**1. Introduction**

The ground motion that can be recorded at the free surface of a terrain is the final result of a series of phenomena that can be grouped into three fundamental typologies: the source mechanism, the seismic wave propagation till the bedrock interface below the investigated site and the site effects (Fig. 1). The first two features define the kind of seismic input whereas the third represents all modifications that can occur as a consequence of the interaction between seismic waves and local characteristics of the investigated site. The physical and mechanical properties of terrains as well as their morphologic and stratigraphic features appreciably affect the characteristics of the ground motion observed at the surface. The whole process of modifications undergone by a given seismic input in terms of amplitude, frequency content and duration, as a consequence of local characteristics, is generally termed the "local seismic response". It is indeed well known that the spectral composition of a seismic event is modified first during the source-bedrock path (attenuation function), and second, when the seismic input interacts with the soft terrains layered between the bedrock and the free surface (Fig. 1a). This latter effect, significantly changes the spectral content so that it is extremely important for estimating the final input to which all structures built in the study area will be subjected.

The influence of local geologic features on the ground motion peculiarities and damage due to earthquakes is well known since years. Studies of Wood (1908) and Baratta (1910) concerning the San Francisco 1906 and the Messina 1908 earthquakes, respectively, pointed out, since the beginning of the last century, that the damage distribution is a function of different site conditions existing in various areas affected by the same shock. Similar effects have been observed by several authors during all destructive earthquakes occurred up till the present.

© 2013 Panzera et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Panzera et al.; licensee InTech. This is a chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Panzera et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

are partially reflected again at the deposit-basement interface. The amount of reflected energy that is "trapped" into the deposit increases with the seismic impedance contrast between the terrains forming the deposit and the basement. Besides, the trapped waves interfere between themselves and the incident waves as a consequence of the geometric features of the deposit, the physical properties of the terrains and the frequency content of the seismic input. Strati‐ graphic site effects are therefore mainly connected to seismic wave trapping phenomena inside the deposit due to reflections as well as interference and resonance effects between incident and reflected waves. The local seismic response, becomes, of course, more complex when the basement-deposit interface has a more irregular geometry, or in the presence of faults, cavities

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Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 103

**Figure 2.** a) Simplified geological map of Mt Etna showing the main structural features (modified from Neri *et al*., 2007), RFS = Ragalna fault system, PFS = Pernicana fault system, PF = Piedimonte fault, TFS = Timpe fault system. In the inset map, the Malta Escarpment (ME) is shown. (b) Sketch geological map of the Maltese Islands (modified from Vari‐

ous Authors, 1993). The black square indicates the investigated area.

and particular topographic conditions.

**Figure 1.** a) Sketch showing the influences that can affect a seismic signal propagating from the source to the free surface of a terrain. b) Scenaria for local seismic response.

Site effects occur as a result of several physical phenomena such as multiple reflections, diffraction, focusing, resonance etc., to which the incoming wavefront is subjected. This is a consequence of the various mechanical properties of terrains, the presence of heterogeneities and discontinuities, as well as the geometry of shallower layers and the existence of topo‐ graphic irregularities both in the basement and the surface. In Figure 1b are shown the principal morphologic and/or structural features that contribute to characterize the local hazard scenario. They are grouped in effects linked to the layers' geometry (a), effects linked to the possible presence of water-bearing strata, landslides, structural discontinuities and cavities (b) and effects linked to the topography (c).

Generally stratigraphic effects are schematized as the modifications affecting a seismic motion that propagates almost vertically inside a deposit having a flat free surface, horizontal layers and negligible lateral heterogeneities. The theoretical analysis of such problem was tackled by Kramer (1996), and however, considering the described simplified assumptions, it can be postulated that the incident waves at the base of the deposit that are reflected at its free surface, are partially reflected again at the deposit-basement interface. The amount of reflected energy that is "trapped" into the deposit increases with the seismic impedance contrast between the terrains forming the deposit and the basement. Besides, the trapped waves interfere between themselves and the incident waves as a consequence of the geometric features of the deposit, the physical properties of the terrains and the frequency content of the seismic input. Strati‐ graphic site effects are therefore mainly connected to seismic wave trapping phenomena inside the deposit due to reflections as well as interference and resonance effects between incident and reflected waves. The local seismic response, becomes, of course, more complex when the basement-deposit interface has a more irregular geometry, or in the presence of faults, cavities and particular topographic conditions.

Site effects occur as a result of several physical phenomena such as multiple reflections, diffraction, focusing, resonance etc., to which the incoming wavefront is subjected. This is a consequence of the various mechanical properties of terrains, the presence of heterogeneities and discontinuities, as well as the geometry of shallower layers and the existence of topo‐ graphic irregularities both in the basement and the surface. In Figure 1b are shown the principal morphologic and/or structural features that contribute to characterize the local hazard scenario. They are grouped in effects linked to the layers' geometry (a), effects linked to the possible presence of water-bearing strata, landslides, structural discontinuities and cavities (b)

**Figure 1.** a) Sketch showing the influences that can affect a seismic signal propagating from the source to the free

Generally stratigraphic effects are schematized as the modifications affecting a seismic motion that propagates almost vertically inside a deposit having a flat free surface, horizontal layers and negligible lateral heterogeneities. The theoretical analysis of such problem was tackled by Kramer (1996), and however, considering the described simplified assumptions, it can be postulated that the incident waves at the base of the deposit that are reflected at its free surface,

and effects linked to the topography (c).

surface of a terrain. b) Scenaria for local seismic response.

102 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**Figure 2.** a) Simplified geological map of Mt Etna showing the main structural features (modified from Neri *et al*., 2007), RFS = Ragalna fault system, PFS = Pernicana fault system, PF = Piedimonte fault, TFS = Timpe fault system. In the inset map, the Malta Escarpment (ME) is shown. (b) Sketch geological map of the Maltese Islands (modified from Vari‐ ous Authors, 1993). The black square indicates the investigated area.

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 of directional effects.

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

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Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 105

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

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

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

frequency whereas it appears less reliable for the estimate of the amplitude values.

frequency peak (Cara *et al*., 2003).

on frequency intervals of ± 5% of the central frequency.
