**5. Site effects linked to the presence of cavities**

The presence of either natural or artificial cavities in the shallower part of various lithotypes is an important aspect whose effects, in terms of the local seismic response evaluation, are still not fully investigated. Grottos can originate from different processes and affect rocky litho‐ types that at the surface appear very stiff and characterized by good elastic properties. It is for instance possible to observe the development of cavities, several meters wide and hundreds of meters long, inside basaltic lava flows, that are related to the cooling of the shallower part of lavas while the still fluid portion flows underneath. Also typical is the presence of extensive cavities in calcareous formations, due to the development of karstic phenomena. Local seismic effects related to such conditions therefore need to be investigatedsince, although the lithology often belongs to the bedrock type, they cannot be considered free from significant modifica‐ tions of both amplitude and frequency content of the seismic input.

The scientific literature concerning these phenomena is rather poor. Studies were performed by Nunziata *et al*. (1999) in some cavities existing inside the pyroclastic terrains of the Napoli downtown area where the authors observed, through numerical modeling, an amplitude decrease of the ground motion at the top of the investigated cavities. Experimental studies, using both earthquake and ambient noise records were recently performed in south-eastern Sicily by Lombardo and Rigano (2010) and Sgarlato *et al*. (2011) and their results will now be briefly summarized.

The influence of cavities in the evaluation of the local seismic response was studied in some selected sites located in the Hyblean region (in the cities of Lentini, Melilli, Siracusa and Modica) and the urban area of Catania, taking into account both natural and artificial cavities such as railway tunnels. In total, the measurements were carried out in about fifteen cavities. They were selected according to criteria of relatively easy access, different geometric features and possibility of having detailed underground surveys. The majority of the investigated grottoes develops in heavily urbanised areas and in some cases, houses and small edifices are built over, or neighbouring them. About 400 time histories of microtremors were recorded in 90 measurements sites that were located inside and over the vault of each grotto, as well as in its neighbourhood, along short profiles having a few tens of meters length, evaluating the horizontal-to-vertical noise spectral ratio as well as the polarization angle of the horizontal component of motion. Besides, in the Catania area, a cavity (Petralia grotto) was selected to install four seismic stations for recording earthquakes. The grotto is located in the northern part of Catania and roughly trends in E–W direction. It develops at a depth of about 3 m from the topographic surface where its easternmost part is open (Fig. 13). Its cross section has a variable size ranging between 10 and 15 m in width and is about 2.5 m high on average. This cavity is formed by several chambers connected by tight passages and it shows evidence of several collapses, the first of which took place at a few tens of meters from the opening of the cavity. Its origin is connected to the flow, cooling and drainage of a pre-historical Etnean lava that, similarly to other lava flows have covered, till historical times, the Catania urban area terrains. The stations were deployed inside, over the vault, in the vicinity of the grotto (named *incave*, *upcave* e *outcave*, respectively) and in a reference site (*uni*), about 500 m away, located on the bedrock. Data were processed using the earthquake's horizontal to vertical spectral ratio (HVSR) or receiver function technique, and the standard spectral ratio (SSR) to a reference site. A set of 34 seismic events, showing a good signal-to-noise ratio were recorded for five months.

### **5.1. Results and discussion**

The outcomes of the present research allowed us to draw the following considerations:

till several hundred meters distance from the fault line;

118 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

are performed in areas where no fault s are evident.

**5. Site effects linked to the presence of cavities**

tions of both amplitude and frequency content of the seismic input.

synthetic cleavages.

Etnean area.

structures.

**•** In the neighborhood of fault areas, the presence of a damage zone implies the existence of ground motion amplifications and persistent directional effects of the horizontal component of motion, set into evidence by both earthquake and ambient noise records, that are observed

**•** The directional site effects and the polarization angles observed for all the investigated structures are always non-parallel to the fault strike making a simple explanation in terms of fault-trapped waves not convincing. To attempt a possible explanation for this recurrent ground motion property we postulate the existence of a tight relationship with the expected

**•** The directional resonance and the TF polarization analysis set into evidence that fault effects appear concentrated in the frequency range 1.0-6.0 Hz. The stability of this frequency interval, observed both in the western and in the eastern flanks of the volcano, encourage us to affirm that it is a possible marker for observing site effects in fault zones, at least in the

**•** Polarization directions coming from both rotated spectral ratios and polarization diagrams tend to become randomly distributed and/or uniformly scattered when noise measurements

Finally, it seems important to point out that present results give further support to findings from previous studies (e.g. Rigano *et al*., 2008; Di Giulio *et al*., 2009; Pischiutta *et al*., 2012) concerning site effects in fault zones and promote the use of ambient noise recordings as a fast technique for preliminary investigations about angular relations between fractures field and directions of amplified ground motion and for preliminary quick surveys in area where the urbanization or the presence of shallow sedimentary deposits hide the evidence of tectonic

The presence of either natural or artificial cavities in the shallower part of various lithotypes is an important aspect whose effects, in terms of the local seismic response evaluation, are still not fully investigated. Grottos can originate from different processes and affect rocky litho‐ types that at the surface appear very stiff and characterized by good elastic properties. It is for instance possible to observe the development of cavities, several meters wide and hundreds of meters long, inside basaltic lava flows, that are related to the cooling of the shallower part of lavas while the still fluid portion flows underneath. Also typical is the presence of extensive cavities in calcareous formations, due to the development of karstic phenomena. Local seismic effects related to such conditions therefore need to be investigatedsince, although the lithology often belongs to the bedrock type, they cannot be considered free from significant modifica‐

Examples of the HVNR results obtained in the various investigated cavities are reported in Fig. 14a. As the plots summarize, it is not possible to observe a unique behavior. The results show the lack of H/V spectral ratio peaks inside some cavities, as shown in the examples #6, and #1\* where spectral ratio peaks do not reach the amplitude of three units. On the other hand, H/V obtained from measurements performed in some other cavities (#1, #3, #2\* and #3\*) tend to reach a significant amplitude (>2 units) in some frequency bands. Such behavior appears related to the size of each cavity. It is in fact observed that a tendency towards H/V significant peaks is evident in cavities whose height is not less than 3-4 metres. It is also

**Figure 13.** Sketch map of the eastern end of the Petralia grotto and location of permanent and mobile stations; white and grey squares refer to ambient noise recording sites located respectively inside or over and outside the grotto.

interesting to note that in some cases considerable effects are observed in H/V spectral ratios from measurements performed over the vault of the cavity (i.e. #3, #3\*), while in other cases (#1 and #2\*) pronounced peaks are observed in measurements performed both inside and over the grotto.

Some interesting considerations can be inferred from investigating possible directional effects. The plots in Fig. 14b show that the peaks centered at 2.5 Hz and 1.5 Hz, for cavities #1 and #2\* respectively, as well as the peak in the range 4.0 – 6.0 Hz, observed for the cavities #6 and #3\*, are markedly directional. The peak values increase up to 3 - 4 units, at directions of 90° and 180° that are nearly coincident with the strikes of the investigated cavities (see the strikes reported in the panels of Fig. 14a).

> To validate the reliability of these ambient noise measurements, a comparison was made with findings from the HVSR and SSR of earthquake data recorded in a test site (Petralia grotto). All spectral ratios (Fig. 15a, b) obtained from records at incave, upcave and outcave stations, show moderate peaks that reach at most a value of 3 units. The HVSR show peaks in two frequency ranges, namely 1.2–1.8 Hz and 3.0–7.0 Hz. It has to be noted that in complex situations the identification of main resonance frequencies through HVSR analysis can be biased by the presence of deamplification phenomena in the vertical component of the ground motion. For this reason the ratios between the vertical component spectra of records at the local permanent stations and at the reference one were calculated (Fig. 15c) in order to highlight the frequency band at which the results can be considered reliable. In Fig. 15c no evident deamplification phenomena are observed, but the vertical component at outcave station, especially in the frequency range 3.0 – 7.0 Hz, shows a slight tendency to deamplification which could explain the spectral peaks observed, in the same frequency range, in the HVSR. The

formed at *Della Chiesa* (# 1), *Petralia* (# 6), *C.le Palma* (# 2\*) and *Speri* (# 3\*) grottoes.

**Figure 14.** a) H/V spectral ratios obtained from ambient noise measurements performed in different grottoes (num‐ bers refer to the grottoes listed in Tab. 1); the ellipse represents the dimensions of the vertical section of each cavity while the arrows indicate in which direction the grotto develops underground. (b) Contours of the geometric mean of ambient noise spectral ratios as a function of frequency (x-axis) and direction of motion (y-axis) for recordings 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 121


**Table 1.** List of investigated cavities.

Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities, Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 121

interesting to note that in some cases considerable effects are observed in H/V spectral ratios from measurements performed over the vault of the cavity (i.e. #3, #3\*), while in other cases (#1 and #2\*) pronounced peaks are observed in measurements performed both inside and over

**Figure 13.** Sketch map of the eastern end of the Petralia grotto and location of permanent and mobile stations; white and grey squares refer to ambient noise recording sites located respectively inside or over and outside the grotto.

120 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

Some interesting considerations can be inferred from investigating possible directional effects. The plots in Fig. 14b show that the peaks centered at 2.5 Hz and 1.5 Hz, for cavities #1 and #2\* respectively, as well as the peak in the range 4.0 – 6.0 Hz, observed for the cavities #6 and #3\*, are markedly directional. The peak values increase up to 3 - 4 units, at directions of 90° and 180° that are nearly coincident with the strikes of the investigated cavities (see the strikes

**. Name Locality H (m) No. Name Locality H (m)** Della Chiesa Catania 5 1\* De Cristoforis Lentini 2 Micio Conti Catania 3 2\* C.le Palma Lentini 5 Di Bella Catania 8 3\* Speri Lentini 6 Caflish Catania 2 1 Ipogeo Siracusa 8 Ciancio Catania 4 1 Mastro Pietro Melilli 8 Petralia Catania 2 1 Barriera Melilli 10 Novalucello Catania 1 1 Cava Modica 6

the grotto.

**No**

reported in the panels of Fig. 14a).

8 Magna Catania 2.5

**Table 1.** List of investigated cavities.

**Figure 14.** a) H/V spectral ratios obtained from ambient noise measurements performed in different grottoes (num‐ bers refer to the grottoes listed in Tab. 1); the ellipse represents the dimensions of the vertical section of each cavity while the arrows indicate in which direction the grotto develops underground. (b) Contours of the geometric mean of ambient noise spectral ratios as a function of frequency (x-axis) and direction of motion (y-axis) for recordings per‐ formed at *Della Chiesa* (# 1), *Petralia* (# 6), *C.le Palma* (# 2\*) and *Speri* (# 3\*) grottoes.

To validate the reliability of these ambient noise measurements, a comparison was made with findings from the HVSR and SSR of earthquake data recorded in a test site (Petralia grotto). All spectral ratios (Fig. 15a, b) obtained from records at incave, upcave and outcave stations, show moderate peaks that reach at most a value of 3 units. The HVSR show peaks in two frequency ranges, namely 1.2–1.8 Hz and 3.0–7.0 Hz. It has to be noted that in complex situations the identification of main resonance frequencies through HVSR analysis can be biased by the presence of deamplification phenomena in the vertical component of the ground motion. For this reason the ratios between the vertical component spectra of records at the local permanent stations and at the reference one were calculated (Fig. 15c) in order to highlight the frequency band at which the results can be considered reliable. In Fig. 15c no evident deamplification phenomena are observed, but the vertical component at outcave station, especially in the frequency range 3.0 – 7.0 Hz, shows a slight tendency to deamplification which could explain the spectral peaks observed, in the same frequency range, in the HVSR. The comparison of HVSR obtained at the three sites shows a tendency toward slightly more pronounced peaks at outcave station with respect to stations located both over and inside the cavity, the last one, in particular, showing always smaller spectral peaks for the EW component in the frequency band 3.0–7.0 Hz (Fig. 15a). A similar behavior is also observed in the SSR (Fig. 15b) where the spectral ratios obtained for the incave station show, especially in the same frequency range, smaller amplifications in both EW and NS components. Such a tendency is also shown in the results of ambient noise measurements (Fig. 15d and e). Moreover, it is interesting to point out that both HVSR and HVNR show, at upcave station, a striking amplitude decrease, in the frequency band 7.0–10.0 Hz, of EW component of motion (see Fig. 15a, d, e). Such behavior appears related to the amplitude increase of the vertical component of motion at the upcave station, as confirmed by the V/Vref shown in Fig. 15c. This implies that, at highest frequencies, both the HVSR and the HVNR show higher values at incave station rather than at upcave station. On the other hand, being evident that in the above mentioned frequency band, at upcave station the EW component of motion has a low amplitude, the SSR shows also a striking decrement (Fig. 15b) and the spectral ratios at incave are lower than the same spectral ratios obtained at upcave. This last observation is a consequence of the low amplitude values at the denominator of the SSR, with respect to that of HVSR, as expected for the horizontal component of motion at the reference site. The SSR however shows amplifica‐ tion mostly in the range 3.0–7.0 Hz (Fig. 15b).

Other noise measurements were performed to the south of the cavity at distances of about 40 m and 100 m from the cavity entrance in order to test how the spectral features previously described appear at increasing distance from the grotto. The obtained HVNR (see inset in Fig. 15e) shows that the 1.2-1.6 Hz spectral ratio peaks disappear already at a distance of about 40 m from the cavity, therefore indicating that they seem to belong mostly to specific features of the grotto area. On the other hand, the peaks at 3.0 – 7.0 Hz, although less pronounced, are still observed, implying that they are linked, at least in part, also to structures extending in a

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A polarization analysis was performed to investigate directivity effects related to the cavity. The azimuthal directions of the horizontal component of motion were obtained after filtering the signal in three frequency bands (1.0 – 3.0, 3.0 – 7.0 and 7.0 – 10.0 Hz) aiming to investigate the frequency ranges observed in the H/V spectral ratios. It can be clearly observed (Fig. 16) that almost all rose diagrams show a sharp polarization in the EW direction. Only polarizations obtained at the incave station by filtering the signal in the range 1–3 Hz appear highly scat‐ tered. Polarization effects were also investigated, using ambient noise records, in the sites located at 40 and 100 m away from Petralia grotto (see bottom panels in Fig. 16). The results obtained show that such polarization effects become less evident as the distance from the cavity increases.

**Figure 16.** Polarization of the horizontal component of motion obtained by filtering the recorded seismic events at 1.0–3.0 Hz (upper panels), at 3.0–7.0 Hz (middle panels) and 7.0–10.0 Hz (lower panels); the bottom panels refer to polarization from ambient noise measurements performed at sites located 40 and 100 m away from the grotto.

wider area around the grotto.

**Figure 15.** Spectral ratios HVSR (a) and standard spectral ratios SSR (b) of all events recorded at Petralia grotto; V/ Vref) of the vertical component of seismic events recorded at the local permanent stations and at the reference one (c); HVNR recorded at the sites of permanent stations (d) and average of all measurements performed in different sites located inside, over and outside the investigated grotto (e), the location of all recording sites is shown in Fig. 14. The inset shows the HVNR from ambient noise recorded in two sites located at distance of about 40m (grey curve) and 100 m (black curve) from the grotto.

Other noise measurements were performed to the south of the cavity at distances of about 40 m and 100 m from the cavity entrance in order to test how the spectral features previously described appear at increasing distance from the grotto. The obtained HVNR (see inset in Fig. 15e) shows that the 1.2-1.6 Hz spectral ratio peaks disappear already at a distance of about 40 m from the cavity, therefore indicating that they seem to belong mostly to specific features of the grotto area. On the other hand, the peaks at 3.0 – 7.0 Hz, although less pronounced, are still observed, implying that they are linked, at least in part, also to structures extending in a wider area around the grotto.

comparison of HVSR obtained at the three sites shows a tendency toward slightly more pronounced peaks at outcave station with respect to stations located both over and inside the cavity, the last one, in particular, showing always smaller spectral peaks for the EW component in the frequency band 3.0–7.0 Hz (Fig. 15a). A similar behavior is also observed in the SSR (Fig. 15b) where the spectral ratios obtained for the incave station show, especially in the same frequency range, smaller amplifications in both EW and NS components. Such a tendency is also shown in the results of ambient noise measurements (Fig. 15d and e). Moreover, it is interesting to point out that both HVSR and HVNR show, at upcave station, a striking amplitude decrease, in the frequency band 7.0–10.0 Hz, of EW component of motion (see Fig. 15a, d, e). Such behavior appears related to the amplitude increase of the vertical component of motion at the upcave station, as confirmed by the V/Vref shown in Fig. 15c. This implies that, at highest frequencies, both the HVSR and the HVNR show higher values at incave station rather than at upcave station. On the other hand, being evident that in the above mentioned frequency band, at upcave station the EW component of motion has a low amplitude, the SSR shows also a striking decrement (Fig. 15b) and the spectral ratios at incave are lower than the same spectral ratios obtained at upcave. This last observation is a consequence of the low amplitude values at the denominator of the SSR, with respect to that of HVSR, as expected for the horizontal component of motion at the reference site. The SSR however shows amplifica‐

**Figure 15.** Spectral ratios HVSR (a) and standard spectral ratios SSR (b) of all events recorded at Petralia grotto; V/ Vref) of the vertical component of seismic events recorded at the local permanent stations and at the reference one (c); HVNR recorded at the sites of permanent stations (d) and average of all measurements performed in different sites located inside, over and outside the investigated grotto (e), the location of all recording sites is shown in Fig. 14. The inset shows the HVNR from ambient noise recorded in two sites located at distance of about 40m (grey curve) and

tion mostly in the range 3.0–7.0 Hz (Fig. 15b).

122 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

100 m (black curve) from the grotto.

A polarization analysis was performed to investigate directivity effects related to the cavity. The azimuthal directions of the horizontal component of motion were obtained after filtering the signal in three frequency bands (1.0 – 3.0, 3.0 – 7.0 and 7.0 – 10.0 Hz) aiming to investigate the frequency ranges observed in the H/V spectral ratios. It can be clearly observed (Fig. 16) that almost all rose diagrams show a sharp polarization in the EW direction. Only polarizations obtained at the incave station by filtering the signal in the range 1–3 Hz appear highly scat‐ tered. Polarization effects were also investigated, using ambient noise records, in the sites located at 40 and 100 m away from Petralia grotto (see bottom panels in Fig. 16). The results obtained show that such polarization effects become less evident as the distance from the cavity increases.

**Figure 16.** Polarization of the horizontal component of motion obtained by filtering the recorded seismic events at 1.0–3.0 Hz (upper panels), at 3.0–7.0 Hz (middle panels) and 7.0–10.0 Hz (lower panels); the bottom panels refer to polarization from ambient noise measurements performed at sites located 40 and 100 m away from the grotto.

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 is dug.

**•** Findings from HVSR and SSR at the test site #6 point out the lack, or the modest presence, of amplification effects at the recording site located inside the cavity with respect to stations placed over and outside the grotto. This effect could be explained in the frame of the constructive interferences between direct and reflected waves that can take place at the free surface, so that, as it would be generally expected, a decrement of amplitude oscillations

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**•** In all investigated cavities, significant directivity effects were observed, pointing to the existence of a marked polarization of the horizontal component of the ground motion in a direction parallel to the main axis of the grotto. Such evidence is not negligible in the planning of buildings to be erected in the neighboring areas. It is however remarkable that experimental data show that at distances greater than about 100 m from the cavity, the polarization analysis shows azimuths no more coincident with the strike of the grotto. It is however important to denote that further investigations need to be performed in other cavities having various height, performing also 2D/3D modeling in order to simulate the variations that a wavefront undergoes when propagating through a terrain having a strong

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

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

can be observed at depth.

impedance contrast due to the existence of a hollow space.

**6. Site effects in landslide zones**

sections.

**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 can be inferred:


It is however important to denote that further investigations need to be performed in other cavities having various height, performing also 2D/3D modeling in order to simulate the variations that a wavefront undergoes when propagating through a terrain having a strong impedance contrast due to the existence of a hollow space.
