**4. Effects connected to the presence of faults**

these approaches are very efficient in overcoming the bias linked to the denominator behavior that could occur in the HVNR's technique and at the same time, longer time-series are processed therefore reducing the problems that may be linked to signal-to-noise ratio. In the TD approach, a direct estimate of the polarization angle is achieved by computing the polarization ellipsoid through the eigenvalues and eigenvectors of the covariance matrix obtained by three-component data (Jurkevics, 1988). The polarization ellipsoid of the analyzed signal is estimated by band-pass filtering it in the interval 1.0 - 10.0 Hz, using the whole recordings and considering a moving window of 1 s with 20% overlap, therefore obtaining the strike of maximum polarization for each moving time window. The results are finally plotted in circular histograms (rose diagrams) showing the polarization azimuths in intervals of ten degrees. In the TF method, a continuous wavelet transform for signal time-frequency decom‐ position, is firstly used. Subsequently, the polarization analysis on the complex wavelet amplitude for each time-frequency pair, is applied. In particular, histograms of the polarization parameters are created over time for each frequency. Polar plots are then adopted for depicting

the final results, which illustrate the combined angular and frequency dependence.

South-Eastern Sicily is located in a complex tectonic region being at the boundary between African and European plates (see inset map in Fig. 2a). Along this border, Mt. Etna, a basaltic volcano more than 3300 m high and with a diameter of about 40 km, resulting from the interaction of regional tectonics and local scale volcano-related processes (McGuire and Pullen, 1989), is sited. The island of Malta is placed in the Hyblean foreland, belonging to the African plate. Most of the formations here outcropping were deposited during the Oligocene and Miocene when the whole area was part of the Malta - Ragusa platform and, as such, attached

The whole study area is delineated by the crossing of lithosphere structures that give rise to the origin of Mt. Etna and by the presence of the Malta Hyblean fault system that runs down the Sicilian coast towards the Ionian sea (ME in the inset map of Fig. 2a). A series of horst and graben structures, NW-SE and NNW-SSE oriented, that are linked to the Malta-Hyblean

On Malta, the geo-structural pattern is dominated by two intersecting fault systems which alternate in tectonic activity. An older ENE-WSW trending fault, the Victoria Lines Fault (or Great Fault), traverses the islands and is crossed by a younger NW-SE trending fault, the Maghlaq Fault (Fig. 2b), parallel to the Malta trough which is the easternmost graben of the Pantelleria Rift system. The faults belonging to the older set, all vertical or sub vertical, are part of a horst and graben system of relatively small vertical displacement (Illies, 1981; Reuther et al. 1985). As concerns Mt. Etna, its eastern flank is the more tectonically active part. Here, several NNW and NNE-trending fault segments (Timpe fault system, TFS), arranged in a 30 km long system (Fig. 2a), control the present topography and show steep escarpments with very sharp morphology (Monaco *et al*., 1997). This system represents the northernmost prolongation of

**3. Geologic and tectonic features of the studied areas**

106 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

escarpment, characterize indeed the tectonic setting of this area.

to the African margin (Pedley *et al.*, 1978).

Fault zones are generally characterized by a highly fractured low-velocity belt (damage zone), hundreds of meter wide, bounded by higher-velocity area (host rock) that can broaden for some kilometres (Ben-Zion *et al.* 2003; Ben-Zion and Sammis 2003, 2009 and references therein). Such geometrical setting and impedance contrast is theoretically similar to the well known situations, widely studied in engineering geology and seismology, when soft sediments overly stiff rock. In the presently depicted case, the discontinuity is almost vertically oriented and, as described by Irikura and Kawanaka (1980), it is in principle proficient to produce local amplification of ground motion (Peng and Ben-Zion, 2006; Calderoni *et al*., 2010; Cultrera *et al*., 2003; Seeber *et al*., 2000), as well as to support the development of fault zone trapped waves (e.g. Li *et al*., 1994; Mizuno and Nishigami, 2006).

There is a large number of papers that describe propagation properties of fault-guided waves in terms of ground motion amplification having a propensity to be maximum along the faultparallel direction. These observations, both in theoretical and experimental approaches deal with almost pure strike slip faults such as the S. Andreas and the Anatolian faults (see Li *et al*., 2000; Ben Zion *et al*., 2003). In the Anatolian fault, Ben Zion *et al*. (2003) observed fault guided waves and an almost constant time delay in the shear waves arrival for different epicentral distances. The authors interpreted such observation as a consequence of a trapping mechanism in the shallower part of the fault between the first 3 and 5 kilometers. Lewis *et al*. (2005) come to a comparable conclusion through the observation of hundreds of seismograms of small magnitude events recorded close to the San Jacinto fault in California. In Italy, Rovelli *et al*. (2002), investigating the Nocera Umbra fault, observed a shallow trapping zone (1-1.5 km) with evidence of measurable amplification effects, as well as pronounced polarization of the ground motion in a direction parallel to the fault strike. On the other hand, Boore *et al*. (2004) in the Calaveras fault, California, noticed that directional effects observed during earthquakes, were occurring parallel to the fault strike and not perpendicularly as it would be expected for a strike-slip mechanism of faulting. Studies about local seismic response nearby fault zones have been performed in Italy and in California by Cultrera *et al*. (2003), Calderoni *et al*. (2010), Pischiutta *et al*. (2012) who observed evidence of ground motion amplification in the fault zone environments and strong directional effects with high angle to the fault strike. Similar studies, performed by Rigano *et al*. (2008) and Di Giulio *et al*. (2009) documented the presence of a systematic polarization of horizontal ground motion, near faults located on the eastern part of the Etnean area, that was never coincident with the strike of the tectonic structures. These directional effects were observed both during local and regional earthquakes, as well as using ambient noise measurements, therefore suggesting the use of microtremors for investigating ground motion polarization properties along and across the main tectonic structures.

In the present study, the results of studies performed in the Etnean area and in South eastern Sicily by Rigano and Lombardo (2005), Lombardo and Rigano (2006), Rigano *et al*. (2008), Di Giulio *et al*. (2009) will be briefly summarized and the outcomes from new investigations carried out in fault areas located both in the Etnean area and in Malta will be shown. Moreover, several measurements were performed in areas significantly distant from the studied tectonic structures (Piano dei Grilli, Etna and the Malta area), in order to observe how directional effects can change at increasing distance from the fault lines.

#### **4.1. Results and discussion**

In Figure 3, some examples of polarization plots obtained by Rigano *et al*. (2008), from microtremor measurements performed along and across the Pernicana and Tremestieri faults, are reported. The authors analyzed both earthquake and ambient noise records and found that in all investigated sites the ground motion polarization azimuths were principally NW-SE and NE-SW oriented, for the Pernicana (Fig. 3a) and the Tremestieri (Fig. 3b) faults respectively. It is interesting to observe that such directions were never found coincident with the strike of the investigated faults. Tests were also performed by the authors in order to verify that the observed polarizations were not affected by features of local sources, such as the volcanic tremor, and were stable in time.

The new ambient noise measurements described in the present study, were performed along four short profiles, having recording points spaced about 50m from each other, crossing the Masseria Cavaliere and the Ragalna fault (see respectively Tr#1, Tr#2 and Tr#3, Tr#4 in Fig. 4a). Results of directional effects investigations (Fig. 5) show that at these sampling sites, located in close proximity to the tectonic structures, the H/V spectral ratios have a tendency to increase in amplitude, in the frequency range 1.0-6.0 Hz, at angles of about 80°-90° for Masseria Cavaliere and 60°-70° for Ragalna faults (specify whether angles are from N, or with fault trace). In general, H/V spectral ratios show a broad band frequency effect with multiple adjacent peaks pointing out a preferential direction which is the typical behavior of directional resonances.

**Figure 3.** Examples of horizontal polarization angles obtained from ambient noise recorded along and nearby the

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Pernicana (a) and the Tremestieri (b) faults (modified from Rigano *et al*., 2008).

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ground motion in a direction parallel to the fault strike. On the other hand, Boore *et al*. (2004) in the Calaveras fault, California, noticed that directional effects observed during earthquakes, were occurring parallel to the fault strike and not perpendicularly as it would be expected for a strike-slip mechanism of faulting. Studies about local seismic response nearby fault zones have been performed in Italy and in California by Cultrera *et al*. (2003), Calderoni *et al*. (2010), Pischiutta *et al*. (2012) who observed evidence of ground motion amplification in the fault zone environments and strong directional effects with high angle to the fault strike. Similar studies, performed by Rigano *et al*. (2008) and Di Giulio *et al*. (2009) documented the presence of a systematic polarization of horizontal ground motion, near faults located on the eastern part of the Etnean area, that was never coincident with the strike of the tectonic structures. These directional effects were observed both during local and regional earthquakes, as well as using ambient noise measurements, therefore suggesting the use of microtremors for investigating

ground motion polarization properties along and across the main tectonic structures.

can change at increasing distance from the fault lines.

108 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**4.1. Results and discussion**

tremor, and were stable in time.

resonances.

In the present study, the results of studies performed in the Etnean area and in South eastern Sicily by Rigano and Lombardo (2005), Lombardo and Rigano (2006), Rigano *et al*. (2008), Di Giulio *et al*. (2009) will be briefly summarized and the outcomes from new investigations carried out in fault areas located both in the Etnean area and in Malta will be shown. Moreover, several measurements were performed in areas significantly distant from the studied tectonic structures (Piano dei Grilli, Etna and the Malta area), in order to observe how directional effects

In Figure 3, some examples of polarization plots obtained by Rigano *et al*. (2008), from microtremor measurements performed along and across the Pernicana and Tremestieri faults, are reported. The authors analyzed both earthquake and ambient noise records and found that in all investigated sites the ground motion polarization azimuths were principally NW-SE and NE-SW oriented, for the Pernicana (Fig. 3a) and the Tremestieri (Fig. 3b) faults respectively. It is interesting to observe that such directions were never found coincident with the strike of the investigated faults. Tests were also performed by the authors in order to verify that the observed polarizations were not affected by features of local sources, such as the volcanic

The new ambient noise measurements described in the present study, were performed along four short profiles, having recording points spaced about 50m from each other, crossing the Masseria Cavaliere and the Ragalna fault (see respectively Tr#1, Tr#2 and Tr#3, Tr#4 in Fig. 4a). Results of directional effects investigations (Fig. 5) show that at these sampling sites, located in close proximity to the tectonic structures, the H/V spectral ratios have a tendency to increase in amplitude, in the frequency range 1.0-6.0 Hz, at angles of about 80°-90° for Masseria Cavaliere and 60°-70° for Ragalna faults (specify whether angles are from N, or with fault trace). In general, H/V spectral ratios show a broad band frequency effect with multiple adjacent peaks pointing out a preferential direction which is the typical behavior of directional

**Figure 3.** Examples of horizontal polarization angles obtained from ambient noise recorded along and nearby the Pernicana (a) and the Tremestieri (b) faults (modified from Rigano *et al*., 2008).

**Figure 5.** Examples of the contours of the geometric mean of the spectral ratios as a function of frequency (x axis) and direction of motion (y axis) obtained at selected ambient noise recording sites located on the Tr#1, Tr#2, Tr#3 and

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In order to better quantify the horizontal polarization of the ground motion, the covariance matrix method in the time and in the time-frequency domain were applied (Fig. 6). The results give a clear indication that ambient noise is affected by a significant horizontal polarization at the measurement sites along and across the investigated faults. It is interesting to observe that results obtained through the TF method clearly show that the recorded ambient noise is polarized in a narrow frequency band (1.0-6.0 Hz), following a roughly east-west and north‐ east-southwest trend, for the Masseria Cavaliere and the Ragalna faults respectively. The TD results confirm the same polarization trend although in some cases the rose diagrams show polarizations that are not sharply oriented (see e.g. #1, #2, #16 and #17, in Fig. 6). It is in our opinion clear that possible contributions of high-frequency noise (6.0-10.0 Hz), related to

Tr#4 transects performed on the masseria Cavaliere and the Ragalna fault, respectively.

**Figure 4.** Location of the noise measurement sites. a) transects performed on the Ragalna fault system; b) measure‐ ment points on the Piedimonte fault; c) ambient noise recording sites in an area far from the investigated faults.

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**Figure 5.** Examples of the contours of the geometric mean of the spectral ratios as a function of frequency (x axis) and direction of motion (y axis) obtained at selected ambient noise recording sites located on the Tr#1, Tr#2, Tr#3 and Tr#4 transects performed on the masseria Cavaliere and the Ragalna fault, respectively.

In order to better quantify the horizontal polarization of the ground motion, the covariance matrix method in the time and in the time-frequency domain were applied (Fig. 6). The results give a clear indication that ambient noise is affected by a significant horizontal polarization at the measurement sites along and across the investigated faults. It is interesting to observe that results obtained through the TF method clearly show that the recorded ambient noise is polarized in a narrow frequency band (1.0-6.0 Hz), following a roughly east-west and north‐ east-southwest trend, for the Masseria Cavaliere and the Ragalna faults respectively. The TD results confirm the same polarization trend although in some cases the rose diagrams show polarizations that are not sharply oriented (see e.g. #1, #2, #16 and #17, in Fig. 6). It is in our opinion clear that possible contributions of high-frequency noise (6.0-10.0 Hz), related to

**Figure 4.** Location of the noise measurement sites. a) transects performed on the Ragalna fault system; b) measure‐ ment points on the Piedimonte fault; c) ambient noise recording sites in an area far from the investigated faults.

110 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

transients or to small scale geologic heterogeneities of a site, may imply incorrect results as shown for instance in Figure 7 where the TD rose diagrams dramatically change their polari‐ zation directions whereas the TF polar diagrams indicate that changes occur at higher frequencies only.

**Figure 7.** Polarization test on the measurements performed in two different periods (January 2009 and April 2011) in

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The other important structure that we investigated is the Piedimonte fault system (PF). The major fault, striking ENE-WSW, is located at the end of the Pernicana fault. It seems to represent an old structural element with a marked morphologic scarp, but no evidence of activity in historical times (Azzaro *et al.*, 2012). The minor faults which spread out from the main structure, striking mostly WNW-ESE, are related with the movement affecting the Pernicana fault. The directional resonance plots (Fig. 8), obtained by rotating the NS and EW components of motion seems to highlight the presence of two different structural behaviors. The results of measurements performed on the footwall of the fault (Fig. 8a) highlight strong directional effects in the frequency range 1.0-6.0 Hz with a NE- SW strike. In the hanging wall (Fig. 8b) the azimuths, the frequency bands and the amplitudes of the HVNRs vary at each measurement point in relation with the small scale geologic framework of each site. Polariza‐ tion results (Fig. 9) confirm the frequency range and azimuths observed through the rotated spectral ratios. Indeed, on the footwall of the major Piedimonte fault the recorded ambient noise is polarized in a narrow frequency band (1.0-6.0 Hz), similarly to the faults in the western area of the volcano, but with an angle of about 45°. In the hanging wall a rather scattered

the site #10 of the transect Tr#2 of the Masseria Cavaliere fault.

distribution of polarizations is observed (e.g. #13, #15, #16, #23).

**Figure 6.** Results of horizontal polarization angles computed through the TD (rose diagrams) and TF analysis consider‐ ing the frequency range of 1.0-10.0 Hz (polar plots), for Ragalna fault system.

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transients or to small scale geologic heterogeneities of a site, may imply incorrect results as shown for instance in Figure 7 where the TD rose diagrams dramatically change their polari‐ zation directions whereas the TF polar diagrams indicate that changes occur at higher

112 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**Figure 6.** Results of horizontal polarization angles computed through the TD (rose diagrams) and TF analysis consider‐

ing the frequency range of 1.0-10.0 Hz (polar plots), for Ragalna fault system.

frequencies only.

**Figure 7.** Polarization test on the measurements performed in two different periods (January 2009 and April 2011) in the site #10 of the transect Tr#2 of the Masseria Cavaliere fault.

The other important structure that we investigated is the Piedimonte fault system (PF). The major fault, striking ENE-WSW, is located at the end of the Pernicana fault. It seems to represent an old structural element with a marked morphologic scarp, but no evidence of activity in historical times (Azzaro *et al.*, 2012). The minor faults which spread out from the main structure, striking mostly WNW-ESE, are related with the movement affecting the Pernicana fault. The directional resonance plots (Fig. 8), obtained by rotating the NS and EW components of motion seems to highlight the presence of two different structural behaviors. The results of measurements performed on the footwall of the fault (Fig. 8a) highlight strong directional effects in the frequency range 1.0-6.0 Hz with a NE- SW strike. In the hanging wall (Fig. 8b) the azimuths, the frequency bands and the amplitudes of the HVNRs vary at each measurement point in relation with the small scale geologic framework of each site. Polariza‐ tion results (Fig. 9) confirm the frequency range and azimuths observed through the rotated spectral ratios. Indeed, on the footwall of the major Piedimonte fault the recorded ambient noise is polarized in a narrow frequency band (1.0-6.0 Hz), similarly to the faults in the western area of the volcano, but with an angle of about 45°. In the hanging wall a rather scattered distribution of polarizations is observed (e.g. #13, #15, #16, #23).

**Figure 8.** Examples of the contours of the geometric mean of the spectral ratios as a function of frequency (x axis) and direction of motion (y axis) obtained at selected ambient noise recording sites located on the footwall a) and hanging wall b) of the Piedimonte fault.

**Figure 9.** Horizontal polarization angles computed through the TD (rose diagrams) and TF analysis considering the

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frequency range of 1.0-10.0 Hz (polar plots), for the Piedimonte fault.

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**Figure 9.** Horizontal polarization angles computed through the TD (rose diagrams) and TF analysis considering the frequency range of 1.0-10.0 Hz (polar plots), for the Piedimonte fault.

**Figure 8.** Examples of the contours of the geometric mean of the spectral ratios as a function of frequency (x axis) and direction of motion (y axis) obtained at selected ambient noise recording sites located on the footwall a) and hanging

wall b) of the Piedimonte fault.

114 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

Ambient noise recordings were also performed in some fault segments belonging to the Great Fault system of the Malta area, in order to investigate site effect features of tectonic structures located in a non volcanic area. Results obtained (Fig. 10) indicate the existence of directional effects similar to those found in the investigated Etnean structures, although a slightly more complicated pattern, probably linked to the influence of the complex lithology existing at both sides of the fault, is observed. However, the polarization plots obtained from records near the fault show a prevailing direction that is not parallel to the structure strike.

Finally, tests were performed to check if the observed directional effects keep the same orientation in areas significantly distant from fault lines. For this reason, ambient noise was recorded in the PG (Piano dei Grill) area, located in the western flank of Mt. Etna, as well as in an area in the northern Malta island. The results (Fig. 11) show that ground motion directions coming from both rotated spectral ratios and polarization diagrams tend to become randomly distributed and/or uniformly scattered. Such findings further support the observations of Rigano and Lombardo (2005) that performed ambient noise measurements in proximity and at few kilometers distance from the Mt. Tauro fault, located in south-eastern Sicily (Fig. 12).

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**Figure 11.** Examples of directional resonances and horizontal polarization azimuths obtained at selected ambient

noise recording sites located on the PG area (#1, #2, #5) and in the northern Malta area (#3, #17, #41).

**Figure 12.** Ambient noise polarization plots in the Augusta area (modified from Rigano and Lombardo, 2005).

**Figure 10.** Examples of directional resonances and horizontal polarization azimuths obtained at selected ambient noise recording sites located on the Malta Great Fault system.

It is not an easy task to interpret all experimental observations when these exhibit prevailing polarization directions that are sharply changing and are always non-coincident with the fault strike. In a recent paper, Pischiutta *et al*. (2012), find that the mean polarization azimuth turns out to be perpendicular to the expected synthetic cleavages. We have to remember that four types of fractures can develop in fault zone (Riedel, 1929; Tchalenko, 1968): a) extensional fracture (T); b) synthetic cleavage (R); c) antithetic cleavage (R'); d) pressure solution surfaces (P). The orientation of these fractures depends on the direction of the resulting stress localized around the fault. Therefore, an interpretation of the present results could, in our opinion, be proposed by focusing our findings in the frame of the brittle rheology of the hosting rock (Pischiutta *et al*., 2012).

Finally, tests were performed to check if the observed directional effects keep the same orientation in areas significantly distant from fault lines. For this reason, ambient noise was recorded in the PG (Piano dei Grill) area, located in the western flank of Mt. Etna, as well as in an area in the northern Malta island. The results (Fig. 11) show that ground motion directions coming from both rotated spectral ratios and polarization diagrams tend to become randomly distributed and/or uniformly scattered. Such findings further support the observations of Rigano and Lombardo (2005) that performed ambient noise measurements in proximity and at few kilometers distance from the Mt. Tauro fault, located in south-eastern Sicily (Fig. 12).

Ambient noise recordings were also performed in some fault segments belonging to the Great Fault system of the Malta area, in order to investigate site effect features of tectonic structures located in a non volcanic area. Results obtained (Fig. 10) indicate the existence of directional effects similar to those found in the investigated Etnean structures, although a slightly more complicated pattern, probably linked to the influence of the complex lithology existing at both sides of the fault, is observed. However, the polarization plots obtained from records near the

**Figure 10.** Examples of directional resonances and horizontal polarization azimuths obtained at selected ambient

It is not an easy task to interpret all experimental observations when these exhibit prevailing polarization directions that are sharply changing and are always non-coincident with the fault strike. In a recent paper, Pischiutta *et al*. (2012), find that the mean polarization azimuth turns out to be perpendicular to the expected synthetic cleavages. We have to remember that four types of fractures can develop in fault zone (Riedel, 1929; Tchalenko, 1968): a) extensional fracture (T); b) synthetic cleavage (R); c) antithetic cleavage (R'); d) pressure solution surfaces (P). The orientation of these fractures depends on the direction of the resulting stress localized around the fault. Therefore, an interpretation of the present results could, in our opinion, be proposed by focusing our findings in the frame of the brittle rheology of the hosting rock

noise recording sites located on the Malta Great Fault system.

(Pischiutta *et al*., 2012).

fault show a prevailing direction that is not parallel to the structure strike.

116 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**Figure 11.** Examples of directional resonances and horizontal polarization azimuths obtained at selected ambient noise recording sites located on the PG area (#1, #2, #5) and in the northern Malta area (#3, #17, #41).

**Figure 12.** Ambient noise polarization plots in the Augusta area (modified from Rigano and Lombardo, 2005).

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

**•** 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 till several hundred meters distance from the fault line;

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

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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.

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

briefly summarized.

**5.1. Results and discussion**


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 structures.
