Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna: Observations

and Fracture Orientation Modelling

F. PANZERA,1,3 M. PISCHIUTTA,2 G. LOMBARDO,1 C. MONACO,1 and A. ROVELLI2

Abstract—Ambient noise measurements performed on the

western flank of Mt. Etna are analyzed to infer the occurrence of

directional amplification effects in fault zones. The data were

recorded along short (\500 m) profiles crossing the Ragalna Fault

System. Ambient noise records were processed to compute the

horizontal-to-vertical noise spectral ratio as a function of frequency

and direction of motion. Wavefield polarization was investigated in

the time–frequency domain as well. Peaks of the spectral ratios

generally fall in the frequency band 1.0–6.0 Hz pointing out

directional amplifications that are also confirmed by the results of

the time–frequency analysis, the largest amplification occurring

with high angle to the fault strike. A variation of the frequency of

the spectral peak is observed between the two sides of the fault,

possibly related to a damage fault asymmetry. Measurements

performed several kilometers away from the fault zone do not show

behavior that is as systematic as in the fault zone, and this suggests

that the observed directional effects can be ascribed to the fault

fabric. We relate the polarization effect to compliance anisotropy in

the fault zone, where the presence of predominantly oriented

fractures makes the normal component of ground motion larger

than the transversal one. In order to test the direction and the type

of fractures that are expected in the fault zone, we modeled the

brittle deformation pattern of the investigated fault. Theoretical

results are in good agreement with field observations of the fracture

strike.

Key words: Directional resonance, fault zone,

Wavefield polarization, Mt. Etna.

1. Introduction

Fault zone rocks exhibit locally reduced elastic

moduli that may lead to amplification of horizontal

ground motion during earthquakes (BEN-ZION 1998).

These effects were observed, among others, by

SPUDICH and OLSEN (2001), within a *1–2 km wide

low-velocity zone around the rupture of the 1984

Morgan Hill earthquake; by SEEBER et al. (2000) and

PENG and BEN-ZION (2006) in the rupture zone of the

1999 Izmit earthquake on the Karadere branch of the

North Anatolian Fault; and by CALDERONI et al.

(2010) along the Paganica-San Demetrio fault during

the April 2009 L’Aquila earthquake sequences in

central Italy.

When the low velocity fault zone layer is coherent

over length scales of several km or more, it produces

trapped waves that are the effect of constructive

interference of critically reflected phases (BEN-ZION

and AKI 1990; LI and LEARY 1990; LI et al. 1994).

Trapped waves have been observed along many

active faults (e.g., LI et al. 1990; MIZUNO and NISHI-

GAMI 2004; LEWIS et al. 2005), as well as near a

dormant fault (ROVELLI et al. 2002; CULTRERA et al.

2003). In trapped waves, the amplified motion is

predominantly fault-parallel and vertical (LEWIS AND

BEN-ZION 2010). In several recent studies, amplified

motions near faults were found to have a high angle

to the fault strike, indicating a mechanism different

than for trapped waves. In four faults of the eastern

flank of Mt. Etna (the Tremestieri, Pernicana, Mo-

scarello and Acicatena faults), RIGANO et al. (2008)

observed that seismic signals are strongly polarized,

but not fault-parallel as would be expected for trap-

ped waves. Using both volcanic tremor and local

earthquakes, FALSAPERLA et al. (2010) found a strong

polarization at seismological stations in the crater

area of Mt. Etna, with polarization directions varying

site by site, but everywhere transversal to the orien-

tation of the predominant local fracture field.

1 Dipartimento di Scienze Biologiche, Geologiche e Ambi-

entali, Sezione Scienze della Terra, Universita degli studi di

Catania, Catania, Italy. E-mail: panzerafrancesco@hotmail.it2 Istituto Nazionale di Geofisica e Vulcanologia, Seismology

and Tectonophysics, Rome, Italy.3 Present Address: Physics Department, Icelandic Meteoro-

logical Office, Reykjavık, Iceland.

Pure Appl. Geophys. 171 (2014), 3083–3097

� 2014 Springer Basel

DOI 10.1007/s00024-014-0831-x Pure and Applied Geophysics

Similarly, DI GIULIO et al. (2009) found very stable

polarization angles on Mt. Etna, in the NE rift seg-

ment and in the Pernicana fault at Piano Pernicana,

with horizontal polarization that again was not par-

allel to the fault strike. PISCHIUTTA et al. (2012, 2013a, b,

2014) interpreted the directional amplification of the

horizontal ground motion in many fault zones as due

to the crack orientation, which causes a larger rock

compliance transversal to fractures. The similar

results obtained in the above mentioned papers by

using earthquake records and ambient noise indicate

that microtremors are a valid tool to investigate

ground motion polarization properties.

In the present study, the results of new measure-

ments of ambient noise performed in the western

flank of Mt. Etna are shown and interpreted in the

frame of local stress field and fault kinematics. The

data were recorded across the tectonic structures of

Figure 1Geological map of the investigated area (modified from BRANCA et al. 2011). The inset map shows the main structural features of Mt. Etna

(modified from NERI et al. 2007). The white square (PG) indicates the Piano dei Grilli area. From North to South: PFS Pernicana Fault

System, TFS Timpe Fault System, RFS Ragalna Fault System, SFS Southern Fault System, BOL Belpasso-Ognina Lineament, TMF

Tremestieri-Mascalucia Fault, and TCF Trecastagni Fault

3084 F. Panzera et al. Pure Appl. Geophys.

the Ragalna Fault System (RFS). Several measure-

ments were also performed at Piano dei Grilli, up to

thousand of meters from the RFS, in order to check

the expected change far from the damage fault zone.

We followed the approach proposed by PISCHIUTTA

et al. (2013a), who interpreted variations of ground

motion polarization across the Pernicana fault, on the

eastern flank of Mt. Etna, in terms of fracture fields in

the fault damage zone, polarization being perpen-

dicular to the predominant fracture direction.

Therefore, following PISCHIUTTA et al. (2013a), we

have modeled the brittle deformation pattern expec-

ted for the investigated fault using the FRAP package

(SALVINI et al. 1999).

1.1. Tectonics of the Study Area

Mt. Etna is a 3,300 m high basaltic volcano

located along the Ionian coast of Sicily (Fig. 1), at the

boundary between the African and European Plates.

The volcanic edifice shows a diameter of about

40 km and lies at the front of the Sicilian thrust belt

and at the footwall of the northern sector of the Malta

escarpment. Its tectonic setting results from the

interaction of regional tectonics and local-scale

volcano-related processes (MCGUIRE and PULLEN

1989).

The eastern and south-eastern sectors of the

volcano are the most tectonically active, being

affected by normal-oblique faulting, related to

WNW-ESE regional extension, and by large sliding

processes (NERI et al. 1991; LO GIUDICE and RASA

1992; RUST and NERI 1996; MONACO et al. 1997;

FROGER et al. 2001; AZZARO et al. 2012). Geodetic

surveys indicate a relative movement toward E-ESE

(BONFORTE et al. 2008; ALPARONE et al. 2011). The

sliding sector is limited to the North by the active

Pernicana Fault System (PFS in Fig. 1), a left-lateral

strike-slip that represents one of the most pronounced

and active tectonic lineaments of the Mt. Etna

structure (AZZARO et al. 2001; NERI et al. 2004).

Moving to the South, the 30 km long Timpe Fault

System (TFS in Fig. 1) is composed by ENE-dipping

and NNW-SSE striking segments, characterized by

normal-dextral motion and vertical offsets up to

200 m. Most of these faults have a high seismogenic

potential and generate shallow earthquakes, as well as

coseismic cracks and creeping (MONACO et al. 1997;

AZZARO 1999; Azzaro et al. 2012). The South Fault

System (SFS in Fig. 1) is composed of two main,

NW–SE trending en-echelon normal-dextral seismo-

genic segments, the Tremestieri-Mascalucia (TMF)

and the Trecastagni (TCF) faults (BARRECA et al.

2013), whose motion is accommodated to the east by

an aseismic, W–E striking, fracture alignment.

According to geological and geochemical surveys

(LO GIUDICE and RASA 1992; ACOCELLA et al. 2003;

NERI et al. 2004; BONFORTE et al. 2013) and remote

sensing data (BORGIA et al. 2000; NERI et al. 2009;

SOLARO et al. 2010; BONFORTE et al. 2011), these

structures, together with another sub-parallel arc-

shaped aseismic alignment, the Belpasso-Ognina

Lineament, represent the southern boundaries of the

SE-ward sliding sector of Mt. Etna.

Conversely, the western flank of the volcanic

edifice is characterized by moderate tectonic activity.

The main structure of this sector is represented by the

RFS (in Fig. 1). According to NERI et al. (2007), field

evidence, together soil radon emission data and

geodetic measures, suggest that the RFS is an active

right-lateral oblique structure with a minimum long-

term dip-slip component. The RFS is formed by two

distinct fault segments: the SW–NE striking Calcer-

ana fault and the N–S striking Masseria Cavalieri

fault (AZZARO 1999; RUST and NERI 1996). The

Calcerana fault shows a smoothed scarp, which is less

evident compared to the previous one that develops

for about 1.4 km with morphological offsets of about

10 m. The Masseria Cavalieri fault is characterized

by a fresh east-facing escarpment up to 20 m high

and 5 km long. Along this structure, fractures are

well visible on buildings and concrete road walls, and

show centimetric-millimetric oblique-dextral offset

(see Fig. 2). The tectonic activity reported in the last

two centuries seems to be mostly related to fault

creep, even though coseismic ground fracturing was

observed during the 1982 earthquake (macroseismic

magnitude Mm = 3.4; AZZARO 1999). At its southern

end, the Masseria Cavalieri fault is characterized by

the occurrence of cinder cones aligned in a N–S

direction. To the south, the fault loses its morpho-

logic evidence and a 3 km long fracture zone

develops with a SSW–NNE direction towards Santa

Maria di Licodia. The relationship between the

Vol. 171, (2014) Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna 3085

distinct fault segments is unclear, and there is lack of

apparent continuity in the field. Finally, there is no

field evidence of the NE–SW trending fault reported

by ROMANO et al. (1979) and RASA et al. (1981) in the

area between Ragalna and Santa Maria di Licodia

(see also MONACO et al. 2010; AZZARO et al. 2012).

1.2. Dataset and Polarization Analysis Method

In order to investigate the effect of the damage

zone of the RFS on ground motion, ambient noise

was recorded along 200–400 m long transects cross-

ing the fault (see Figs. 1, 3). In each of the two fault

segments, two transects were carried out, each one

consisting of four to six measurement sites (Fig. 3).

Measurements were also performed in the Piano dei

Grilli (PG) area (Fig. 1) in order to record data in

districts significantly far (up to 7 km) from the

investigated tectonic structures. This allowed us to

observe how directional amplification effects change

moving away from the fault zone. Ambient noise was

recorded using a 3-component 1-Hz velocimeter

Mark L4C 3-D connected to a 12-bit analog-to-

digital converter. Time series were 30 min long, with

a sampling rate of 100 Hz. The signals were

processed computing the horizontal-to-vertical noise

spectral ratios (HVNSR). According to the guidelines

suggested by SESAME (2004), time windows of 30 s

were selected in the stationary part of the records,

eliminating transients associated to local distur-

bances. Fourier spectra were calculated and

smoothed using a triangular average on frequency

intervals of ±5 % of the central frequency. HVNSRs

were calculated after rotating the NS and EW

Figure 2Evidence of movements linked to the RFS activity, as observed in some man-made structures

3086 F. Panzera et al. Pure Appl. Geophys.

components of motion by bins of 10 from 0� (north)

to 180� (south). They are plotted in Fig. 4 using

contour plots of amplitude, as a function of frequency

(x-axis) and direction of motion (y-axis). This

approach is powerful in enhancing, if any, the

occurrence of site-specific directional effects. It was

applied to earthquake data (SPUDICH et al. 1996;

PISCHIUTTA et al. 2010) to study directional reso-

nances. A similar procedure was applied by PANZERA

et al. (2011a, 2012) and BURJANEK et al. (2012) using

ambient noise to identify the site-response directivity

on ridges and unstable slopes of rock blocks.

Moreover, this technique was adopted by RIGANO

et al. (2008), DI GIULIO et al. (2009) and PISCHIUTTA

et al. (2012), to study directional resonances in fault

zones, using both earthquake and noise recordings.

However, in presence of lateral and vertical

heterogeneities or velocity inversion, the HVNSR

can be ‘‘non-informative’’ due to the occurrence of

amplification on the vertical component of motion

(DI GIACOMO et al. 2005; PANZERA et al. 2011b, 2013).

Thus, in this study, we also applied the time–

frequency (TF) polarization analysis proposed by

VIDALE (1986) and exploited by BURJANEK et al.

(2012). This technique can provide quite robust

results, overcoming the bias that could be introduced

by the denominator spectrum in the HVNSR

calculation.

Following BURJANEK et al. (2010, 2012), the

continuous wavelet transform (CWT, see KULESH

et al. 2007) is applied to signals in order to select

time windows whose length matches the dominant

period: signals are thus decomposed in the time–

frequency domain and the polarization analysis is

applied. For each time–frequency pair, polarization

is characterized by an ellipsoid and is defined by two

angles: the strike (azimuth of the major axis

projected to the horizontal plane from North) and

the dip (angle of the major axis from the vertical

axis). Another important parameter is ellipticity that

is defined, according to VIDALE (1986), as the ratio

between the length of the minor and major axes: this

Figure 3Location and name of ambient noise recording sites along the four transects carried out across the RFS

Vol. 171, (2014) Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna 3087

parameter approaches 0 when ground motion is

linearly polarized. As stressed by BURJANEK et al.

(2010, 2012), the chosen wavelet in the CWF

analysis affects all the polarization parameters as

well as the analysis resolution. Polarization strike

and dip obtained all over the time series analyzed

are cumulated and represented using polar plots

where the contour scale represents the relative

frequency of occurrence of each value, and the

distance to the center represents the signal frequency

in Hz. In order to assess whether ground motion is

linearly polarized, the ellipticity is also plotted

versus frequency.

2. Results

2.1. Observations

In Fig. 4, we show some results of HVNSRs

obtained at selected ambient noise recording sites,

along the four short profiles crossing the Masseria

Figure 4Examples of contours of the spectral ratios geometric mean, as a function of frequency (x-axis), direction of motion (y-axis) and HVNSR

contour plot amplitudes obtained at sites located on the RFS (see locations in Figs. 1 and 3)

3088 F. Panzera et al. Pure Appl. Geophys.

Cavalieri fault and the inferred Ragalna-S.M. Licodia

structure. The HVNSRs depict a significant ampli-

tude increment in the frequency band 1.0–6.0 Hz, at

angles of about 80�–90� for the Masseria Cavalieri

and 60�–70� for the Ragalna-S.M. Licodia, respec-

tively, showing several adjacent peaks that indicate a

preferential and site-dependent direction of horizontal

ground motion amplification. It is also interesting to

remark that in the Masseria Cavalieri fault, the results

related to the stations on the west side of the fault

show significant spectral ratio amplitude at frequency

values higher than those observed at the stations

located on the east side of the fault (Fig. 5). This

effect could be an evidence of fault damage asym-

metry (DOR et al. 2006; DUAN 2008 and references

therein). Since the peaked frequency is controlled by

the ratio of shear velocity by the thickness of the

fractured layer, Fig. 5 suggests that, in the Masseria

Cavalieri fault, the damage zone has a smaller

velocity (higher damage) or the volume of cracks is

larger (or both) in the eastern part where lower

frequencies are peaked in the spectral ratios.

To get more precise information on the ground

motion horizontal polarization, the TF polarization

Figure 5Cross-sections obtained combining the ambient noise measurements along Tr#1 and Tr#2 profiles of the Masseria Cavalieri fault as a function

of distance (x-axis), frequency (y-axis) and HVNSR contour plot amplitudes. The dotted line indicates the position of the fault

Vol. 171, (2014) Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna 3089

analysis was applied (Fig. 6). It is worth noting that

polar plots visibly confirm a pronounced polarization

in narrow frequency bands that generally fall in the

range 1.0–6.0 Hz, with maxima roughly trending in

the W–E and SW–NE direction for the Masseria

Cavalieri and the Ragalna-S.M. Licodia segments,

respectively. In order to test whether the amplifica-

tion effects are strictly fault-dependent, we performed

some measurements several kilometers away from

the RFS, in the Piano dei Grilli area (see location in

Figs. 1, 7). Amplification directions obtained from

HVNSRs, as well as the polar plots obtained from the

time–frequency (TF) polarization analysis, appear to

be randomly distributed and/or uniformly scattered

(Fig. 7), showing a not-uniform behavior between

measuring sites in Piano dei Grilli area. In this way,

we can exclude a source-dependence and a time-

dependence of ambient noise in the investigated

volcanic area.

The experimental results suggest two different

polarization patterns at the two investigated struc-

tures. In the Masseria Cavalieri area, we observe

quite pronounced amplitudes of the HVNSR peaks

(e.g. #1, #4, #5, #8, #10 in Fig. 4). Moreover, the TF

polarization analysis revealed values of the ellipticity

reaching a minimum in a wide frequency band

(1.0–6.0 Hz), as well as dip values showing an

horizontal trend in the same frequency band (see

examples in Fig. 8a). We stress that in this area,

geological investigation found clear evidence of the

presence of the fault, as fractures on buildings and

concrete road walls.

On the other hand, along the Ragalna-S.M.

Licodia alignment we still observe directional effects,

Figure 6Polar plots, computed through the TF polarization analysis, of the strike angles from ambient noise measurements along the RFS transects (see

location in Figs. 1 and 3)

3090 F. Panzera et al. Pure Appl. Geophys.

Figure 7Locations and name of the noise measurement sites at the Piano dei Grilli area and corresponding results obtained through the contours of the

spectral ratios geometric mean and the TF polarization analysis (higher and lower and panels, respectively)

Vol. 171, (2014) Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna 3091

although they show different features. Indeed, we do

not have a decrease of the ellipticity values in a wide

frequency range, but a minimum in particular

frequency values (see examples in Fig. 8b). Follow-

ing these findings and since the Ragalna-S.M. Licodia

structure does not show any field morphologic

evidence, we focus on the Masseria Cavalieri fault.

We model the brittle deformation, proposing an

interpretation of the observed directional resonance

effect in terms of fracture fields.

2.2. Interpretation of Ground Motion Polarization

in Terms of Fractures in the Fault-Zone

By analyzing several faults, PISCHIUTTA et al.

(2012, 2013a, b) found a near orthogonal relation

between the horizontal polarization and the orienta-

tion of the main fracture field produced by the fault

kinematics. They modeled the expected brittle defor-

mation pattern in the fault damage zone through the

Frap Package (SALVINI et al. 1999), that uses a

combination of numeric and analytic tools, and

interpreted the directional amplification as produced

by stiffness anisotropy, with larger stiffness parallel

to the dominant sets of fractures. This interpretation

is in agreement with results of numerical simulations

of uniaxial compression tests on fractured rocks

(GRIFFITH et al. 2009) and laboratory experiments of

wet and dry discontinuities (PLACE et al. 2014).

Following the same approach, we investigate the

relation between the horizontal polarization and the

strike of the fracture field across the Masseria

Cavalieri fault. We model the fault as a N–S striking

and 80� east-dipping surface, with dimensions 3 km

(along strike) 9 1 km (along dip). According to the

morphotectonic analysis, the fault kinematics is

assumed to be characterized by oblique right-lateral

strike-slip with a total displacement of 70 m. In order

to allow extension to play a concomitant role with

strike-slip movement, the kinematic vector is fixed to

N145� direction with 45� plunge. The fault surface is

discretized into a grid of quadrangular cells, each one

Figure 8Examples of polar plots and trends of the azimuth, dip and ellipticity in selected sites of the Masseria Cavalieri (a) and the Ragalna-S. M. di

Licodia structures (b)

3092 F. Panzera et al. Pure Appl. Geophys.

being characterized by an attitude and a position in a

reference frame. For each cell, the program computes

four components of stress: (1) the regional stress

tensor rR; (2) the overburden rV; (3) the fluid

isotropic pressure PF; and (4) the ‘kinematic stress’

rK that results from the brittle strain accumulation

due to frictional resistance and failures associated to

the fault. The regional stress tensor is set to be

consistent with the fault development and is oriented

with r1 at N35� and r3 at N125�, both lying on the

horizontal plane (cyan arrows in the top left-hand

panels of Fig. 9), and vertical null r2 axis.

In our first model, we consider the fracture pattern

as produced by all the stress contributors (top panels

of Fig. 9). The resulting stress tensor is calculated at

each cell through the sum of the four stress compo-

nents. Then the resulting stress is compared to the

strength in the cell as predicted by the Coulomb–

Navier Failure Criterion, in order to evaluate the

capability of producing fractures at each cell. This

model predicts that synthetic cleavage is the predom-

inating fracture type, with an average strike of N22�(Fig. 9, top left-hand panel). Its distribution is

computed using the Daisy Package (SALVINI et al.

1999; available at http://host.uniroma3.it/progetti/

fralab/), and its strike is visualized through a violet

wind-rose diagram (Fig. 9, top central and right-hand

panels). Secondary extensional fractures are expected

Figure 9Results of brittle deformation modeling performed for the Masseria Cavalieri fault. The two models differ in the number of stress components

(regional stress rR, overburden rV, fluid pressure PF and kinematic stress in Model 1; only kinematic stress in Model 2). The fault surface is

discretized through a grid of quadrangular cells in a NS–EW reference frame. The cell color depends on the predominant fracture type (red

for synthetic cleavage and blue for extensional fracture). The fault kinematics is assumed to be oblique right-lateral strike-slip and the

kinematic vector is fixed to N145� direction with 45� plunge (yellow arrows). The cyan arrows represent the orientation of the regional

stresses r1 and r3, the violet arrow the overburden rV and the green arrows the fluid isotropic pressure PF. The expected orientation of

synthetic cleavage and extensional fractures is drawn using a 3D-plane visualization projected on the fault surface (the color bars represent the

plane strike). The expected orientation of synthetic cleavage and extensional fractures is visualized through the wind-rose diagrams on the

right side of each panel. The over imposed red rose diagram shows the observed polarization across the Masseria Cavalieri fault

Vol. 171, (2014) Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna 3093

to develop with an average strike of N33� (blue rose

diagram).

In our second model, we estimate the expected

fracture pattern as produced only by the kinematic

stress, which is caused by the frictional motion of the

fault (bottom panels of Fig. 9). To evaluate fracture

production, the comparison with the strength is here

performed only using the kinematic stress values at

each cell. This second model yields predominant

extensional fractures striking at N15� direction, with

synthetic cleavage at N7� direction (Fig. 9, bottom

central and right-hand panels).

Considering that in the Masseria Cavalieri fault,

observed polarization is oriented roughly E–W

(Fig. 9, red rose diagrams), the second model seems

to better explain the experimental observations.

Moreover, these orientations agree with the fracture

strikes as measured in the field along the Masseria

Cavalieri fault zone. These extensional fractures were

recognized on buildings and concrete road walls in

the fault neighborhood, with a centimetric–millimet-

ric oblique-dextral offset. The azimuth distribution

through a red rose diagram as well as the location of

the 11 measuring sites are shown in Fig. 10.

Figure 10Location of the man-made structures where fracture azimuths were measured and corresponding rose diagram

3094 F. Panzera et al. Pure Appl. Geophys.

2.3. Concluding Remarks

We have found new evidence of ground motion

polarization in the western flank of Mt. Etna on fault

zones associated with the RFS.

The analysis was performed using ambient noise

signals through the HVSRs computation and TD

polarization analysis. It revealed the occurrence of a

directional amplification effect at stations installed in

the fault damage zone, and polarization showing a

high angle with the fault strike. This is consistent

with previous studies on other faults (PISCHIUTTA et al.

2012, 2013a, 2014). Noise measurements performed

in an area that is thousand meters away from the fault

structures did not show any amplification effect,

leading to the exclusion of source and time depen-

dence in the investigated volcanic area.

On the two sides of the fault, directional ampli-

fication occurs in different frequency bands, peaked

frequencies being smaller on the eastern side. This

could be related to an asymmetric rock damage.

The observed wavefield polarization is then inter-

preted in terms of fractures associated with the fault

damage zone. Modeling of brittle deformation pattern in

the Masseria Cavalieri fault provides a nearly perpen-

dicular relation between the theoretical fracture field

and the experimentally observed polarization seismic

wavefield. Field observations of fractures on man-made

structures match the results of theoretical models well.

Finally, it is worth noting that the present analysis

on local amplifications in fault zones gives further

support to findings coming out from previous studies

(e.g., RIGANO et al. 2008; DI GIULIO et al. 2009;

PISCHIUTTA et al. 2012). These results and the evidence

of sharp spatial variations of ground motion within

small distances could have important implications on

shaking hazards at the local scale. Accordingly, they

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 in fault zones.

Acknowledgments

The authors are grateful to Dr. J. Burjanek for having

kindly provided access to the time–frequency (TF)

polarization analysis software and for useful expla-

nations. The authors also wish to thank anonymous

reviewers and the Guest Editor Prof. Yehuda Ben-

Zion for constructive comments that contributed to

improving the quality of the paper.

REFERENCES

ACOCELLA, V., BEHNCKE, B., NERI, M., and D’AMICO, S. (2003), Link

between major flank slip and eruptions at Mt. Etna (Italy),

Geophys. Res. Lett., 30(24), doi:10.1029/2003GL018642.

ALPARONE, S., BARBERI, G., BONFORTE, A., MAIOLINO, V., and

URSINO, A. (2011), Evidence of multiple strain fields beneath the

eastern flank of Mt. Etna volcano (Sicily, Italy) deduced from

seismic and geodetic data during 2003–2004, Bull. Volcanol. 7,

869–885, doi:10.1007/s00445-011-0456-1.

AZZARO, R. (1999), Earthquake surface faulting at Mount Etna

volcano (Sicily) and implications for active tectonics, J. Geodyn.,

28, 193– 213, doi:10.1016/S0264-3707(98)00037-4.

AZZARO, R., MATTIA, M., and PUGLISI, G. (2001), Fault creep and

kinematics of the eastern segment of the Pernicana Fault (Mt.

Etna, Italy) derived from geodetic observation and their tectonic

significance, Tectonophys. 333, 401–415.

AZZARO, R., BRANCA, S., GWINNER, K., and COLTELLI, M. (2012), The

volcano-tectonic map of Etna volcano, 1:100.000 scale: an

integrated approach based on a morphotectonic analysis from

high-resolution DEM constrained by geologic, active faulting

and seismotectonic data, Ital. J. Geosci. 131(1), 153–170. doi:10.

3301/IJG.2011.29.

BARRECA, G., BONFORTE, A., and NERI, M. (2013), A pilot GIS

database of active faults of Mt. Etna (Sicily): A tool for inte-

grated hazard evaluation, J. Volcanol. Geotherm. Res. 251(1),

170–186, doi:10.1016/j.jvolgeores.2012.08.013.

BEN-ZION, Y. (1998), Properties of seismic fault zone waves and

their utility for imaging low-velocity structures, J. Geophys. Res.,

103(B6), 12567–12585.

BEN-ZION, Y., and AKI, K. (1990), Seismic radiation from an SH

line source in a laterally heterogeneous planar fault zone, Bull.

seism. Soc. Am. 80, 971–994.

BONFORTE, A., BONACCORSO, A., GUGLIELMINO, F., PALANO. M., and

PUGLISI, G. (2008), Feeding system and magma storage beneath

Mt. Etna as revealed by recent inflation/deflation cycles, J.

Geophys. Res. 113, B05406, doi:10.1029/2007JB005334.

BONFORTE, A., FEDERICO, C., GIAMMANCO, S., GUGLIELMINO, F.,

LIUZZO, M., and NERI, M. (2013), Soil gases and SAR data reveal

hidden faults on the sliding flank of Mt. Etna (Italy), J. Volcan.

Geotherm. Res. 251, 27–40, doi:10.1016/j.volgeores.2012.08.

010.

BONFORTE, A., GUGLIELMINO, F., COLTELLI, M., FERRETTI, A., and

PUGLISI, G. (2011), Structural assessment of Mt. Etna volcano

from Permanent Scatterers analysis, Geochemistry, Geophysics,

Geosystems 12(2), Q02002, doi:10.1029/2010GC003213.

BORGIA, A., LANARI, R., SANSOSTI, E., TESAURO, M., BERARDINO, P.,

FORNARO, G., NERI, M., and MURRAY, J.B. (2000), Actively

growing anticlines beneath Catania from the distal motion of

Mount Etna’s decollement measured by SAR interferometry and

GPS, Geophys. Res. Lett., 27(20), 3409–3412, doi:10.1029/

1999GL008475.

Vol. 171, (2014) Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna 3095

BRANCA, S., COLTELLI, M., GROPELLI, G., and LENTINI, F. (2011),

Geological map of Etna volcano, 1:50000 scale, Ital. J. Geosci.

130(3), 265–291.

BURJANEK, J., GASSNER-STAMM, G., POGGI, V., MOORE, J. R., and

FAH, D. (2010), Ambient vibration analysis of an unstable

mountain slope, Geophys. J. Int., 180, 559–569, doi:10.1111/j.

1365-246X.2009.04451.x.

BURJANEK, J., MOORE, J. R., MOLINA, F.X.Y., and FAH, D. (2012),

Instrumental evidence of normal mode rock slope vibration,

Geophys. J. Int., 188(2), 559–569, doi:10.1111/j.1365-246X.

2011.05272.x.

CALDERONI, G., ROVELLI, A., and DI GIOVAMBATTISTA, R. (2010),

Large amplitude variations recorded by an on-fault seismologi-

cal station during the L’Aquila earthquakes: evidence for a

complex fault-induced site effect, Geophys. Res. Lett. 37,

L24305, doi:10.1029/2010GL045697.

CULTRERA, G., ROVELLI, A., MELE, G., AZZARA, R., CASERTA, A., and

MARRA, F. (2003), Azimuth dependent amplification of weak and

strong ground motions within a fault zone, Nocera Umbra,

Central Italy, J. Geophys. Res. 108(B3), 2156–2170, doi:10.

1029/2002JB001929.

DI GIACOMO, D., GALLIPOLI, M.R., MUCCIARELLI, M., PAROLAI, S., and

RICHWALSKI, S.M. (2005), Analysis and modeling of HVSR in the

presence of a velocity inversion: the case of Venosa, Italy, Bull.

Seism. Soc. Am. 95(6), 2364–2372, doi:10.1785/0120040242.

DI GIULIO, G., CARA, F., ROVELLI, A., LOMBARDO, G., and RIGANO, R.

(2009), Evidence for strong directional resonances in intensely

deformed zones of the Pernicana fault, Mount Etna, Italy, J.

Geophys. Res., 114, doi:10.1029/2009JB006393.

DOR, O., ROCKWELL, T. K., BEN-ZION, Y. (2006), Geological

observations of damage asymmetry in the structure of the San

Jacinto, San Andreas and Punchbowl faults in southern Cali-

fornia: A possible indictor for preferred rupture propagation

direction, Pure Appl. Geophys., 163, 301–349.

DUAN, B. (2008), Asymmetric off-fault damage generated by

bilateral ruptures along a bimaterial interface, Geophys. Res.

Lett. 35, L14306, doi:10.1029/2008GL034797.

FALSAPERLA, S., CARA, F., ROVELLI, A., NERI, M., BEHNCKE, B., and

ACOCELLA, B. (2010), Effects of the 1989 fracture system in the

dynamics of the upper SE flank of Etna revealed by volcanic

tremor data: the missing link? J. Geophys. Res. 115, B11306,

doi:10.1029/2010JB007529.

FROGER, J.L., MERLE, O., and BRIOLE, P. (2001), Active spreading

and regional extension at Mount Etna imaged by SAR interfer-

ometry, Earth Planet. Sci. Lett. 187, 245–258, doi:10.1016/

S0012-821X(01)00290-4.

GRIFFITH, W.A., SANZ, P.F., and POLLARD, D.D. (2009), Influence of

outcrop scale fractures on the effective stiffness of fault damage

zone rocks, Pure Appl. Geophys., 166, 1595–1627.

KULESH, M., DIALLO, M.S., HOLSCHNEIDER, M., KURENNAYA, K.,

KRUGER, F., OHRBERGER, M., and SCHERBAUM, F. (2007), Polari-

zation analysis in the wavelet domain based on adaptive

covariance method, Geophys. J. Int. 170(2), 667–678, doi:10.

1111/j.1365-246X.2007.03417.x.

LEWIS, M.A., PENG, Z., BEN-ZION, Y. and VERNON, F.L. (2005),

Shallow seismic trapping structure in the San Jacinto fault zone

near Anza, California, Geophys. J. Int., 162, 867–881, doi:10.

1111/j.1365-246X.2005.02684.x.

LEWIS, M., and BEN-ZION, Y. (2010), Diversity of fault zone damage

and trapping structures in the Parkfield section of the San

Andreas Fault from comprehensive analysis of near fault

seismograms, Geophys. J. Int., 183(3), 1579–1595, doi:10.1111/j.

1365-246X.2010.04816.x.

LI, Y.G., and LEARY, P. C. (1990), Fault zone trapped seismic

waves, Bull. Seismol. Soc. Am., 80, 1245–1271.

LI, Y. G., LEARY, P. C., AKI, K. and MALIN, P. (1990), Seismic

trapped modes in the Oroville and San Andreas fault zones,

Science, 249, 763– 765, doi:10.1126/science.249.4970.763.

LI, Y.G., AKI, K., ADAMS, D., HASEMI, A., and LEE, W.H.K. (1994),

Seismic guided waves trapped in the fault zone of the Landers,

California, earthquake of 1992, J. Geophys. Res. 99,

11705–11722.

LO GIUDICE, E., and RASA, R. (1992), Very shallow earthquakes and

brittle deformation in active volcanic areas: the Etnean region as

example, Tectonophysics 202, 257–268.

MCGUIRE, W.J., and PULLEN, A.D. (1989), Location and orientation

of eruptive fissures and feeder-dykes at Mount Etna: influence of

gravitational and regional stress regimes, J. Volcanol. Geo-

therm. Res. 38, 325–344.

MIZUNO, T., and NISHIGAMI, K. (2004), Deep structure of the Moz-

umi-Sukenobu fault, central Japan, estimated from the

subsurface array observation of fault zone trapped waves, Geo-

phys. J. Int., 159(2), 622–642, doi:10.1111/j.1365-246X.2004.

02458.x.

MONACO, C., TAPPONNIER, P., TORTORICI, L., and GILLOT, P.Y.

(1997), Late Quaternary slip rates on the Acireale–Piedimonte

normal faults and tectonic origin of Mt. Etna (Sicily), Earth

Planet. Sci. Lett. 147, 125–139.

MONACO, C., DE GUIDI, G., and FERLITO, C. (2010), The Morpho-

tectonic map of Mt. Etna, Ital. J. Geosci., 129, 3, 408–428.

NERI, M., GARDUNO, V.H., PASQUARE, G., and RASA, R. (1991),

Studio strutturale e modello cinematico della Valle del Bove e

del settore nord-orientale etneo, Acta Vulcanologica, 1, 17–24

(in Italian).

NERI, M., ACOCELLA, V., and BEHNCKE, B. (2004), The role of the

Pernicana Fault System in the spreading of Mt. Etna (Italy)

during the 2002–2003 eruption, Bull. Volcanol. 66, 417–430,

doi:10.1007/s00445-003-0322-x.

NERI, M., GUGLIELMINO, F., and RUST, D. (2007), Flank instability

on Mount Etna: Radon, radar interferometry, and geodetic data

from the southwestern boundary of the unstable sector, J. Geo-

phys. Res., 112, B04410, doi:10.1029/2006JB0047.

NERI, M., CASU, F., ACOCELLA, V., SOLARO, G., PEPE, S., BERARDINO,

P., SANSOSTI, E., CALTABIANO, T., LUNDGREN, P., and LANARI, R.

(2009), Deformation and eruptions at Mt. Etna (Italy): a lesson

from 15 years of observations, Geophy. Res. Lett. 36(2), L02309,

doi:10.1029/2008GL036151.

PANZERA, F., D’AMICO, S., LOTTERI, A., GALEA, P., and LOMBARDO,

G. (2012), Seismic site response of unstable steep slope using

noise measurements: the case study of Xemxija bay area, Malta,

Nat. Haz, Earth Sci. 12, 3421–3431, doi:10.5194/nhess-12-3421-

2012.

PANZERA, F., LOMBARDO, G., and MUZZETTA, I. (2013), Evaluation of

buildings dynamical properties through in situ experimental

techniques and 1D modelling: the example of Catania, Italy, J.

Phys. Chem. Earth, doi:10.1016/j.pce.2013.04.00.

PANZERA, F., LOMBARDO, G., and RIGANO, R. (2011 a), Evidence of

topographic effects analysing ambient noise measurements: the

study case of Siracusa, Italy, Seismol. Res. Lett. 82(3), 385–391,

doi:10.1785/gssrl.82.3.385.

PANZERA, F., RIGANO, R., LOMBARDO, G., CARA, F., DI GIULIO, G.,

ROVELLI, A. (2011b), The role of alternating outcrops of

3096 F. Panzera et al. Pure Appl. Geophys.

sediments and basaltic lavas on seismic urban scenario: the

study case of Catania, Italy, Bulletin of Earthquake Engineering

9(2), 411–439. doi:10.1007/s10518-010-9202-x.

PENG, Z., and BEN-ZION, Y. (2006), Temporal changes of shallow

seismic velocity around the Karadere-Duzce Branch of the North

Anatolian Fault and strong ground motion, Pure Appl. Geophys.

163, 567–600.

PISCHIUTTA, M., CULTRERA, G., CASERTA, A., LUZI, L., and ROVELLI,

A. (2010), Topographic effects on the hill of Nocera Umbra,

central Italy, Geoph. J. Inter., 182(2), 977–987, doi:10.1111/j.

1365-246X.2010.04654.x.

PISCHIUTTA, M., SALVINI, F., FLETCHER, J., ROVELLI, A., and BEN-

ZION, Y. (2012), Horizontal polarization of ground motion in the

Hayward fault zone at Fremont, California: dominant fault-high-

angle polarization and fault-induced cracks, Geophys. J. Int.,

188(3), 1255–1272, doi:10.1111/j.1365-246X.2011.05319.x.

PISCHIUTTA, M., ROVELLI, A., SALVINI, F., DI GIULIO, G., and BEN-

ZION, Y. (2013a), Directional resonance variations across the

Pernicana Fault, Mt. Etna, in relation to brittle deformation

fields, Geophys. J. Int., 193(2), 986–996, doi:10.1093/gji/ggt031.

PISCHIUTTA, M., ANSELMI, M., CIANFARRA, P. ROVELLI, A., and SAL-

VINI, F. (2013b), Directional site effects in a non-volcanic gas

emission area (Mefite d’Ansanto, southern Italy): Evidence of a

local transfer fault transversal to large NW–SE extensional

faults? J. Phys. Chem. Earth, 116–123, doi:10.1016/j.pce.2013.

03.008.

PISCHIUTTA, M., PASTORI, M., IMPROTA, L., SALVINI, F., and ROVELLI,

A. (2014), Orthogonal relation between wavefield polarization

and fast S-wave direction in the Val d’Agri region: an inte-

grating method to investigate rock anisotropy, J. Geophys. Res,

119, 1–13, doi:10.1002/2013JB010077.

PLACE, J., BLAKE, O., RIETBROCK. A., and FAULKNER, D. (2014), Wet

fault or dry fault? A laboratory approach to monitor at distance

the hydromechanical state of a discontinuity using controlled

source seismics, Pure Appl. Geophys, this volume.

RASA, R., ROMANO, R., and LO GIUDICE, E. (1981), Morphotectonic

map of Mt. Etna, 1:100.000 scale, Progetto Finalizzato Geodin-

amica, Istituto Internazionale di Vulcanologia, CNR, Catania,

Italy.

RIGANO, R., CARA, F., LOMBARDO, G., and ROVELLI, A. (2008),

Evidence of ground motion polarization on fault zones of Mount

Etna volcano, J. Geophys. Res. 113, B10306, doi:10.1029/2007-

JB005574.

ROMANO, R., STURIALE, C., and LENTINI, F. (1979), Geological map

of Mt. Etna, 1:50.000 scale, Progetto Finalizzato Geodinamica,

Istituto Internazionale di Vulcanologia, CNR, Catania, Italy.

ROVELLI, A., CASERTA, A., MARRA, F., and RUGGIERO, V. (2002), Can

seismic waves be trapped inside an inactive fault zone? The case

study of Nocera Umbra, central Italy, Bull. Seism. Soc. Am.,

92(6), 2217–2232, doi:10.1785/0120010288.

RUST, D., and NERI, M. (1996), The boundaries of large-scale

collapse on the flanks of Mount Etna, Sicily. In: Volcano Insta-

bility on the Earth and Other Planets, edited by W.M. McGuire,

A.P. Jones and J. Neuberg, Spec. Pub. Geol. Soc. London, 110,

193–208.

SALVINI, F., BILLI, A., and WISE, D.U. (1999), Strike-slip fault-

propagation cleavage in carbonate rocks: the Mattinata Fault

Zone, Southern Apennines, Italy, J. Struct. Geol., 21, 1731–1749.

SEEBER, L., ARMBRUSTER, J. G., OZER, N., AKTAR, M., BARIS, S.,

OKAYA, D., BEN-ZION, Y., and FIELD, E. (2000), The 1999

Earthquake Sequence along the North Anatolia Transform at the

Juncture between the Two Main Ruptures, in The 1999 Izmit and

Duzce Earthquakes: preliminary results, Edited by Barka A.,

Kazaci, O., Akyuz, S., and Altunel, E., Istanbul Technical Uni-

versity, 209–223.

SESAME (2004), Guidelines for the implementation of the H/V

spectral ratio technique on ambient vibrations: Measurements,

processing and interpretation, SESAME European Research

Project WP12, deliverable D23.12, at http://sesame-fp5.obs.ujf-

grenoble.fr/Deliverables, 2004.

SOLARO, G., ACOCELLA, V., PEPE, S., RUCH, J., NERI, M., and SANSOSTI,

E. (2010), Anatomy of an unstable volcano from InSAR: multiple

processes affecting flank instability at Mt. Etna, 1994–2008, J.

Geophys. Res., 115, B10405, doi:10.1029/2009JB000820.

SPUDICH, P., HELLWEG, M., and LEE, W.H.K. (1996), Directional

topographic site response at Tarzana observed in aftershocks of

the 1994 Northridge, California, earthquake: implications for

mainshock motions, Bull. Seism. Soc. Am., 86(1B), S193–S208.

SPUDICH, P., and OLSEN, K.B. (2001), Fault zone amplified waves as

a possible seismic hazard along the Calaveras Fault in central

California, Geophys. Res. Lett. 28(13), 2533–2536, doi:10.1029/

2000GL011902.

VIDALE, J.E. (1986), Complex polarization analysis of particle

motion. Bull. Seism. Soc. Am. 76, 1393–1405.

(Received September 17, 2013, revised March 10, 2014, accepted March 12, 2014, Published online March 27, 2014)

Vol. 171, (2014) Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna 3097