental frequency of the site whereas, it is less reliable as regards its amplitude. Nevertheless, the HVNR curve contains valuable information about the underlying structure, especially as concerns the relationship between VS of the sediments and their thickness (Ibs-Vonseth and Wholenberg 1999; Scherbaum *et al*. 2003). Recordings of ambient noise and the use of the HVNR technique has recently had widespread use in studying landslides (*e.g.* Del Gaudio *et al*., 2008; Burjánek *et al*., 2010; Del Gaudio and Wasowski, 2011; Burjánek *et al*., 2012).

We recorded ambient noise at 27 sites using a 3-component seismometer (Tromino, www.tro‐ mino.eu) and directional effects were investigated by rotating the NS and EW components of motion by steps of 10 degrees starting from 0° (north) to 180° (south). Moreover, a direct estimate of the polarization angle by using the covariance matrix method (Jurkevics, 1988) and the time-frequency (TF) polarization method (Burjánek *et al*., 2010; 2012) was achieved.

#### **6.1. Results and discussion**

lateral extension of cohesive rock masses lying over a deforming mass of softer material where

**Figure 18.** Geo-lithologic map of the north-eastern part of the Xemxija bay (modified from MEPA Various Authors, 2004). The insets show the location of the study area and a sketch of lateral spreading effects along the Xemxija coast.

A preliminary study of the area, focusing our attention on the risk of landsliding and rockfalls, was carried out using the ambient noise HVNR technique in order to characterize the rock masses behavior in the presence of fractures linked to the landslide body. This type of measurements can be done quickly and with a high spatial density, providing a fast tool for setting the dynamic behavior of the rock outcropping. Generally, this spectral ratio exhibits a peak, corresponding to the fundamental frequency of the site whereas, it is less reliable as regards its amplitude. Nevertheless, the HVNR curve contains valuable information about the underlying structure, especially as concerns the relationship between VS of the sediments and their thickness (Ibs-Vonseth and Wholenberg 1999; Scherbaum *et al*. 2003). Recordings of ambient noise and the use of the HVNR technique has recently had widespread use in studying landslides (*e.g.* Del Gaudio *et al*., 2008; Burjánek *et al*., 2010; Del Gaudio and Wasowski, 2011;

the controlling basal shear surface is often not well defined.

126 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

Burjánek *et al*., 2012).

A dense microtremor measurement survey was carried out focusing attention on the NE part of the bay, in which there is major evidence of slope instability and in which a high level of cliff fracturing is evident. Recording sites were located in order to sample the area as uniformly as possible. Moreover, several recording sites where chosen on a linear deployment for investigating the role of the fractures in the HVNR behaviour. The measurements were performed in three different zones: indeed, we carried out one set of recordings in a stable platform and far away from the cliff edge, a set of measurements in the fractured area along the cliff, and another set on the landslide body (see Fig. 18 for measurement points location). Examples of the spectral ratios obtained in the three different zones are shown in Figure 19a. The results allowed us to identify a region, away from the cliff edge, where the HVNR peaks are around a stable frequency of about 1.5 Hz. These fundamental peaks may be generally associated with the interface separating the BC layer from the underlying carbonates of the Globigerina Limestone formation (GL). Moreover, the presence of the BC layer gives rise to a velocity inversion since it has a lower shear wave velocity with respect to the overlying UCL formation. This causes the HVNR values to drop below 1 unit over a wide frequency range (Di Giacomo et al., 2005). The origin of the resonance peak as linked to the BC/GL interface was confirmed by the results obtained through a 1D modelling, performed by computing the synthetic HVNR curves (Fig. 19b). To compute the synthetic spectral ratios we considered that ambient vibrations wavefield can be represented by the superimposition of random multi modal plane waves moving in all the directions at the surface of a flat 1-D layered visco-elastic solid, as in Herrmann (2002) formulation, extending the modal summation up to the fifth mode. We also applied initial constraints on the thicknesses and elastic parameters of the layers using borehole logs data and we took shear waves velocity values from a separate preliminary study carried out in the same area using the ReMi, MASW and Refraction methods (Panzera *et al.,* 2011a). Similar behaviour to the above is observed in the spectral ratio obtained at sites close to the cliff edge and all around the identified fractures, but with slightly different features at the high-frequency interval. We observe indeed a clear and predominant peak at around 1.5 Hz, which is associated with the interface between BC and GL, and several smaller peaks at higher frequency (> 9.0 Hz). The fact that these peaks are not visible in the unfractured region leads us to postulate that they may be associated with the presence of fractures and of blocks almost detached from the cliff and therefore free to oscillate. Finally, the sites on the rock-fall area show a different HVNR behaviour with respect to the measurements taken on the plateau. In this area it is possible to identify HVNRs showing bimodal dominant peaks at low frequen‐ cy, in the range 1.0-3.0 Hz, as well as pronounced peaks at about 3.0 Hz and at frequency higher than 9.0 Hz. The bimodal peaks at low frequency (1.0-3.0 Hz) can, in our opinion, be associated

with the contact between the rockfall and detritus unit and the BC formation, as well as to the interface between BC and the underlying GL formation.

to the vibration of smaller blocks can be observed at higher frequencies (9.0-40.0 Hz). Further‐ more, we obtained a direct estimate of the polarization angle through the full use of the threecomponent vector of the noise wave-field. General behavior of the noise wave-field, in this frequency range is shown through rose diagrams, whereas, in order to distinguish between properties of low and high frequency components of the signal, strike versus frequency polar plots were obtained. The examples shown in Figure 19d and e show that the maxima of the horizontal polarization occur in the north-east to east-north- east direction, although in some cases the high frequency directionality is more complex. As observed by Burjánek et al. (2010), high-frequency ground motion can indeed reflect the vibration of smaller blocks that imply both different resonant frequencies and directions. The polarization observed for the sites located away from the unstable areas (see e.g. #25 in Figure 19) show a trend with more dispersed and variable directions. The boundaries of the landslide area therefore appear well defined by the polarization pattern and as postulated by Kolesnikov et al. (2003) the landslide activity is characterized by strong horizontal polarization in a broad frequency band. In our study, a tendency seems evident for the entire landslide body to generally vibrate with a northeast azimuth and it may be assumed that, during a strong earthquake the ground motion would be amplified in this direction. Studies of Burjánek *et a*l. (2010; 2012) point out that the ambient noise polarization take place at about 90 degree angle to the observed fractures which are perpendicular to the sliding direction. In the present study the polarization angle is parallel to the opening cracks, which appears in contrast to the above mentioned results. A possible explanation of our findings is that there exists a prevailing north-easterly sliding direction of the landslide body which is strongly affecting the polarization direction especially in the 1-10

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Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 129

In Figure 20 we summarize the above results into a tentative draft profile, located as shown in Fig. 18, which illustrates the main geological features and hypothesizes the shape of the landslide body. The bottom panel shows a 2-D diagram obtained by combining all the ambient noise measurements along the profile. Moving along the profile from measurement point #5 to #17, it is interesting to observe the increasing amplitude of the HVNR at frequencies greater than 6.0-7.0 Hz. It can also be noticed that, especially between 50 and 60 m along the profile (sites #1, #2 and #3) the influence of the fracture zone is evident (see dashed area in Fig. 20). This is associated with the vibrational mode of the almost detached blocks. Along the cross section, at distances ranging from 60 to 100 m, it is possible to note both the presence of the bimodal peak associated with the two interfaces detritus/BC and BC/GL, as well as the high frequency peaks most probably associated with the vibration of large blocks that have been

detached from the cliff-face and are now partially or totally included in the BC.

As final considerations, it is noteworthy to observe that this study highlights the importance of evaluating the local seismic response in presence of slope instabilities related to landslide hazard. The instability processes that could be potentially triggered are linked to both slow mass movements, which might normally occur in tens or hundreds of years, and to sudden rockfall in the case of ground shaking due to moderate-to-strong earthquake activity. It is

Hz frequency range.

worth noting that:

**Figure 19.** a) Example of HVNR results obtained at some recording sites located in the not fractured zone (#25), in the cliff area (#13), and in the landslide zone (#14); (b) results obtained through a 1D modelling, performed in sites locat‐ ed on the non-fractured zone, by computing the synthetic HVNR curve; (c) rotated HVNR, (d) rose diagrams and (e) polar plots obtained at recording sites #25, #13 and #14.

Inspection of directional effects (see examples in Fig. 19c) show that they are clearly evident in the cliff fracture zones,at an angle of about 40°-60° N in all the considered frequency range, although some variability in azimuth is observed at high frequency (>9 Hz) at site #13. On moving away from the cliff edge, the rotated HVNR show a slight change of the directional resonance angle and an amplitude decrease of the rotated spectral ratios at high frequency. Such a behavior could be linked to the increase of rock stiffness and a reduction of the blocks' freedom to oscillate. Finally, it is evident that the directionality pattern observed in the rotated HVSRs performed on the landslide body is quite complex. The general trend has a prevailing direction of about 40°-60° N at low frequency (1.0-9.0 Hz), similarly to what is observed in the fractured zone, whereas different resonant frequencies and directions that could be ascribed to the vibration of smaller blocks can be observed at higher frequencies (9.0-40.0 Hz). Further‐ more, we obtained a direct estimate of the polarization angle through the full use of the threecomponent vector of the noise wave-field. General behavior of the noise wave-field, in this frequency range is shown through rose diagrams, whereas, in order to distinguish between properties of low and high frequency components of the signal, strike versus frequency polar plots were obtained. The examples shown in Figure 19d and e show that the maxima of the horizontal polarization occur in the north-east to east-north- east direction, although in some cases the high frequency directionality is more complex. As observed by Burjánek et al. (2010), high-frequency ground motion can indeed reflect the vibration of smaller blocks that imply both different resonant frequencies and directions. The polarization observed for the sites located away from the unstable areas (see e.g. #25 in Figure 19) show a trend with more dispersed and variable directions. The boundaries of the landslide area therefore appear well defined by the polarization pattern and as postulated by Kolesnikov et al. (2003) the landslide activity is characterized by strong horizontal polarization in a broad frequency band. In our study, a tendency seems evident for the entire landslide body to generally vibrate with a northeast azimuth and it may be assumed that, during a strong earthquake the ground motion would be amplified in this direction. Studies of Burjánek *et a*l. (2010; 2012) point out that the ambient noise polarization take place at about 90 degree angle to the observed fractures which are perpendicular to the sliding direction. In the present study the polarization angle is parallel to the opening cracks, which appears in contrast to the above mentioned results. A possible explanation of our findings is that there exists a prevailing north-easterly sliding direction of the landslide body which is strongly affecting the polarization direction especially in the 1-10 Hz frequency range.

with the contact between the rockfall and detritus unit and the BC formation, as well as to the

**Figure 19.** a) Example of HVNR results obtained at some recording sites located in the not fractured zone (#25), in the cliff area (#13), and in the landslide zone (#14); (b) results obtained through a 1D modelling, performed in sites locat‐ ed on the non-fractured zone, by computing the synthetic HVNR curve; (c) rotated HVNR, (d) rose diagrams and (e)

Inspection of directional effects (see examples in Fig. 19c) show that they are clearly evident in the cliff fracture zones,at an angle of about 40°-60° N in all the considered frequency range, although some variability in azimuth is observed at high frequency (>9 Hz) at site #13. On moving away from the cliff edge, the rotated HVNR show a slight change of the directional resonance angle and an amplitude decrease of the rotated spectral ratios at high frequency. Such a behavior could be linked to the increase of rock stiffness and a reduction of the blocks' freedom to oscillate. Finally, it is evident that the directionality pattern observed in the rotated HVSRs performed on the landslide body is quite complex. The general trend has a prevailing direction of about 40°-60° N at low frequency (1.0-9.0 Hz), similarly to what is observed in the fractured zone, whereas different resonant frequencies and directions that could be ascribed

interface between BC and the underlying GL formation.

128 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

polar plots obtained at recording sites #25, #13 and #14.

In Figure 20 we summarize the above results into a tentative draft profile, located as shown in Fig. 18, which illustrates the main geological features and hypothesizes the shape of the landslide body. The bottom panel shows a 2-D diagram obtained by combining all the ambient noise measurements along the profile. Moving along the profile from measurement point #5 to #17, it is interesting to observe the increasing amplitude of the HVNR at frequencies greater than 6.0-7.0 Hz. It can also be noticed that, especially between 50 and 60 m along the profile (sites #1, #2 and #3) the influence of the fracture zone is evident (see dashed area in Fig. 20). This is associated with the vibrational mode of the almost detached blocks. Along the cross section, at distances ranging from 60 to 100 m, it is possible to note both the presence of the bimodal peak associated with the two interfaces detritus/BC and BC/GL, as well as the high frequency peaks most probably associated with the vibration of large blocks that have been detached from the cliff-face and are now partially or totally included in the BC.

As final considerations, it is noteworthy to observe that this study highlights the importance of evaluating the local seismic response in presence of slope instabilities related to landslide hazard. The instability processes that could be potentially triggered are linked to both slow mass movements, which might normally occur in tens or hundreds of years, and to sudden rockfall in the case of ground shaking due to moderate-to-strong earthquake activity. It is worth noting that:

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

major axis of the ridge, are often observed (Spudich *et al.*, 1996).

formed by sand and gravel, laying over a marly clays basement.

2002; Pagliaroli *et al.*, 2007).

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

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Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 131

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,

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,

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 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‐ ing sites located across the profile.

