**2. Methodologies**

Several different methodological approaches are commonly adopted to quantitatively assess the local seismic response. In practice, it is evaluated with respect to a reference site represented by the outcrop of a rocky basement (either real or supposed) existing in the investigated area. In other words, the local ground motion is compared with the one relative to a reference bedrock outcrop.

The site response can be evaluated through various approaches, also collateral between themselves, each of them having specific advantages and/or drawbacks well known in literature (see Pitilakis, 2004). Main methodologies can be grouped into two categories: numerical methods and experimental methods.

Numerical methods are founded on the use of computer codes that simulate wave propagation through soft deposits, from the bedrock to the free surface. Such codes allow the modelling of the dynamic behaviour of a terrain by adopting linear, equivalent-linear or non- linear models (e.g. SHAKE, Schnabel *et al*, 1972; EERA, Bardet *et al*, 2000; DESDRA, Lee e Finn, 1978) that can be either mono or multi-dimensional (Hudson *et al*, 1994). These methods provide in output the time history of involved seismic parameters and require as input data a detailed knowledge of the site geometry, the geotechnical properties of terrains and the stress-strain relationships.

The experimental methods allow us to evaluate the local seismic response using the records of seismic signals that be generated by earthquakes, artificial seismic sources or ambient noise. They are only moderately expensive and take implicitly into account all site effects, although their drawback is linked to the use of low or very low energy events, so that the seismic response evaluation is performed at low deformation levels and entirely in the linear field.

The results described in the present study, draw from the use of spectral ratios evaluated through comparison between the investigated site and the reference one (SSR *Standard Spectral Ratio* technique) and/or by calculating the spectral ratios between the horizontal and the vertical components of motion at the investigated site (HVSR *Horizontal to Vertical Spectral Ratio* and HVNR *Horizontal to Vertical Noise Ratio* or Nakamura method).

The SSR technique (Borcherdt 1970) consists in computing the Fourier spectral ratio of the same seismic waves (generally S waves) simultaneously recorded by the horizontal components of two seismic stations, one of which is located on a bedrock outcrop. The main difficulty associated with this technique is a proper choice of the reference site that has to be a flat outcrop of the bedrock. Moreover, the correct use of the SSR technique requires that the distance between test and reference sites has to be significantly smaller than the epicentral distance. The earthquake HVSR, or receiver function technique, does not need a reference station and consists in the computation of the horizontal-to-vertical spectral ratio of the components of motion recorded at one seismic station only (Lermo and Chavez-Garcia 1993). This technique is founded on the assumption that the vertical component of motion is not affected by the local geological conditions. It is applied both to the time window of shear waves and to the entire seismic record and has shown to be a good approach for the evaluation of the site fundamental frequency whereas it appears less reliable for the estimate of the amplitude values.

In this study the characteristics of the local seismic response, linked in particular to the presence of discontinuities such as faults and cavities, as well as topographic irregularities and landslide phenomena, are investigated. Case-studies of sites located both in South-eastern Sicily and in Malta are described, illustrating, besides local amplification phenomena, the possible presence

Several different methodological approaches are commonly adopted to quantitatively assess the local seismic response. In practice, it is evaluated with respect to a reference site represented by the outcrop of a rocky basement (either real or supposed) existing in the investigated area. In other words, the local ground motion is compared with the one relative to a reference

The site response can be evaluated through various approaches, also collateral between themselves, each of them having specific advantages and/or drawbacks well known in literature (see Pitilakis, 2004). Main methodologies can be grouped into two categories:

Numerical methods are founded on the use of computer codes that simulate wave propagation through soft deposits, from the bedrock to the free surface. Such codes allow the modelling of the dynamic behaviour of a terrain by adopting linear, equivalent-linear or non- linear models (e.g. SHAKE, Schnabel *et al*, 1972; EERA, Bardet *et al*, 2000; DESDRA, Lee e Finn, 1978) that can be either mono or multi-dimensional (Hudson *et al*, 1994). These methods provide in output the time history of involved seismic parameters and require as input data a detailed knowledge of the site geometry, the geotechnical properties of terrains and the stress-strain relationships. The experimental methods allow us to evaluate the local seismic response using the records of seismic signals that be generated by earthquakes, artificial seismic sources or ambient noise. They are only moderately expensive and take implicitly into account all site effects, although their drawback is linked to the use of low or very low energy events, so that the seismic response evaluation is performed at low deformation levels and entirely in the linear field. The results described in the present study, draw from the use of spectral ratios evaluated through comparison between the investigated site and the reference one (SSR *Standard Spectral Ratio* technique) and/or by calculating the spectral ratios between the horizontal and the vertical components of motion at the investigated site (HVSR *Horizontal to Vertical Spectral*

*Ratio* and HVNR *Horizontal to Vertical Noise Ratio* or Nakamura method).

The SSR technique (Borcherdt 1970) consists in computing the Fourier spectral ratio of the same seismic waves (generally S waves) simultaneously recorded by the horizontal components of two seismic stations, one of which is located on a bedrock outcrop. The main difficulty associated with this technique is a proper choice of the reference site that has to be a flat outcrop of the bedrock. Moreover, the correct use of the SSR technique requires that the distance between test and reference sites has to be significantly smaller than the epicentral distance.

of directional effects.

**2. Methodologies**

bedrock outcrop.

numerical methods and experimental methods.

104 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

The Nakamura technique (HVNR) (Nakamura, 1989) uses as a seismic input the ambient noise and computes the spectral ratio between the horizontal and the vertical components of motion. Ambient noise has, in recent years, become widely used for site amplification studies. Its use appears opportune for significant reductions in field data acquisition time and costs. The evaluation of site response using the HVNR technique is largely adopted since it requires only one mobile seismic station with no additional measurements at rock sites for comparison. Besides, it does not require the long and simultaneous deployment of several instruments which is necessary to collect a useful earthquake data set. The basic hypothesis of using ambient noise is that the resonance of a soft layer corresponds to the fundamental mode of Rayleigh waves, which is associated with an inversion of the direction of Rayleigh waves rotation (Nogoshi and Igarashi, 1970; Lachet and Bard, 1994). Thus, the ratio between the horizontal and vertical spectral components of motion can reveal the fundamental resonance frequency of the site. Reliability of such approach has been asserted by many authors (e.g. Lermo and Chavez-Garcıa, 1993; Bard, 1999) who have stressed its significant stability in local seismic response estimates. It is commonly accepted that, although the single components of ambient noise can show large spectral variations as a function of natural and cultural distur‐ bances, the H/V spectral ratio tends to remain invariant, therefore preserving the fundamental frequency peak (Cara *et al*., 2003).

In the present study, ambient noise records were performed using a Tromino instrument (www.tromino.it), a compact 3-component velocimeter with a reliable instrumental response in the frequency range 0.5-10 Hz. The signals were processed by evaluating the horizontal-tovertical noise spectral ratios (HVNR). Following the guidelines suggested by the European project Site EffectS assessment using AMbient Excitations (SESAME, 2004), time windows of 30 s were considered, selecting the most stationary part and excluding transients associated to very close sources. Fourier spectra were calculated and smoothed using a triangular average on frequency intervals of ± 5% of the central frequency.

The potential presence of directional effects in the ground motion recorded at the surface was also investigated. Such investigations can be done by computing the spectral ratios (SSR, HVSR, HVNR) after rotating the horizontal components by steps of 10° starting from 0° (north) to 180° (south) and plotting the contours of the spectral ratio amplitudes as a function of frequency and direction of motion. This approach (Spudich *et al*., 1996) is powerful in enhanc‐ ing, if any, the occurrence of site specific directional effects. A direct estimate of the polarization angle, for noise data, can be achieved through two different methods. The time domain method (TD) by Jurkevics (1988) and the time-frequency (TF) polarization analysis by Burjánek *et al.* (2010 and 2012). The results obtained through polarization techniques are quite robust since 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.

the Malta Escarpment and forms a system of parallel step-faults having vertical offsets up to 200 m that down-throw towards the sea. Most of these faults are highly seismogenic and generate shallow earthquakes as well as co-seismic cracks in the soil and creep phenomena (Azzaro, 1999). In the north eastern part of the area, the active Pernicana fault system (PFS) represent the most significant tectonic structure. It is a strike-slip fault roughly E-W oriented with a length of about 18 km from the NE rift to the coastline (Neri *et al*., 2004; Azzaro *et al*., 2001; Acocella and Neri, 2005). At the end of this structure, close to the coast line, the Calata‐ biano and the Piedimonte faults (PF) can be considered, following Lentini *et al.* (2006), as the

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

Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 107

neotectonic structures of the basement outcropping in north-eastern Sicily (Fig. 2a).

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

(e.g. Li *et al*., 1994; Mizuno and Nishigami, 2006).

The western flank of the volcano is affected by a moderate tectonic activity, the Ragalna fault system (RFS) being the main structure (Fig. 2a). This system is formed by three distinct fault segments the Calcerana and the Ragalna faults trending NE-SW and the N–S striking Masseria Cavaliere fault (Azzaro, 1999; Rust and Neri, 1996). This latter structure is a fresh east-facing escarpment up to 20 m high and 5 km long. Less evident compared to the previous one, the NE-SW striking Calcerana fault and the NE-SW trending structure, reported by some authors in the area between Ragalna and Biancavilla, do not show strong field manifestations.

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

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
